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Russell, H. M
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OfRAR'
TECHNICAL SERIES, No. 23, PART II. TATE PI.Ar OAa
U. S. DEPARTMENT OF AGRICULTURE,
BUREAU OF ENTOMOLOGY.
L. O. HOWARD, Entomologist and Chief of Bureau.



MISCELLANEOUS PAPERS.



AN INTERNAL PARASITE OF THYSANOPTERA.




BY

H. M. RUSSELL,
Entomological Assistant.




ISSUED APRIL 27, 1912.














WASHINGTON:
GOVERNMENT PRINTING OFFICE.
S i i























BUREAU OF ENTOMOLOGY.

L. O. HOWARD, Entomologist and Chief of Bureau.
C. L. MARLATT, Entomologist and Acting Chief in Absence of Chief.
R. S. CLIFTroN, Executive Assistant.
W. F. TASTET, Chief Clerk. F. II. CHITTENDEN, in charge of truck crop and stored product insect investigations. A. D. HOPKINS, in charge of forest insect investigations. W. D. HUNTER, in charge of southern field crop insect investigations. F. M. WEBSTER, in charge of cereal and forage insect investigations. A. L. QUAINTANCE, in charge of deciduousfruit insect investigations. IE. F. PHILLIPS, in charge of bee culture. D. M. Ro(ERS, in charge of preventing spread of moths, field work. ROLLA P. ('URRIE, in charge of editorial woArk. MAIABEL (COLCORD, in charge of library.

TRUCK CROP AND STORED PRODUCT INSECT INVESTIGATIONS.
F. H. CHITTENDEN, in charge.

II. M. RUSSELL, C(. II. POPENOE, IVILLIAM B. PARKER, II. O. MARSH, M HI. Hn,
FRED A. JOHNSTON, JOHN E. GRAF, entomological assistants. I. J. CONDIT, collaborator in California. P. T. COLE, collaborator in tidewater Virginia. W. N. ORD, collaborator in Oregon. THOMAS H. JONES, collaborator in I'orto Rico. MARION T. VAN HORN, PAULINE M. JOHNSON, jreparators.
II







C() N "I" E N T S

Page.
Introduction .................................................. ........... 25
history of the discovery .................................................... 26
Classification anld original descrij)tiou ...... --....................... ..... 27
Life-history summary of the origiil host (Ileliothripsfsciatus Pergande) 28 Methods of coiletijg for evidence of parasitisn ............................. 28
Methods of rearing parasites for life-history study --------------------------- 29
Stage of thrips showing parasitisI- .......................................... 30
Appearance of )arasitized prepipp ---------------------------------------- 30
Behavior of parasitized thrips --------------------------------------------- 31
Time between first indication of parasitism and f,)rmation of parasite pupw under laboratory conditions ------------------------------------------------ 31
Effect of cold on parasitized prepup:e ------------------------------------- 31
The pupa of the parasite -------------------------------------------------- 3
Formation ----------------------------------------------------------- 1
Description --------------------------------------------------------- 32
Length of pupal stage ------------------------------------------------ 33
Effect of cold on pupal stage ------------------------------------------ 33
The adult parasite -------------------------------------------------------- 33
Emergence ---------------------------------- ------------------------- 33
Description of newly emerged, live adult ................................ 34
Relation and proportion of sexes -------------------------------------- 35
Parthenogenesis ....................................................... 35
Method and period of oviposition -------------------------------------- 36
Habits in the open ---------------------------------------------------- 38
Effect of oviposition in different stages of host --------------------------- 38
Number of eggs and length of oviposition period ------------------------- 38
Double parasitism ---------------------------------------------------- 39
Activity ------------------------------------------------------------- 39
Flight -------------------------------------------------------------- 40
Sight ---------------------------------------------------------------- 40
Behavior on different kinds of leaf surfaces ----------------------------- 40
Host relations -------------------------------------------------------- 41
Ileiothripsfascia tus Pergande -------------------------------------- 41
Thrips tabaci Lindeman ------------------------------------------ 41
Euthrips tritici Fitch --------------------------------------------- 42
Ileliothrips h.morrhoidalis Bouch ---------------------------------- 42
lleliothrips femoralis Router -------------------------------------- 42
Miscellaneous observations ---------------------------------------- 43
Summary of experiments to determine host relations ---------------- 43
Length of life in confinement ------------------------------------------ 43
Causes of mortality in the open ---------------------------------------- 44
Effect of oviposition in different ages of host larvae ------------------------- 44
Life history -------------------------------------------------------------- 44
Length of time fr.)m ovipositiop to first indication of parasitism -----------45
Length of time from oviposition to pupation of the parasite -------------- 45
Length of life cycle -------------------------------------------------- 46
First appearance in the spring ----------------------------- ------------ 47
Last appearance in the fall ------------------------------------------- 47
Number of generations ------------------------------------------------ 47
Hibernation ---------------------------------------------------------- 47
Occurrence ------------------------------------------------------------- 48
Distribution ------------------------------------------------------------- 48
Percentage of parasitism in different localities ------------------------------ 49
Means of colonizing the parasite ........................................... 51
III


























ILLUSTRATIONS.

Page.
1'(u. 1. Vials used for the study of thrips parasites .......- -.... 29
2. (,'age for rearing parasites.......................................... 30
3. Parasitized prepupas of Ileliothripsfasciatus.......................... 31
4. Diagram illustrating length of time from appearance of parasitism to
formation of parasite pupae of Thripoctenus russelli, July-October,
1911 ............................................................ 32
5. Thripoctenus russelli: Pupa, ventral view............................. 33
6. Diagram illustrating length of pupal stage in 146 individuals of Thripoctenus russelli .................................................... 34
7. Thripoctenus russelli: Adult ......................................... 35
8. Diagram illustrating length of time from oviposition to first evidence of
parasitism for 353 individuals of Thripoctenus russelli---------------............... 47
9. D)iagrain illustrating length of time from oviposition to pupation for 241
individuals of Thripoctenus russelli............................... ------------------------------48
10. Diagram illustrating total number of days in the life cycle of Thripoctenus russelli; summary of all rearing experiments ................. 49
11. Mailing tubes used in shipment of thrips parasites.................... -------------------51
IV











U. S. D. A., B. E. Tech. Ser. 23, Pt. IT. Issued April 27, 1912.

MISCELLANEOUS PAPERS.


AN INTERNAL PARASITE OF THYSANOPTERA.
By H. M. RUSSELL,
Entomological Assistant.
INTRODUCTION.
Although many of the injurious forms of thrips have been the subject of exhaustive research, not only in the United States but throughout the entire world, no internal parasites of importance had been reared until 1911. In July the writer reared a parasite from the prepupa of Heliothrips fasciatus Pergande, which Mr. J. C. Crawford has described under the name Thripoctenus russelli. At the present time the already-published literature on thrips parasites is not so abundant as to preclude a brief review in this paper. In 1860 Mrs. Charlotte Taylor I described a parasite of thrips on wheat under the name Pezomachus thripites. This insect was figured and described as a small wingless creature, a little larger than its host. The thorax consisted of 2 nodes, while the antenna were madeup of 40 joints and the palpi of 6. These parasites were observed emerging from thrips larvve in some numbers. They copulated in a short time, after which the female deposited her eggs in the larvoe of the thrips.
Unfortunately this description is so meager that this insect could hardly be recognized unless actually reared from its host again. It represents, therefore, one of our lost species.
In June, 1911, G. del Guercio published a short note on a chalcidid parasite of Phiwothrips olewe (Costa) Targioni.2 He gave a short description of the appearance of a parasitized thrips and an excellent figure of the same. According to Del Guercio, the parasitic larva seen within the host is of an oval shape, reddish in color, and resembles a dipterous larva. The adult was mentioned only as "an elegant chalcidid," which he stated he would describe, as soon as he had enough specimens. Nothing was written concerning the biology, habits, or relative abundance of this insect.
Del Guercio, in September, 1911, published a further account of this parasite of the olive thrips, to which he gave the name TetrastiAmerican Agriculturist, vol. 19, p. 300, 1860. 2 Rcdia, vol. 8, pp. 65-68, 1911.
25





26 MISCELLANEOUS PAPERS.

clus gentilii.1 In this paper he described the adult, of which he stated he had both sexes. According to his observations, these copulate frequently, after which the female seeks out the young larvae of the host and oviposits in them. The larger larva, prepupa, pupa, and adult stages are passed by in all cases. This insect, he states, passes the winter as larva in its dead host and completes its transformation in the spring. It has been very abundant in Liguria. Del Guercio suggested that by cutting off limbs of the olive infested with parasitized material and shipping the same, this species could be diffused in new localities. It appears from his description of the adult and its habits that he has a larger species than the one referred to in the present paper and with quite different life history and habits.
Dr. L. O. Howard,2 December 7, 1911, gave a short note on the discovery of the parasite treated in this paper, which is mentioned here as it was the first published note on this insect.
Mr. J. C. Crawford, of the United States National Museum, in December, 1911, described Thripoctenus russelli as a new genus and new species.3 At the same time, the writer gave a brief summary of the life history and habits of this insect.
The present paper, in the light of the information so far obtained on this parasite, aims to present the life history, habits, and economy of Thripoctenus russelli. The writer has been ably assisted in the work by Mr. John E. Graf and every point of importance discovered in the life history of this insect has been separately verified by either M r. Graf or the writer. The writer also wishes to express his appreciation to Dr. L. 0. Howard, Dr. F. II. Chittenden, and Mr. J. C. Crawford for their assistance in producing this paper.
HISTORY OF THE DISCOVERY.
The writer, while engaged in the study of truck-crop and sugarbeet insects, at Compton, Cal., began, in May, 1910, an investigation of the life history, habits, and means of control of the bean thrips (Heliothripsfasciatus Pergande). From.that time until the following fall, frequent collections of the young of this insect were made for study and rearing purposes. On November 10, 1910, the last collection of larva of this thrips found that year was made in the yard of the laboratory, for the purpose of determining in what stage this insect passed( the winter. These fall-collected larvm were placed in vials and( kept under daily observation and on December 10 four specimens of the prepupal stage of Heliothrips fasciatus gave unmisSAtti D)i Georgofili Di Firenw, 5 ser., vol. 8, pp. 222-227, 1911.
2 Report of the Entomologist for 1911, U. S. Department of Agrieutlture.
a Proc. Ent. Soc. Wa-sh., vol. 13, p. 233, December 29, 1911.
SI bid., pp. 235-238.






AN INTERNAL PARASITE OF THYSANOPTERA. 27

takable signs of parasitism. Two of these died, but on December 13, from the other two, parasitic larva had emerged and formed naked pupo. These parasitic pupe were sent to Dr. Howard in the hope that the adult could be reared and identified, but unfortunately both died and it was impossible to identify the species. At that time Dr. Howard wrote to the author and advised him of the scientific interest attached to this insect, and requested him to make careful collections of the host larvo in order to recover the parasite, if possible.
This work was immediately taken up, but it was not until the following year, during February, 1911, that the larvW of Heliothrips fasciatus were again found in the field. From that time until July 1, 1911, large numbers of the larva of this thrips were collected and reared to adults, but in no case was there a sign of parasitism. However, on July 3 one prepupa of Heliothripsfasciatus.gave unmistakable signs of parasitism, but it died before the parasite matured.
While collecting thrips material at Hollywood, Cal., on June 29, 1911, a very minute hymenopteron was observed on the underside of a bean leaf in company with larvm of this same thrips; and while it was being carefully examined through a small hand lens it appeared to oviposit once in a thrips larva. After this, as the insect was not again observed to come into contact with thrips larve, it was captured and put in 50 per cent alcohol. When the minute size of this insect was taken into consideration, it was at once evident that the single case of apparent oviposition must be more or less doubtful, and that it was very essential that the insect be reared to support this observation. This was actually accomplished on July 29, 1911, when an adult of this same species was reared from a larva of Ieliothripsfasciatus, collected at Hollywood on June 29, 1911. From then until November this parasite has been collected and reared in large numbers.
CLASSIFICATION AND ORIGINAL DESCRIPTION.
Thripoctenus russelli Crawford is a minute insect belonging to the hymenopterous family Chalcididw and to the subfamily Tetrastichinm. Mr. Crawford' has erected a new genus and species for this insect. He places the genus near Winnemana, but separates it from the latter by the absence of two longitudinal grooves on the scutellum.
The following technical description of the genus and species is taken from Mr. Crawford's paper: Belongs to the tribe Tetrastichini; ante~noe with two joints in the funicle, one very small ring joint, club three-jointed, the joints fused; parapsidal furrows indistinct, Loc. cit.






28 MISCELLANEOUS PAPERS.
incomplete; mesonotum without a median groove; scutellum without longitudinal grooves; propodeum without a median carina; spiracles round; submarginal vein very short, marginal vein long, stigmal short, the knob almost subsessile; postmarginal vein developed though not showing distinctly, as it is, as are the other veins, colorless; marginal fringes of anterior wings almost two-thirds as long as the greatest width of the wings.
Female.-Length about 0.6 mm. Head and thorax black, the abdomen brown, with a large basal flavous spot; head thin anterio-posteriorly, collapsing after death; antennae testaceous, the pedicel longer than the two joints of the funicle combined; second joint of the funicle longer than the first; hairs on antennae long; head and thorax smooth, polished; legs including the coxve testaceous; wings hyaline, the veins colorless.
Male unknown.
LIFE-HISTORY SUMMARY OF THE ORIGINAL HOST.
(Heliothrips fasciatus Pergande.)
Heliothrips fasciatus Pergande, the original host of this parasite, is found throughout the entire State of California. In the vicinity of Compton, during the months of July and August, the following life-history notes were obtained.
The eggs are laid in the leaves of the food plant and hatch in from 13 to 19 days. The larva immediately begin to feed on the leaves of the host plant, generally on the underside, but when crowded appear to thrive on the upper surface of leaves as well. This stage requires from 10 to 12 days, and as soon as the larvae are full grown, most of them leave the plant to pupate in rubbish or cracks. The prepupal and pupal stages together occupy from 7 to 12 days, making the total length of the life cycle from 30 to 44 days. The insect is thus exposed to the attack of the parasite during the larval stage only, or for from 10 to 12 days.
METHODS OF COLLECTING FOR EVIDENCE OF PARASITISM.
The small size of both the host and the parasite makes it imperative that special methods be adopted in the rearing of these insects. In making collections for evidence of parasitism the bean leaves badly infested by the host thrips were picked and brought into the laboratory in tin boxes. The thrips larvae were then removed with camelshair brushes and placed on freshly picked leaves of the food plant. These were then placed in vials (100 mm. in length by 28 mm. in diameter), which had been lined at the bottom with filter paper. (See fig. 1, at left.) It was found that the filter paper absorbed the surplus moisture and the material transformed better than in cases where the filter paper was omitted. These vials were then tightly closed with cotton plugs, labeled, and put into an outdoor insectary. Each day the material was examined, and the larva,, when necessary, were removed to fresh leaves by means of the brushes. As soon as full





AN INTERNAL PARASITE OF rIL1YSANOIIllIA. 29

grown the thrips larvve either pushed in between the vial an(I edge of the cotton plug or in between the filter paper and the vial, in order to pupate in protected places.
Because of this habit it was very easy to determine paras~itismI and the amount of material affected. It was found that many of the smaller thrips larvea died before completing their growth, but this mortality, fortunately, did not retard the investigation.
When it was desirable to discover if material was parasitized or to get adult parasites, the leaves infested with thrips larva were put into small wooden boxes with holes bored in the sides for glass tubes.
























FIG. 1.-Vials used for the study of thrips parasites. (Original.)
(See fig. 2.) These openings were kept closed until the thrips larvaw had all changed, and then the tubes were put in. The parasites upon emerging immediately sought the light and were readily removed from the tubes.
METHODS OF REARING PARASITES FOR LIFE-HISTORY STUDY.
In studying the life history and habits of this parasite, use was made of the large vials described above. In this case, however, the filter paper was left out of the tubes until after the parasite had oviposited in its host. (See fig. 1, at right.) Whenever adults of
26003-12-2





30 MISCELLANEOUS PAPERS.

this parasite emerged they were placed in these vials with fresh leaves heavily infested with healthy larve of this species of thrips. In this way oviposition could be carefully watched, and when desired the parasites were removed to fresh vials. The larve in which the parasites had oviposited were carefully changed to fresh leaves whenever necessary until they became mature and pupated.
STAGE OF THRIPS SHOWING PARASITISM.
Although hundreds of p)arasitized thrips have been observed in the laboratory, in no case has the parasitism ever been evident in any stage but that of the prepupa. The larve have been carefully examined at regular intervals but until they have changed to prepupae it is impossible to ascertain which of the larvae are actually parasitized. In the p)repupal stage parasitism develops rapidly, the growth of the host is retarded, and its death follows shortly. In every instance in which a prepupa has changed to a pupa an adult thrips has emerged from this stage, so that, in so far as our present knowledge is complete, the
prepupal stage of the thrips
is the only one in which
parasitism is evident.
-APPEARANCE OF PARASITIZED PREPUPM.
Parasitism first becomes
evident two or three days
after the thrips larve have
changed to the prepupal
stage, and, in case unparasitized propups are in the
FIG. 2.-Cage for rearing parasites. (From Banks.) sitized prepupe are in the vial, often after these have
further changed to the pupal stage. Parasitism is first indicated in the semitransI)arent host prepupa when the color contents of the antenna, head, and anal end of the thrips are removed through the feeding of the internal parasite, leaving the parts hyaline. At the same time there is seen an internal deepening of color in the center of the body. As the feeding of the parasitic larva progresses, the color is more and more directed to the center of the body, and the edges of the body begin to appear hyaline. At about this time the legs of the host larva collapse and sprawl in all directions. A few hours after parasitism is first noticed, the body of the thrips prepupa is swollen and rounded, the original shape being lost. The body is entirely hyaline or colorless except an inner cylinder of deep crimson in IleliothIi s fasciatuis (fig. 3). or yellow in Thrips tabaci Lindeman and Eit/hrins tritici Fitch.





AN INTEltNAL PARASITE OF TIYSANOPTEIA. 31
BEHAVIOR OF PARASITIZED THRIPS.
The larva, of thrips that have been p)arasitize(d require a few more days for development before changing to the prepupal stage than healthy individuals. It has also been noticed that parasitizedI larva are more apt to change to pireipupa under thie paper in the bottom of the vials, while normal larvae generally crawl up into the edge of the cotton plug before pu)ating. After parasitism becomes evident in the prepupal stage the doomed insects still have the power of locomotion and will crawl around to some extent. But soon the contents of the limbs are drawn from themni and as these collapse all motion in the host thrips ceases. TIME BETWEEN FIRST INDICATION OF PARASITISM AND FORMATION OF PARASITE PUPE UNDER LABORATORY CONDITIONS.
After p)arasitism becomes evident in the thrips prepup'T, the parasitic larvwe grow very rapidly and within two days, in a few cases, have changed to pupat of Thripoctenus russelli; but the majority change in three and four days. In a few cases this period has taken as long as ten days, but this is unusual and apparently due to some condition unknown at this time. The diagram, figure 4, has been FIG.3.-Parasitized prepupe prepared to show the time between the ap- of Heliothrips fasciatus,
Sb lateral and dorsal views.
pearance of parasitism and the formation of Greatly enlarged. (Origithe parasite puppy. nal.)
In a total of 223 specimens, 104, or nearly 50 per cent, changed to pups, in three days, and 85, or over 38 per cent, in four days.

EFFECT OF COLD ON PARASITIZED PREPUPZE.
A few experiments were made with parasitized prepupae of the host thrips to determine the effect of cold on the parasitic larvae within. On October 19 several parasitized prepupae were placed in cold storage in a dry atmosphere kept at a constant temperature of 310 F. This material was removed November 27, 1911, and when examined on December 2, by Mr. Graf, all of the parasitic larvae were found to have emerged and changed to pupae.

THE PUPA OF THE PARASITE.
FORMATION.
As has been already stated, this parasite invariably completes its growth in the prepupa of its host, and for this reason it changes to the pupal stage in the localities which may bave been chosen by its





32 MISCELLANEOUS PAPERS.
host for pupation. This, of course, will vary with the different species of tlhrips attacked. Two years constant work with Heliothrips fasciatus has shown that, at least in southern California, the majority of the larvae of this species pupate in rubbish, in cracks, or under clods of earth. Therefore, when Heliothrips fasciatus is the host of this parasite the latter will be found to form its pupe in the same places. Thrips tabaci and Euthrips tritici both enter the ground and form oval cells in which to pupate, and thus, when either of these thrips is the host, the /0 pupe of this parasite will necessarily
9- be formed underground.
--- The parasitic larva, when ready
8-- ------ to pupate, splits the skin of the
host larva at the anterior end and 7 then gradually works the skin off
7 at the anal end. This molted skin
in many cases may be seen lying behind the parasitic pupa.

DESCRIPTION.
When newly formed, the pupa
of Thripoctenus russelli is almost
3 white, but the intestinal tract in
30 ---- the thorax and anterior part of the
S- abdomen is a bright crimson, sinm20 ---- -ilar to the spot in the prepupa of 4f --s- --- --the host that, as stated previously,
/o- -- first indicates parasitism. After a
---- --- short exposure to the air the edges
o c o of the body and appendage cases
S. of the parasitic pupa begin to
FIG. 4.-Diagram illustrating length of time darken. At first this darker color from appearance of parasitism to formation
of parasite pupm of Thripoctenus russelli, is of a bluish tint, but it changes July-October, 1911. (Original.) more and more to black until the pupa is shining black excepting that portion where the reddish color still shows through. The pupa (fig. 5), which is flattened oval, is rounded in front, but with a slight notch at the center, and is followed by a distinct neck the sid(les of which are convex. Posterior to this it is abruptly widened to its fullest extent and forms prominent shoulders. The sid(les then converge very slightly to the beginning of the abdominal segments. The abdomen forms a well-rounded oval, the appendages all being closely appressed to the underside of the pupal shell.






AN INTERNAL PARASITE OF TIIYS-AN0P'TERlA. 3

LENGTH OF PUPAL STAGE.
The pupal stage of this hymenopteron requires over one-half 'of the time of the total life cycle. During the months of July, August, and September, this stage is from 16 to 28 days, but in the majority of cases from 17 to 20 days are required. The diagram, figure 6, has been prepared from the records of 146 indlividluals, andl indicates the length of this stage and the number of individuals for each different length.
EFFECT OF COLD ON PUPAL STAGE.
In order to test the effect of coldl, pupae brought into Washington in September were exposed to outside conditions for over a month. Durin-g this time the pupae remained dormant. About the middle of October these were divided into two lots, one of which was exposed while the other was placed in the greenhouse. After a number of (l4ys the adults began to emerge from the pupae in the greenhouse, in which location the temperature was above the prevailing temperature on the outside, wilie the lot which had been placed on the outside remained dormant.
On October 19 Mr. Graf placed a number of pupae of this species in cold storage at a temperature of 31' F. in both dry and Moist atmospheres. On November 28 he removed some of these pupae and shipped them to the writer in Washington, D. C. On December 4 these were divided into two. lots, one of which was placed in the outdoor insectary while the other was placed in the insectary green- FI,. 5. Thripoctenus rsel:Pupa, ventral
house. The temperature maintained in this green- view. G re atly enhouse is moderately low and so did not tend greatly larged. (Original.) to hasten development in the pupae. However, on February 14 and 17, adults emerged 79 and 82 days after the pupae were removed from cold storage, in which they had been kept for 41 days. During the three weeks previous to emergence of the adults the average mean temperature in the greenhouse was 65.5' F. February 20 the pupae in the outdoor insectary were still unchanged.
THE ADULT PARASITE.
EMERGENCE.
The emergence of the adult parasite is best described by Mr. Graf's notes as taken at the time of observation, August 9, 1911. Several adult parasites were emerging from the pupal cases and were carefully watched. In every case noted, the anterior end of the pupal case, covering the face






34 MISCELLANEOUS PAPERS.

of the contained parasite, split off and the parasite began to emerge. In addition to the anterior opening, the pupal case, in mary instances, split down the back, although this did not hold true with all. In one case the split-off anterior end of the pupal case adhered to the face and antenare of the newly emerged insect and it required fully an hour from the time the parasite was entirely freed from the pupal case until it was freed from that portion which covered its face. The longest time in the period of emergence was consumed by the insect in freeing its head and thorax from the pupal case. After this was accomplished the insect rapidly worked the case from its abdomen by the aid of its posterior legs. The time of emergence noted varied greatly, depending probably upon the strength of the individual and the hardness of the pupal case. The shortest time noted was 41 minutes from the time the pupal case first began to split until the adult parasite was entirely free. The longest period noted was 1 hours. These both seemed to be exceptional cases, as five others were timed as follows: (1) 21 minutes, (2) 33 minutes, (3) 25 minutes, (4) 28 minutes, and (5) 36 minutes.
For a short time after freeing itself from the pupal case, the adult parasite remains in one position, busily engaged in cleansing its body. The head and antenna are cleansed by the fore legs and the wings and body by the hind legs. These
.--are then cleansed by drawing 44 -them through the mandibles. k -M- When the adult first emerges
30 --- there is a suggestion of pink
25- -------- color in the abdomen. This is
o- ------ explained by the fact that this
Z -\ parasite in the larval stage does
/ -- not expel any excrement, and
------ this accumulation of waste
0 17 2 2 Z2 25 XZ 2 7 Z*8
4.- vs matter is of necessity still conFIG. (.-Diagramn illustrating length of pupal stage in tained in the abd(lomen and(I im146 individuals of Thripcttnus russeli. (Original.) parts to it the crimson color so
noticeable in parasitized prepupa. During the change from larva to pupa this waste product still remains and is, in fact, as before stated, in the abdomen of the adult upon emergence. In a short time, however, the a(dult expels this semiliquid, and it forms several minute re(d spots on the surface beneath.

DESCRIPTION OF NEWLY EMERGED, LIVE ADULT.

The living adult of Thripoctenus russelli is a very minute insect, being about 0.7 mm. long and 0.2 mm. wide. The head is black, with (lark red eyes and light yellow antenna. The thorax is bluish black with the wings hyaline and the legs light yellow, while the ab(lomen is light yellow with more or less black toward thie anal end. The ovipositor is nearly concealed when not in use, but when extended is very long, slen(ler, and light yellow. This insect is shown in figure 7, so that with the technical descriptionn on page 35 a more lengthy, description at this time is unnecessary.







AN INTEI NAL PARASVIHE (), 'I'll YSAN()PTEIRA. 35

RELATION AND PROPORTION 0F SEXES.

During the course of this investigation hundreds of specimens of this insect have been reared, but among this number it has been impossible to find a single male. Furthermore, up to October 1, 1911, a generation of females had been reared from a female and these in turn had oviposited in their host. Regarding the length of time this insect might continue to multiply without the introduction of the male, or what proportion of males might be produced, it is impossible to determine at the present writing.

PARTHLENOGENESIS.

This insect reproduces parthenogenetically to a large extent, for, as the accompanying table indicates, in every one of a number of

























FIG. 7.-Thripoctenus russelli: Adult. Greatly enlarged. (Original.)

experiments performed to determine this point, the fact was clearly proven by the development of the parasite in thrips larvT, ovipositel in by unfertilized females.

TABLE 1.-Results of oviposition by unfertilized females of Thripoctenus rMsselli.


Date of
oviposi- First Number First Number
Experiment No. tion by sign of of host adult of
unfer- para- para- parasite parasite CI ergd.parasites
tilized sitism. sitized. emerged. emerged. female.

I .......................................... Aug. 13 Aug. 20 56 Sept. 11 Sept. 20 44
2.................................... ...do Aug. 21 35 Sept. 12 Sept. 18 25
3 .......................................... Aug. 14 Aug. 20 10 Sept. 11 Sept. 13 7
4 .......................................... Aug. 15 Aug. 21 6 Sept. 12 Sept. 15 5
5 ........................................ Aug. 19 Aug. 26 7 Sept. 18 Sept. 28 5
6 .......................................... Aug. 20 Aug. 27 31 Sept. 19 Oct. 2 18







36 MISCELLANEOUS PAPERS.

These six experiments with six unfertilized females gave a total of 135 parasitized forms, and from these were reared 104 parasite adults.
In many other species of parasitic insects where adults have been reared from eggs deposited by unfertilized females, the offspring have all been males.' However, in so far as we have progressed in the rearing of this parasite artificially, the offspring Qf unfertilized females have been females, for in every case these insects, when placed separately in vials with thrips larva, 'began to oviposit in their host. In one case at least the prepupa of the thrips developed signs of parasitism from the unfertilized egg of the second generation.
Table II shows results of 9 experiments to show effect of parthenogenesis in offspring of the first generation.

TABLE II.-Effect of parthenogenesis on offspring of first generation of Thripoctenue russelli.

Exp. Date. Experiment. Action. Result.
No.

1 Sept. 13 Unfertilized parasite, reared from egg Insect proved itself to be Sept. 24, 1 pardeposited by unfertilized female, female by ovipositing im. asitized preplaced in vial with thrips larvawe mediately. pupa a ppeared.
2 ...do ..... .... do .......................................do................. Noresult.
3 ...do ....... do ....................................... do ...................... Do.
4 ... ....................................... do ..................... Do.
5 Sept. 14 ....................................... do ...................... Do.
6 Sept. 15 .... do ....................................... do ...................... Do.
7 Sept. 16 .....do ....................................... do ...................... Do.
8 Sept. 18 .do.................................... do ..................... Do.
9 Sept. 23 do .............................. do..................... Do.


METHOD AND PERIOD OF OVIPOSITION.

Oviposition was observed under laboratory conditions in many cases, and in nearly all the operation was identical. The females of this insect, within a few hours after emerging from the pupa, and in all cases under observation, without being fertilized by the males, when placed on a leaf with thrips larvo began to oviposit as soon is they had located their hosts. The time required for this, of course, varied, as some species of thrips were not as active as others, nor as easily disturbed, so that the parasite had little trouble in finding them. The process is probably best described by the author's notes on oviposition in Heliothripsfasciatus taken at the time of observation. The female, in ovipositing, crawled very slowly over the leaf with her antenna diveroing but horizontal and constantly moving from side to side. In this manner she moved over the leaf more or less in a circle until she came in contact with the larva of Ilcliothripsfusiiatus. At this point the antenna, were drawn closely together and moved carefully and slowly over the larva from one end to the other; then, if she I See pp. 257 -2.8, Bi. 91, Bur. Ent., U. S. Dept. Agr. The Importation into the United States of the Parasites of the Gipsy Moth and the Brown-tail Moth. Dy L. 0. Howard and W. F. Fike.






AN INTERNAL PARASITE OF THYS.ANOPTERA. 37

appeared satisfied, the antenna were dropped downward, touching the larva. The abdomen was curved under the body and a long, slender, yellowish ovipositor extended from near the anal end of the body. This was quickly thrust into the body of the thrips larva, and as soon as the egg was deposited the ovipositor was withdrawn. Generally this was easily done, but in a few cases a short struggle was necessary before it could be accomplished. The ovipositor was usually inserted into the abdomen near the anal end, but sometimes it is thrust into the side of the thorax. In one instance, where the larva had been approached from in front, the ovipositor was inserted into the front of the body. After the egg was deposited the parasite in some instances would immediately select a new victim or she would rest while cleansing her legs, antennae, and body and then resume her quest. In other cases, after carefully examining a host larva she would leave it without attempting to oviposit. On eome occasions the female was observed to deposit an egg in a larva and after leaving it would shortly return to the same one and oviposit a second or even a third time. In the majority of cases the larve of Heliothripsfasciatus did not seem to be in the least disturbed by this action. While the writer was engaged in observing oviposition he timed the parasite from its first touching of the host larve with its antennte until the ovipositor was finally withdrawn and in three cases this procedure occupied 20, 50, and 30 seconds each. One female was carefully watched and in 5 minutes oviposited in 4 different larvae and rejected two others after short examination. DurinIg the second 5 minutes this female deposited 2 more eggs. Another female, recently emerged, was placed in a vial with a large number of larve of this thrips at 3.15 p. nm. At 3.25 p. m. she had deposited her first egg and during the next hour, under the constant observation of the writer, she deposited 20 eggs in 18 larve in a period of 35 minutes, and 38 eggs in 36 larve in 60 minutes.
Oviposition in the larve of Thrips tabaci was identical with that in H. fasciatus, but the thrips larve were more active and more easily alarmed so that they either moved away or violently threw the tip of the abdomen around. In such cases, the parasite retreated but in a short time again approached the larva and in most cases after two or three failures would eventually succeed min ovipositing in the larve. The same thing was noticed with the larva of Euthrips
tritici, but in both cases the parasite by returning to the attack generally accomplished her purpose. The same parasite was often placed with one after another of these three species of thrips and would oviposit in each one without appearing to notice the difference. In one case a parasite was observed trying to oviposit in a dead thrips larva that was still more or less soft. Still another adult was observed attempting oviposition in a soft lump of dirt in the vial.
This parasite does not seem to prefer any particular size of larva, as it has been seen to oviposit in all sizes, from larve not over a day or two old to larve ready to change to prepupe. If, while examining the leaf for thrips larve, it encounters a larva from in front, the parasite generally moves around to the side and then oviposits in the region of the thorax or the abdomen.






38 MISCELLANEOUS PAPERS.

HABITS IN THE OPEN.
Oviposition as observed in the field is not very different from that in the laboratory. The following notes were taken while watching the parasite on foliage of turnip infested with Thrips tabaci: The female was observed crawling slowly along with the antennae, in motion and turned first to one side and then to the other; the body also was turned to one side or the other. At other times she ran very rapidly across the leaf. Finally she discovered a larva of tabaci and immediately placed her anterior legs on it and lowered her antenna until they rested on the body. She then oviposited in the host in the same manner that was observed so many times in the laboratory. In nature, however, the thrips larva were more scattered, so that this insect succeeding in finding and ovipositing in only 4 larve during 40 minutes, and to do this she covered nearly 6 square inches of leaf surface.
In other cases where this insect was observed in the open, a slight jar would frighten her and instantly she would take wing.
EFFECT OF OVIPOSITION IN DIFFERENT STAGES OF HOST.

In the course of the work in rearing the thrips parasite it was found that this insect would oviposit in all stages of the host. In order to determine if the parasite would develop in all stages, a number of experiments were performed in which the parasite was placed in a vial with p)repupm and pups of. Heliothrips fasciatus; and in every case the parasite did not hesitate to oviposit freely in these stages. In spite of this not a single form ever developed the slightest sign of parasitism, and the host adult developed in all cases. From these experiments, it al)pears, we can safely say that, although this insect will oviposit in all forms of this thrips, it will develop successfully only when the eggs are deposited in the larval stage of the host.

NUMBER OF EGGS AND LENGTH OF OVIPOSITION PERIOD.
Because of the extremely artificial conditions that were necessary in rearing this very minute hymenopteron, it is impossible even to estimate with any degree of correctness the egg-laying capacity of a female under normal conditions. This is due not only to the fact that the parasite is apt to die before the normal number of eggs is deposited, but also largely to the artificial method of rearing, by reason of which many of the host larvae that have been parasitized die before they show the usual indications of parasitism. In one case a female was observed, under laboratory conditions, to oviposit in 36 larvae of Helothrips fasciatus 38 times in 1 hour. From the oviposition record of another female, 56 p)arasitized el)tl)pupm d(evelOl)e(l, while a third, that un(ldohubtedlly p)arasitized a still larger number, gave a total of 91 p)aransitizedl forms that later developed.
IThe parasites in the vials were obl)served( to ovnposit in thrips, as a rule, during a )eriod of 5 days, although in one instance a female





AN INTERNAL PARASITE OF THYSANOPTERA. 39

was noted that oviposited on the first and eighth (lay while confine(l in a vial with larva of Tb rips tabaci, at Washington, D. C. Tiis oviposition period undoubtedly is shorter than that which occurs under conditions which are normal, for, confined in the vials, the parasites were in close contact with many thrips larvae, their eventual hosts, and so might deposit their eggs faster, while in the open field the parasites were compelled to spend considerable time in searching among the leaves for the thrips larva. Undoubtedly this alone would lengthen the normal period of oviposition to at least double that over which oviposition was observed to occur under artificial conditions in the laboratory.
DOUBLE PARASITISM.
Although it has been frequently observed that this parasite would oviposit in the same host two or even three times, only one adult hymenopteron has ever been reared from these doubly-parasitized specimens, and in nearly every case only one parasitic larva has seemed to develop in them. On one occasion, however, Mr. Graf, late in November, 1911, observed a case in which double parasitism had developed and under such conditions that its success was plainly indicated. At that time he took the following notes: In the prepupa that was doubly parasitized, the parasitic larva occupying the abdomen of the thrips was about full-sized and no great difference in size could be noted in the alimentary canal of this one as compared with the others. The second parasitic larva occupied the thorax of the host prepupa, with one end of its body against one anterior angle of the thorax of the host, while the other end was against the opposite posterior angle. The colored portion was about one-fourth natural siz-e, and it was more of a yellowish red. It was well shaped however, and not in the least distorted by its position. Under the high-power lens there was a plain division between the two and there was no mistaking the fact that there were two distinct hymenopterous larvae occupying the body of the prepupal host at the same time. Judging from the appearance under the lens, the larva occupying the abdomen of the host would have been able to develop and pupate, while the one located in ihe thorax, due to its cramped position and inadequate food supply, would have been unable to reach the pupating stage and would have died.
-Unfortunately this host was killed so that no data were obtained as to whether either of the parasitic larva would have been able to complete its transformation. However, double parasitism must always result in the death of one of the larvae and in such a small host undoubtedly in many cases both must die owing to the insufficient nourishment afforded by such a minute host.
ACTIVITY.
While the adult parasite is usually quiet, during the period just after emerging from the pupal case it is, under normal conditions, like most of the Hymenoptera, a very active insect. At times it has






40 MISCELLANEOUS PAPERS.

been noted to travel over the leaf surface with astonishing speed for so minute a creature, but in the majority of cases which came under observation it seemed to crawl over the leaf in a slow and cautious manner, turning first to one side of the leaf and then to the other, apparently in constant search of its host. !Jnless disturbed it was seldom noted to take flight, but if jarred or frightened it takes wing at once.
FLIGHT.
This insect is quite capable of sustained flight, as shown by the fact that Mr. Graf captured an adult in the outdoor insectary, on a shelf 5 feet above the ground and about 15 feet from the foliage which held the parasitized thrips. On September 27, 1911, Mr. Graf incorporated in his notes the following: On this date two adult parasites were noted in flight. In both instances the parasites flew from the desk to the window, a distance of about 24 inches, and probably could have flown a much greater distance. The characteristic rapidity of their short flights in the vials was lacking and they flew slowly and in a zigzag line. Their flight resembled very much that of a fly in its aimless circuit about a room. During the flight they maintained about the same level, so it is probable that these insects are relatively strong fliers. It is probable that in still air or traveling with an air current they could traverse quite a distance and thus be scattered quite rapidly. This fact is further borne out by the rapidity with which parasitized material of Thrips tabaci was taken from all points of a patch of turnips once it had started to show in one place.
SIGHT.
Although this parasite has large eyes, it does not appear to distinguish objects very plainly or at any great distance. In many cases it was observed, while searching for larvx of its host, to pass near them; so close, in fact, as almost to touch them. In one instance a female, while traveling across a leaf very rapidly, was observed to run halfway over a thrips larva, and then, as if just aware of the presence of its host, to stop, back off, and immediately oviposit in it. It is attracted to light and will travel toward it in the vials so that by turning the bottom toward the window, the cotton plugs may be removed without danger of losing the insect.
BEIIAVIOR ON DIFFERENT KINDS OF LEAF SURFACES.
In the course of the laboratory experiments with the thrips parasite it was observed that it experienced considerable difficulty in moving over the surface of a leaf if the latter was hairy or sticky. On leaves with smooth, glaucous surfaces, such as cabbage, turnip, or wil lettuce, this insect moved with rapidity and without apparent difficulty. But on the underside of leaves such as nasturtium and Chinese gourd, the movements were delayed and often ceased while the insect cleaned its legs. Because of this condition oviposi-





AN INTERNAL PARASITE OF TtIYSANOPTERA. 41

tion was very slow on leaves of the latter type and the percentages of thrips larvw parasitized in such experiments were low. Without doubt this same difficulty arises in nature, so that thrips living on plants with spiny or sticky leafage will not be so highly parasitized as those living on plants with smooth lafage.

HOST RELATIONS.
Iehliothrips fasciatus Pergande.
The parasite in question, Thripoctenus russelli Crawford, was first discovered in the larvw of Heliothrips fasciatus Pergande, and when the adult parasites were first reared they were naturally confined in vials, with the young of this species, and at once oviposited in them. These were carefully manipulated and the parasites later reared in large numbers under laboratory conditions. Collections of this thrips made at various times during 1911 have shown it to be regularly parasitized by this hymenopteron.
Tlirips tabaci Lindeman.
As soon as oviposition was noted in larve of Heliothrips fasciatus, larvea of the onion thrips (Thrips tabaci Lindeman) were substituted for those of Heliothrips as a possible host for the parasite, with the result that oviposition took place at once. From these onion thrips larvoo the parasites were successfully reared in large numbers. On September 2, 1911, this parasite was observed actively ovipositing in larvoo of Thrips tabaci in the field and from collections of larvT" of this species made at that time the parasite was reared in large numbers. (See Table III.)
TABLE III.-Parasitism of Thrips tabaci as shown by field collections at Compton, Cal., 1911.

Number Number Number
Lot No. Date. para- Lot No. Date. para- Lot No. Date. parasitized. sitized. sitized.
1 ........... Aug. 30 52 9 .......... Sept. 15 96 17 .......... Sept. 23 23
2 ........... Sept. 2 16 10 ......... Sept. 16 30 18 .......... Sept. 25 14
3 ........... Sept. 5 24 11 ......... Sept. 18 46
4 ........... Sept. 9 29 12 ......... Sept. 19 40 T o t a 1
5 ........... Sept. 11 45 13 ......... Sept. 20 50 number
6 ........... Sept. 12 20 14 ......... Sept. 21 15 of para7.......... Sept. 13 34 15 ......... Sept. 22 23 sitiz e d
8 ........... Sept. 14 28 16 ......... Sept. 23 17 forms............. 602

The percentage of parasitism of this species in September, 1911, ranged in different collections from 15 per cent to 60 per cent, while the average of 7 lots was 33.5 per cent, as shown in Table IV.





42 MISCELLANEOUS PAPERS.
TABLE IV.-Percentage of parasitism of Thrips tabaci by Thripoctenus russelli at Compton, Cal., 1911.

Date of Number Per cent
Lot No. collec- para- of paration. sitized, sitism.

1.......... ............................. ..................... Aug. 30 5235
2... ......... ..................-----.. Sept. 2 16 15
3........................... ............... ... .............. Sept. 5 24 40
4 ............................... ................... Sept. 11 45 30
5............................................................. Sept. 13 34 30
6................... .......... .............................. .. Sept. 14 28 25
7........................ ...................................... Sept. 15 96 60
Total.............................................................. 295 ......
Average.............. .................................................... 33.5

In October, from the pupa of this parasite sent to Washington, I). C., from Compton, Cal., adults were reared which, when placed in vials with larve of Thrips tabaci, immediately began to oviposit in them. Unfortunately the parasitized larve soon died, the parasites perishing with them, and but few data were secured from this experiment. It is possible that this mortality of the host was due to the cool weather which prevailed at the time of the experiment.
These experiments demonstrate the important fact that the parasite Thripoctenus will oviposit in the larve of Thrips tabaci. This later may be an important asset in the control of this destructive insect.
Euthrips tritici Fitch.
Adults of Thripoctenus, when confined with larve of Euthirips
tlitici Fitch, immediately commenced oviposition. As a result, although the larve of this thrips are more difficult to rear, the parasite was successfully reared in a number of experiments. No evidence of parasitism in the open was observed for this species, but as the larve were at no time very abundant, sufficient data were not secured to determine whether this insect was being parasitized in the field.
leliothrips hxmorrhoidalis Bouch6.
A series of experiments was conducted in which the larva of Heliothrips hmorrihoidalis Bouch6 was confined with this parasite and in every case oviposition was noted many times. However, with the exception of one doubtful example, no indication of parasitism ever appeared. To account for this failure is impossible at the present time unless it was that the host larve were in a far too advanced state.
Heliothrips jemnorais Reuter.
Adults of Thripoctenus ru.sselli that emerged in Washington on October 2, 1911, were confined with the larve of Heliothrips femoralis lieuter .and they immediately commenced oviposition. These





AN INTERNAL PARASITE OF TIHYSANOPTERA. 43

experiments also failed to give any further results, due no doubt to the fluctuating temperature of the building where the experiments were conducted.
MISCELLANEOUS OBSH8ERVATIONS.
Larve of Trichothrips n. sp. were confined with the parasite, but whenever the parasite came into contact with the larve it at once moved away and upon no occasion did it attempt oviposition. The refusal of the parasite to oviposit was no doubt due to the fact that this thrips was much larger, of an advanced stage, and with a tougher skin.
While observing the oviposition of the parasite in thrips larve, one was observed to come into contact with a small red spider (Tetranychus ?). The parasite immediately stopped, examined the mite, and then bent under the abdomen and oviposited in it. Undoubtedly this was a case of wasted energy, as the body of the mite was entirely too small to nourish the larva of the parasite.
Young of Triphleps insidiosus Say were placed in a vial with the parasite, but no attempt to oviposit was observed. In most cases the young bugs moved so rapidly that the parasite could not get near enough to touch them.
In one experiment Mr. Graf placed very young cabbage aphides (Aphis brassics L.) in the vial with the thrips parasite, but it refused to oviposit in them.
SUMMARY OF EXPERIMENTS TO DETERMINE HOST RELATIONS.
To briefly summarize the results of the foregoing experiments, this parasite has been reared from Heliothrips fasciatus Pergande, Thrips tabaci Lindeman, and Euthrips tritici Fitch. It has also been observed to oviposit in larve of Heliothrips hemorrhoidalis Bouch6 and Heliothrips femoralis Reuter, but without results at the present writing. Upon one occasion, as noted above, it oviposited in a red spider, also without results. Although given the opportunity it refused even to attempt oviposition in Trichothrips n. sp., Triphleps insidiosus, or Aphis brassica.
LENGTH OF LIFE IN CONFINEMENT.

Owing to the artificial conditions under which this insect was studied, the normal length of life must have been greatly curtailed. However, adults were often noted in active oviposition for from 3 to 5 days and one was recorded that oviposited on the first and eighth days of its confinement. Adults have been kept alive in the tubes for a period of 10 days. In another case adults were found alive in a shipment of material that was in the mail for 2 days. This insect probably has quite an extended length of life under natural





44 MISCELLANEOUS PAPERS.

conditions, but its minute size renders it almost impossible to determine this point.
Samuel Doten 1 has recently published some interesting information on the relation of food to the length of life of certain parasites kept in confinement. He found that under favorable conditions some species could be kept alive in confinement for months.
CAUSES OF MORTALITY IN THE OPEN.
Many of the adults of this tiny hymenopteron suffer an untimely death, instances of which have been observed in the present investigation. It was a common sight to see two or three entangled in a spider's web on the underside of a single leaf, and these invariably perished, as they were too frail to extricate themselves. On one occasion the larva of a syrphid was observed to seize one of these parasites and kill it. The larva of Triphleps insidiosus was also noted, in one instance, with this insect impaled on its beak.
One female of Thripoctenus russelli was observed on a croton leaf, attempting oviposition in the larva of Heliothrips uemorrhoidais. It was frequently caught by the sticky excrement of its host, and only with great difficulty succeeded in freeing itself. On the next morning this parasite was found fastened to the leaf and dead. Thus the excrement of the larvo of Heliothrips may serve as a protection to the thrips against its natural enemy.
EFFECT OF OVIPOSITION IN DIFFERENT AGES OF HOST LARVZ.
At the present time our observations show that the degree of maturity of the host larvx bears some relation to the length of time between oviposition by the parasite and indication of parasitism in the host. In all cases observed where the parasite oviposited in larvoo that were nearly fully grown either the parasitic larvae did not develop or the length of time required for their development was lengthened a number of days. Also in one experiment where oviposition took place in thrips larvan only 2 days old the length of time required until there were signs of parasitism was 14 daysconsiderably more than the average. It appears that the best results are obtained when the parasite selects host larvw about half grown as its victims.
LIFE HISTORY.
The life history of ,Thripoctenus russelli is taken from records made under artificial conditions, as the material was confined in vials closed by cotton plugs. These were kept in an open-air insectary; hence the other conditions were as nearly natural as it was possible to make them. The fluctuations of temperature were recorded on a
I Tech. Bul. 78, Nevada Agr. Exp. Sta., September, 1911.







AN INTERNAL PARASITE OF THYSANOPTERA. 45

Freize thermograph. The observations covering the life-hislory
studies of the parasite were made during the months of July, August, and September.

LENGTH OF TIME FROM OVIPOSITION TO FIRST INDICATION OF PARASITISM.

Because of the very minute size of this insect and the internal life led by it in the larval stage, the length of the egg and larval stages was not separately determined. The time required from oviposition
by the adult parasite until parasitism becomes evident in the host may be 5 days as a minimum or 17 days as a maximum, but by far the greater number require from 7 to 12 days for this. In Table V
this period is recorded for a series of 19 experiments.

TABLE V.-Time between oviposition and first indication of parasitism, by Thripoctenus russelli.

Number parasitized larvoe observed.
Parasite
ovi- Total
ExpeN- posited August. September. parasitmert No. in host ized.
16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 2 3 4


1.......... Aug. 9..........--4840 4---- ------- --- ------ --- ----20
2 ..........d... d1 1 2 1 1 0 1 -------------------------7
3---------Aug.11------050 3 0 0 2 35 ------ ---------- ------21
4 --------- Aug. 12------- 1 2 2 0 3 1 ----------------------------------- 9
5 ----------do ---------- --- 3 2 1 ------------------------------------------- 7
6 ---------- o-- do 1 0 0 0 .................1
7--------- Aug. 13 ------------ 3 3 8 16 10 13 3- ---- 56
8 ---------- do ------------------- 2 1 14 7 7 4 .... 35
......d-- ---....-------- 4 3 2 1 .......
..........-Aug. 14 4 3 2 110
10 ............d--- --- -4 0 1 1 0 17-------------------7
11 --------------------------------1---- --- --- --- --- --- ----_---- --- --- 2
12 ..........o------ -2 2 1 1 2 2------------ ----10
13 -------- Aug. 15 --------------- 2 5 1 .............. 8
14 ............ do ------------------ 1 1 ... ----...-.. .- --...-......--. 2
15 ........... do ------------- ------ 10 14 13 6 4 4 0" 0 68
16 ............ do.--------------- 5 0 0 0 0 1 .... ......... ....---- --- 6
17........ Aug. 19 ------------------------- ---- -- -- -- 9 8 10 8 3 1 39
18 ---------.-..... do--- -----------------------------------2-------------------- 7
19 ........Aug.-20------------- --------------------- 39 3 6 4 30
Total.. ....... ----.- .-. : -- 345
Total--------- --- --- --- --- --- ---- ---- ---- -----------~

The diagram, figure 8, gives the length of time between oviposition and indication of parasitism for 353 specimens.

LENGTH OF TIME FROM OVIPOSITION TO PUPATION OF THE PARASITE.

The length of time from oviposition to pupation of the parasite, or the period covered by the egg and larval stages, was carefully determined for 241 specimens and varied from.8 days, as a minimum, to 19 days as a maximum; but 13 or 14 days are required by the greater number of individuals. Figure 9 shows this period.
As the length of the pupal stage was taken up in the discussion of the pupa (p. 33) it is not again referred to here.








46 MISCELLANEOUS PAPERS.


LENGTH OF LIFE CYCLE.

The life cycle is shown in Tables VI, VII, and VIII, including

the average mean temperature for the period.

TABLE VI.-Length of life cycle of 44 individuals of Thripoctenus russelli.


Sign of para- Parasite larvae Adult sitism. pupated. emerged.
Date of oviposition.
Date. No. Date. No. Date. No.


Aug. 13 ..................................................... Aug.20. 3 Aug. 22.. 1 Sept. 11. 1
Aug. 21.. 3 Aug. 23.. 2 Sept. 12- 4 Aug. 22. 8 Aug. 24.. 3 Sept. 13. 8 Aug. 23 16 Aug. 25.. 7 Sept. 14. 7 Aug. 24...... Aug. 26.. 9 Sept. 15. 10
Aug.25.. 10 Aug. 27.. 3 Sept. 16- 4 Aug. 26.. 13 Aug. 28.. 3 Sept. 17 .....
Aug. 27.. 3 Aug. 29.. 9 Sept. 18- 8 Aug. 31.. 8 Sept. 19. 1 Sept. 20- 1
Total ................................................ ........... 56 1 45 .......... 44


Total average mean temperature from August 13 to September 20. 65.680 F. Total life cycle for the 44 specimens, 30 to 39 days.

TABLE VII.-Length of life cycle of 4 individuals of Thripoctenus russelli.


Sign of para- Parasite larvve Adult sitism. pupated. emerged.
Date of oviposition.
Date. No. Date. No. Date. No.


Aug. 14 ............................................ Aug. 20.- 4 Aug. 22.. 2 Sept. 11. 2
Aug.21.. 1 Aug.23., 2 Sept. 12.....
Aug. 22...... Aug. 24 ------Sept. 13.
Aug. 23.. 1 Aug. 25.. 1 Sept. 14. 1 Aug. 24 ..... Aug. 20 ...... Sept. 15. 1
Aug. 25. ..... Aug. 27 ......
Aug. 26. 1 Aug. 2S..
Aug. 29. 1

Total ...................................................... 7 ........... 6 .......... 4

Total average mean temperature from August 14 to September 15, 6.5.960 F. Total life cycle for the 4 specimens, 28 to 32 (lays.

TABLE VIII.-Life cycle of Thripoctenus russelli in Thrips tabact as a host, September and October, 1911.


Sign of para- Parasite Adult
sitism. pupated. emerged.
late of oviposition.
Date. No. Date. No. Date. No.

Aug. 28 ......... .................................. Sept. 4. 2 Sept. 8.. 2 Sept.29. 1
Sept. 5.. 1 Sept. 9.. 6 Oct. 2... 1 Sept. 6,. 3 Sept. I I. 2 Oct. 3... 2 Sept. 8.. 7 Sept. 12. 1 Sept. 15. 1
Total..................................................13 ......... 12 .......... 4

TotaI average mean temperature from August 2S to October 3, 64.7+ F. Entire life cycle for the 4 specimens, 32 to 36 days.





AN INTERNAL PARASI'E OF TIYSANOPTiIIA. 47

To summarize all of the rearing experiments con(lucted (lurilig the summer of 1911, the life cycle of this insect required from 28 to 44 days, but the average was between 30 and 34 days. (See fig. 10.)
FIRST APPEARANCE IN TIlE SPRING.
In 1911 this insect was not observed until June 15, although a very careful lookout had been kept from early in the spring. Undoubtedly future work will show that this insect becomes active considerably before this time and probably as early as the mi(dlle of March at least.
LAST APPEARANCE IN TIE FALL.
In 1910 parasitized thrips were found as late as November 10 and after that, although thrips larvi were collected, no signs of parasitism could be found. In 1911
very few adults emerged from
the pupe after October 1, 4 -although adults were observed 403
in the field as late as October _3o
21. On November 10, 1911, .25
a large collection of thrips 20larvoo gave only 9 parasitized ZS -specimens, while a later col- / lection, made December 8, .
gave one parasitized form, S 6 7 8 9 /0 /1213 /4 /5//7
.4 VS.
which would indicate that FG. 8.-Diagram illustrating length of time from ovithis parasite had about ceased position to first evidence of parasitism for 353 indiits work. viduals of Thripoctenus russelli. (Original.)
NUMBER OF GENERATIONS.
The experiments made during 1911 have shown that the female of this insect may begin oviposition in a very few hours after emerging from the pupa. Because of this we can expect a new generation to be begun the same day that the female emerges, provided she finds host larve. Therefore, if we take March 15 as a point when this insect becomes active in the spring and November 10 as the date that this insect ceases to oviposit, we might obtain as many as 8 generations, provided each required only 28 (lays for development, or only 5 generations if the maximum period for development was required.
HIBERNATION.
Thripoctenus russelli passes the winter in the pupal stage, the different individuals evidently hibernating as soon as the temperature drops sufficiently low to check development. PIupw, taken to Washington, D. C., all entered hibernation as early as October 1, while the adults were still emerging at the laboratory in Compton,






48 MISCELLANEOUS PAPERS.
Cal. At the latter place many entered hibernation as early as October 5, and after that date very few adults emerged.
OCCURRENCE.
The adult of this insect was first observed in a bean field at Holly-wood, Cal., on June 29, 1911. At this time one was noted in oviposition and a careful search of the bean foliage for about 3 hours resulted in the capture of 5 more. On July 18, 1911, Mr. Graf and the writer again visited this field and after a very careful examination collected 12 more adults. On August 4, 1911, a visit to this field resulted in the capture of 7 more specimens of this insect, and on August 18 a prolonged examination produced 1 specimen on a leaf of Lactuca scariola and 1 on the
so ------ ---leaf of a lima bean plant.
4- -At Compton, although many
parasitized thrips had been col33 lected for over 1 months, this
/ \ insect was not found in the adult
-_ --- stage until August 26, 1911, when
-k- -- --- 1 was taken on a leaf of Lactuca.
o -- At this time the adults seemed to
-. be very common-on turnips in the o 9 / / z/2 /3 4/s 15 /6 67/9 laboratory yard, for on August FIG. 9.-Diagram illustrating length of time from 28 Mr. Graf collected.1; on August oviposition to pupation for 241 individuals of 29 1; and on August 30, 2. The
Thripoctenus russelli. (Original.) writer on observed
writer, on September 1, observed
7 adults crawling over turnip foliage and on September 2, 4 more were observed. Two of these were actively ovipositing in larva of Thrips taba ci. On September 17 and 23 Mr. Graf observed 4 adults on the turnip foliage.
While Mr. Graf was at Puente, Cal., on October 3, 1911, he found 1 adult of this parasite on a leaf of Sonchus oleraceus badly infested by the bean thrips. At the same place on October 21, 1911, Mr. Graf discovered a small bush of Nicotiana glauca badly infested by the larva of Heliothrips fasciatus and careful examination revealed the presence of this insect. On 15 leaves of this plant infested by the bean thrips he found between 40 and 50 adults of this parasite. These were very active and many were observed in oviposition.
DISTRIBUTION.
This insect was first discovered at Compton, Cal., during December, 1910, and durir)g 1911 it has been found to occur very commonly in that locality. During June, 1911, it was found to occur at Hollywood and later it was reared from material collected at Whittier and Puentfe, Cal. These localities are all situated in Los Angeles County and make up an area of nearly 150 square miles. A few collections







AN INTERNAL PARASITE OF THYSANOPTERA. 49


of the host larvw made at Garden Grove and in Los Angeles failed to show parasitism, but probably more extensive collections would have
revealed the presence of this insect in both places. Comi)ton, which
is near the coast, has a much cooler climate than Puente, which is
more inland, so that this insect aippears to thrive along the coast ani also in the drier and hotter interior valleys.

PERCENTAGE OF PARASITISM IN DIFFERENT LOCALITIES.

The hosts of this parasite are such delicate insects that in all rearing
work many have died from the artificial conditions in the vials.
Therefore it is impos- ....
sible to give the exact 45
percentages of the 6 /40number of thrips par- ----
asitized by this insect i-3o -orthe number of nor-
mal forms. Never- o
theless some idea may %
be obtained of the -------------work this insect is o 1----- ------ --H-I
capable of doing to P.34 $J
check i ost, Hello- FIG. 10.-Diagram illustrating total number of days in the life cycle of check itshot
HiLo- Thripoctcnus russelli; summary of all rearing experiments. (Original.) thripsfasicatus, in Los

Angeles County, Cal. Table IX gives the percentage of parasitism
at Compton for 1911.

TABLE IX.-Percentage of parasitism of Heliothrips fasciatus, Compton, Cal., 1iO-11.
Number
Numra Number Per cen~t
Lot No. Collected. parasi- numal permit
tized normal parasiforms. forms. tized.

1910
1 ............................................................... N ov. 10 4 20 16.66
1911.
2 ............................................................. June 15 1 5 16.66
3 ............................................................. June 29 2 26 7.0+
4 ........................................................... July 6 2 8 20.0
5 ............................................................ July 8 ... -...... 20-30.........
6 ............................................................ July 10 .......... 31 .........
7 ------------------------------------------------------------- July 12 57 12 70.3
8 -------------------------------------------------------------- July 12 5 .0
9 ............................................................... July 14 5 26 16.0+
10 .............................................................. July 17 ......... 16 .........
Ii ----------_--------............. -------- -- ----- July 18 11 .......... 10.0
12 .........................................................--- July 18 11 .......... 7.0
13 ...............................................-- July 31 21 ......... 15.0
14 ----------------------------------------- ----------- July 31 37 --- .---- 25.0
15 ......................... .-.............................. A ug. 3 3 --------- 3.0
16. ......................................................... .. A ug. 7 65 ......... 50.0
17 ----------------------------------------------------------- Aug. 8 74 .. 45.0
18 .............................................................. A ug. 14 54 .......... 40.0
19 .............................................................. A ug. 16 147 .......... 70.0
20 ------------------------------------------------------------- Aug. 21 140 ......... 170.0
21 .............................................................. Aug. 26 80 ----------- (2)
22 ............................................................. Aug. 28 24 -------- (2)
23 .............................................................. A ug. 30 35 .......... (2)
24 ............................................................. Sept. 2 27 ------- (2)
25 ---------------------------------------------------Sept. 4 41 .......... (2)
26 ..................................................----------- Sept. 6 9 .......... (2)
Total parasitized forms of Ileliothrips fasciatus collected
at Compton ............................................ 850 ....................
I About. 2 Not determined.







50 -MISCELLANEOUS PAPERS.

In these collections the percentage of parasitism ranged from 0 to 70.3.
Table X gives the percentage of parasitism at HoUywood. The average percentage for that locality was only 0.78+, which seems very strange, as the adult parasite was frequently collected on foliage at that point. However, this low degree of parasitism may. have been due to collections made at times when the parasitized material was )upating

TABLE X.- Pcrcentage of parasitism (f Ileliothripsfasciatus, at Hollywood, Cal., 1911.

Number Number Per cent
Lot No. Collected. parasl- normal parasitized forms. tized.
forms.

1911.
1 ...... -....................................................... June 29 2 34 5.55
2- ---- ..................................................... July 8 11 38 22.44
3-......... ...................................................... July 8 .......... 25 ..........
4---. ..-------------------- ---_-------------- July 18 ---------- 275
5 ........ -................................ ...................... July 18 ......... 142
6 .............................................................. July 18 -----------50 .........
7_ ............-........-....................................... July 18 ------ --- 66 ........
8---------------------------July 18-------------250.....
9 ............-............................................ July 18 ........ 134. ........
It..........----------------------------------------------- J1uly 18 50... 34 ----lo ...... .. .... ....... ....................................... July 18 .. 50 .........
11- ........................................-- ------- ----- July 18 --------- 150 ..........
12 .................................. ............ A u g. 4 4 (?) (?)
A ............................................................. A g 4 .......... ..40 ........
14 .............................................................. 18 1 5 199
Aotalge per... nt.... ........... ........................ .....18 1,754--- .. 78+.
Average per cent for total--------- -------------------- ---------- --------- --------- .78+


Table XI gives the percentages of parasitism for a number of
collections made during August and September, 1911, at Puente.
In this locality the parasite was working about as actively as at
Compton, and the percentage of parasitism rose as high as 49.5 in
one case that was determined.

TA LE X .- Percentage of parasitism of Jleliothrips fasciatus at Purnte, Cal, 1911.


Number
ized nornial. sitizedo
forms.

1911.
-.. ...... ....... .................. A ug. 1 70 .......... 35
2 ........... ... ... .....-- -... .............. ... do-... 17 40 29.8
.......-................ ..................... Aug. 10 19 (?) (?)
4 ................. ...... ... ... ............. ... do ..... 1 19 5
5---- .....-.......................................... ... do.... 2 (?) Very sm all.
SA_ ..........u................................ ,ug. 29 132 () ( j)
7. .... ................................... .... Sept. 1 84 (?)
-.......-......................... ................... Sept. 6 89 95 49.5
10, ......................... ... ................... .. ,. 1 (to.. ..7
------------------------ (?) ?

T otal .............................................. .......... 561


At Whiittier the lost wus not abundant, and the one collection
maude there ()n A rust 29 1911 of 25 thrips larva, developed parasitisil ill two (11ses, or about 8 per cent.





AN INTERNAL PARASITE ()F T[IYSAN O)PTElA. 51
MEANS OF COLONIZING THE PARASITE.
Experiments, conducted during 1911 between (oimlptoin, Cal., al Washington, D. C., have shown that this parasite can be distrifutcd in two ways. Either the material can be collected( ill the fiChld all(l shipped direct to the point where its introduction is desired, or the material can be placed in cold storage and thus kept until at more favorable period.
In shipping material direct from the field two methods have been used that give good results. Of these the more I)romisiig is to collect material where the parasitism is high and put tie thrips larvo in vials as described under 'Methods of collecting for evidence of parasitism" (p. 28). Then, as soon as the parasitic larvm have





















FIG. 11.-Mailing tubes used in shipment of thrips parasites. Original.
transformed to pupae, they should be packed carefully in cotton, to prevent jarring, and shipped. (See fig. 11.) In the other method the only difference is that the material is shipped as soon as placed in the vials. While this has the advantage of giving more time for shipping, it unfortunately seems to cause greater mortality inl the parasitized material. It is also possible to pick leaves of plants infested by parasitized thrips, and pack these in cigar boxes and ship direct. However, this method has not given as good results during the past year as the other two methods.
During the fall of 1911 much material was placed in cold storage in Los Angeles. In these experiments parasitized prepupe of the thrips were placed in vials or in rubbish and then put in either the





52 MISCELLANEOUS PAPERS.

eggr room or the fruit room. In the same way parasite pupae were also placed in cold storage. In the egg room the temperature is kept at 31' F. and the air is dry, while in the fruit room, while the temperature is the same, the air is rather moist.
From these experiments it seems that the parasitized prepupan aire able to stand exposure in cold storage for at least 41 days, and when removed at the end of that time the parasites will change to P uPa-,.
Likewise, when parasite pupae are placed in cold storage, the period of emergence of the adult may be retarded for a time until conditions are more favorable for its propagation. In these experiinents pupx, were left in cold storage for 41 days and then put in a cool greenhouse and in 79 days the adults began to emerge.
The methods of shipping insects and placing them in cold storage just described are at the most very imperfectly worked out, and it is purposed to experiment along these lines until the behavior under these conditions shall be as completely known as is necessary.




A DDTIONAL COPIIES of this publication
X-may be procured from the SUPERINTN.NDENT OF DocumENTS, Government Printing
Office, Washington, D. C., at 5 cents per copy



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