An experimental cooperative community program for the cultural control of bugs of the genus Lygus on alfalfa seed crops ...

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Title:
An experimental cooperative community program for the cultural control of bugs of the genus Lygus on alfalfa seed crops in the Mohawk area of Arizona in 1939 and 1940
Physical Description:
Book
Creator:
Stitt, Loyd L
United States -- Bureau of Entomology and Plant Quarantine
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U.S. Department of Agriculture, Bureau of Entomology and Plant Quarantine ( Washington, D.C )
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aleph - 030285145
oclc - 779480924
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E-546 August 941
DEPARTMENT190 1
AGRICULTURE
BUREAU OF
ENTOMOLOGY AN D
rLANT QUARANTINE





AN EXPERIMENTAL COOPERATIVE COMMUNITY PROGRAM FOR TI-.'E CUU7 -T CN~O
OF BUGS OF THE GENUS LYGUS ON ALFALFA SEED CROPS IN iE ,iA, E O
ARIZONA IN 1939 and 1940 l/

By Loyd L, titt, Division of Cereal and Forage Insect Investigations



INTRODUCTION

Severe reduction of the alfalfa seed crop caused by plant bi~s of
the genus Lypus in the Mohawk area of Yuma County, Ariz., in 1938, a cm-
pared with the yields obtained in 1936 and 1937, resulted in an appeal by
seed growers for aid in controlling these pests. Investigations during
1938 and preceding years showed a noticeable lack of uniformity in the time
the seed crop was started in different fields, and that this irregularity,
together with failure to clean up weed and alfalfa growth that harbore. the
bugs during critical periods of the year, was favorable to their interfeld
migration, survival, and multiplication. These investigations also indi-
cated that thorough winter clean-up of host plants and the observance of
much closer restrictions on the time of starting the seed crop in all fields
within a locality were not only effective measures for the control of L_ uj,
but also were beneficial in reducing some of the losses of alfalfa seed
caused by the seed chalcid Bruchorhagus gibbus Bch, and thu penta toid
Chlorochroa savi St~l. As a result of these observations an experirinal
cooperative community program for the cultural control of Lyus huB "as
inaugurated in the Mohawk area and followed during the years 1939 a39 940.
During the period the control program was in effect, Lyus piopuictions
became much reduced and increased seed yields were obtained.

l/ The experimental program for the cultural control of LX!us in te
Mohawk area was cooperative between the alfalfa seed growers of the district,
the Agricultural Extension Service of the University of Arizona, worLng
through G. E. Blackledge, County Agent for Yuma County, Ariz., a the
Tempe, Ariz., Laboratory of Cereal and Forage Insect Investigations of the
Bureau of Entomology and Plant Quarantine of the United States Depart~ent of
Agriculture.
The author expresses his gratitude to V. L. Wildermuth, Sciior
Entomologist in charge of the Tempe, Ariz., Laboratory, under whose immediate
supervision this investigation was conducted.





-2-


Two important features in the success of this program were the whole-
hearted-,)operation of the alfalfa seed growers in following the cultural
prs~ctces suggested and the fact that these practices did not involve
'~:~i- variations from those already prevailing in the area.

The apparent success of the Lygus control program, as conducted in
this area, indicates that similar programs may be practicable in other
alfalfa seed-growing districts, under different environmental conditions.
The fo!!oiing discussion of the procedure and results of the community
control experiment in the Mohawk area is therefore given.

REVIEW OF LYGUS INVESTIGATIONS CONDUCTED
IN ARIZONA AND SOUTHERN CALIFORNIA g/

In the alfalfa seed-producing areas of Arizona and southern Cali-
fornia three species of Lygus have been present, namely, L. hesperus Knight,
L elisu~' Vvn Duzee, and L. pratensis oblineatus (Say). 3/ Investigations
for 1- y -s 1935 through .938 showed that these bugs caused severe damage
to the alfalfa buds, flowers, and seeds. High Lygus populations caused an
incre--se in flower fall and in the proportion of brown seeds and a decrease
in seed i ild. Nine positive correlation coefficients computed from data
sh~',in the relationship between the bug population and the proportion of
,ower fill and brown seed were statistically significant at the 5-percent
level cnd six of these were significant at the 1-percent level.

Field observations during May and June 1937 showed that Lygs hesperus
constituted over 85 percent of the total number of Lygus collected in
z'alfaf, L. elisus showed preference for weeds, and the indications were
theft they remained on these hosts as long as the condition of the plants
w>. sui~t2le. L. praensis oblineatus appeared to be more abundant during
the fll, winter, and spring than during the summer, Plant-feeding mirids,
of hich Lus is one genus, normally seek a satisfactory growth of a pre-
ferred host plant for oviposition. Lygu spp. prefer the succulent, tender
terminal parts of their host plants.

,'igrations of the bugs within the cultivated area were found to be
vory iportant in causing high Lygus infestations in the alfalfa seed
fields. The cutting of alfalfa for hay caused the adult bugs to migrate by
fight to neighboring earlier out fields, where they found optimum feeding
conitio s on the new growth. Lack of uniformity in the time of cutting
the first hay crop and in starting the seed crop, in neighboring fields,
p% provided a continuous supply of suitable food and enabled the bugs to survive
cid multiply, with resultant extremely high Lygus populations and severe
cmage to the alfalfa seed crop.

2/ For detailed information on the Lygua investigations see U. S.
ep r .eiit of Agriculture Technical Bulletin 741, Three Species of the
(Ceuin Ly-us and Their Relation to Alfalfa Seed Production in Southern
Arizona sad California.
3/ Order Hemiptera, family Miridae.





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Field studies and laboratory [hlot tests hve shown that high mortality
of the nym1nhal stages is caused wy the removal o. sacculet food, as in lhe
cutting of the alfa hay crop .nd subsequ et e r to th e oli. :tic
conditions of southern [rizcna. When air tvmje( ies reached 9O' to
1000 F an es imated mortality of g0 t 95 percent of tho nymphs occur ed
in test plots. Field observations durin the r vel'~n t of the see6 crop
in Arizona have shown that the La.> prpu>aiow- (1duItv and nymphs) were
reduced in alfalfa fields by clean cutting during Ppril, and in many cases
the populations never again reached the precutting Jevel. Except in felds
into Nx.hich a migratory influ.- occurred, from 2 to 3 weeks elapsed after the
hay crop was cut before even a small increase in the population was detected
in the seed crop immediately following.

LOCATION AND ORGANIZATION OF THE COMMUNITY EXPERIMENT

The cooperative community experiment in the cultural control of Lyiius
bugs on alfalfa was conducted in the Mohawk area of Yuma County, Ariz.,
because of the interest in and desire for it shown by the local seed growers,
G. E. Blackledge, Agricultural Agent for Yuma County, supported the in-
auguration of this movement and gave of both his time and facilities.

The Mohawk area includes the Antelope and Mrohawk Valleys, within
which are situated the towns of Weilton and Roll. The district is in the
Lower Sonoran Life Zone of the mesquite and creosote bush associations
Rainfall is low and temperatures high, and this results in a prevailing low
relative humidity. The temperature range between day and night is great.
The cropped land consists of about 6,500 acres, which is all irrigated by
water pumped from wells. According to the official estimate, 5,430 acres
(approximately 83 percent of the cropped land) produced alfalfa seed during
1939. The University of Arizona report 4/ on the vwater condition and crop
production in the Mohawk district indicated that alfalfa seed was the only
profitable crop.

The first step in the organization of the control program was the
distribution of a questionnaire to the alfalfa seed growers to obtain infor-
mation essential in formulating a plan of action. One of the purposes of
the questionnaire was to ascertain the prevailing cultural practices used
in seed growing. The outstanding information obtained by this means was
as follows: (1) The preferred dates for turning to seed were from April 15
to May 15; (2) the majority of the seed growers irrigated the seed crop
before it bloomed; (3) most of the growers required a period of 6 to 10
days to harvest their hay crop before the seed crop was started; and (4)
over one-half the seed growers in the Mohawk area indicated that they were
able to recognize Ly gus bugs and, in many cases, Vnew their general habits
and the type of injury they cause to the growing alfajfa seed crop.

The replies to the questionnaire indicated that a community control
experiment could be arranged without expense, special operations, or too

4/ University of Arizona. Report of Septemer 16, 1.936, on the Moha,.-k
Municipal Water Conservation District.




-4-


a derrrture from current farm practices used by the majority of the
seel groers. Mr. Elackledge therefore, called a meeting of the growers
i e ':'k area in Januiry 1939, at which 4he insect, its habits, the
k'ind cf i.~ ry it causes, ani certain farm practices which appeared bene-
-rucing L Ygus d S -;e were discussed. Folloing this, the seed
_e e ,duted their own orgDnization for carrying out a program, which
cc. in setting definie ates to start the seed crop, the inauguration
of cle.-u.p measures, and he selection of a comh-tee to handle the plan.
The :ru zation was completed approximately 3 months prior to the dates
se f: a arng the seed ciop, The periods agreed upon during which the
!a crop preceding the seed crop was to be cut were between April 20
S93 mcd bet een April 16 and 26 in 1940. All alfalfa fields in
aitrict were to e cut cleanly without leaving uncu plants, and
re to be cleaned of jl alfalfa and weeds. All fall and winter
o be eliminated by FeruIry 1,. The comitte deemed it advisable
to iicc: orate these requiio lents in an agreement which the seed growers
e 3e d to sign, pledging themselves to ccrry out the program to com-
pleto. Each grower was ftirnished a copy of this agreement so there would
he no a'stion as to the cities he was to perform Cormit.ee meetings were
o lhe <. different times Io discuss Iny problems rising and to review the
progre.s of the work. Any suggestions agreed upon by the committee that
ar~e red o be beneficial to the program here miieographed and sent to
each seed grower.

FARMING PRACTICES) U' IN THE MOHAWK A?,EA EXFEIMENT

ocordiuo to the v rogram adopted, the fnil and winter growth of
.... I na. reecds were to .e cleaned up by psturing "ith ctte or sheep,
oi ,, -in December od nuary. The spring crop of alfalfa was to be
cut or rel off in reh in time to allow approximately 40 days for the
SmnL of the lst Ir y crop and the toree of food i the roots essen-
i he gorous ee y cvovth of the seed c, cir cre was to
he U' i: cutting the f hfa closely and cle. ny a r'der to reduce the
ii t coo supply and ;-i( La the st2~rvn ) the bubs. In Goth 1939
d <40 ,he committee .:cive in secur'ir os' ce by ii the growers
o ne de limits set fcr Lhe c uting of the 1as hny crop.

o I'ears a ehe as cn e on the proper ica of the acreage on
i I i s hay cut i completed a the iep cified in the agree-
I'i ,x oha Valleys) was
,v t, it ws a it''d Ithat vppro i:: .ey 5,00 acres (92 percent)
0 ;acres c' e ole oc jcc rding the agree-
rent in V4' tis *, T, 5u 'cres, or ) ucon, in the Mohawk Valley
... ... .. or 75 pierce i he P~ulcove Valle y tv,,ing to the greater
the c wt }arries in .he n sioee Valley, there
-v iovei e ii K e h' fields. The com-
S 1 cen.d i~hfuilly ,'or he program, and the per-
s o" alln-up shoAu i hese reourds3 cl, ry illustrate the results





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Regulation of irrigations, so as to prevent the production of too
heavy and succulent a growth of the alfalfa plants in a 8eed field, aided
in checking the Lygus increases, as the resultant plant growth was not
optimum for the best Lygus development. Unusually light irrigations during
the winter of 1939-40 prevented abundant growth of alfalfa and, owing to
the shortage of pasture, resulted in an excellent winter clean-up by grazing
stock. The consequent reduction in winter carry-over of Lygus bugs appeared
beneficial in that the Lygus numbers were low in April at the time the seed
crop was started.

METHODS USED IN EVALUATING THE EFFECTIVENESS OF
THE COMMUNITY CULTURAL CONTROL EXPERIMENT

The effectiveness of this control experiment in 1939 was determined
by comparison of records of L~us populations, flowers fallen, proportion of
brown seeds, and seed yields made in the area that year with similar records
made in the same fields the previous year. For the year 1940, in addition
to the comparisons with the records of the two preceding years in the
Mohawk area, a comparison was also made with data obtained in the South
Gila area, where no community control program was conducted. This seed
district of 2,900 acres is located 35 miles west of the Mohawk area and in
past years has been similar to it in alfalfa culture and seed yields.
Both Yuma County and State yields of alfalfa seed have also been used in
comparisons with the yields of the controlled Mohawk area for 1938, 1939,
and 1940.

Lygus Population Surveys

Alfalfa fields were selected at random, in which systematic weekly
surveys were made during the development of the seed crop. Ljyus popula-
tions, stage of plant growth, Lyus damage, and abundance of other insects
were recorded for each field surveyed. Field surveys were made before
the seed crop was started, and the data showed that fields exhibiting the
higher Lygu populations, in the hay crop prior to the seed crop, usually
contained the higher populations in the seed crop. In most cases these
higher populations seemed attributable to the fact that the alfalfa hay crop
had grown for more than 40 days, which had permitted greater numbers of
Lygus bugs to develop.

In making the survey records of Lygua population the same system
was followed in all fields. Six fields in the Mohawk area were studied
in detail in 1938, prior to the inauguration of the community program.
Eleven fields were studied in that area in 1939, and in 1940 records were
kept on 13 fields in the Mohawk area and 7 in the South Gila area, and were
available for comparisons. The population records in 1940 for field No. 7
are given in detail in table 1 and show the type of records taken during
the development of the seed crop.




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Table l.--Lyvgus sweeping records on alfalfa in field No. 7, Mohawk area, 1940

Date of Type of Numbers captured per 100 sweeps
record plant growth Adults Nymphs Total

1940
Mar. 29 New growth 7 8 15
Apr. 5 New growth 6-8" tall 31 4 35
Apr. 12 New growth 3-10" tall 21 1 22
Apr. 24 Pasture stubble 4 4 8

Seed crop

May 2 New growth 8 20 28
May 9 New growth 17 16 33
May 15 New growth 12-18" tall 41 10 51
May 22 Bud 48 28 68
May 29 Blooming 46 106 152
June 6 Blooming 50 198 248
June 13 Full bloom 12 14 26
June 20 Full bloom 18 2 20
June 24 Late bloom 44 4 48
July 2 Late bloom and regrowth 84 52 136
July 17 Late bloom and regrowth 24 16 40
July 23 Maturing 0 4 4
854

The populations used in the following comparisons were based on the
total number of Lygus captured during the development of the seed crop.
The estimated number of LyZus captured in field No. 6 was as follows: 5,192
in 1938, 1,302 in 1939, and 1,040 in 1940. In the Mohawk area the average
number of Lygus captured per 100 sweeps during the seed-crop development was
275.8 in 1938, 75,1 in 1939, and 83.2 in 1940. In the South Gila area, in
1940, 437.9 Lyg2s were captured per 100 sweeps. The important information
derived from these population records was that high populations were present
where no community program was functioning, that is, for the years 1938 in
the Mohawk area and 1940 in the South Gila area, and that low Lyg popu-
lations prevailed where the community program was in operation, that is,
for the years 1939 and 1940 in the Mohawk area. Field observations made
during these years also showed that where high Lvgus populations occurred,
severe Lyg2s damage was present. and where the Lygus numbers were low, the
seed crop usually was satisfactory.

The predators, mainly bugs of the genus Geocoris, probably affected
the populations somewhat in 1940, although the numbers of Lyus bugs per
100 s .eeps were lower in 1939 when few predators were observed. Many obser-
vations in 1940 indicated the apparent absence of the first and second
instars of Lygus, which is not the normal condition, and it was considered
th"t the predators had devoured the nymphs because predators had been
obser'ed feeding on the nyitihs. Nevertheless, the main reason for the lower




-7-


Lygus population in 1939 than in 1940 appeared to be the more zealous clean
cutting of the alfalfa in 1939., and past observations indicate that the
predators cannot be depended upon to serve as a means of controlling the
Lygus bugs.

Raceme Sampling

To check the field observations., raceme samples (150 to 200 racemes)
were collected from every field in which the systematic sweeping records
were obtained. These raceme samples were taken at random, when the seed
crop was mature, from the same parts of the field as were the sweeping
records. From the raceme samples the numbers and percentages of flowers
forming pods or failing to do so and the numbers and percentages of good
or injured seeds were determined.

Flower Fall and Pod Set

To ascertain the pod set and flower fall, 50 racemes, taken at random
from the sample, were examined under the binocular microscope. In this way
a determination could be made of the flowers which produced pods and the
flowers which failed to do so, or "flower fall." A bract on the raceme
indicates the location of every bloom. When a seed pod is produced a
definite enlargement of the basal attachment of the pedicel occurs. The
presence or absence of this characteristic pedicel development permits an
accurate count of the number of flowers producing seed pods, and the flower
fall.

Detailed studies previously published 5/ showed that an average
of 52.50 percent of the alfalfa flowers failed in fields of very low Lygus
infestation, whereas in fields of high population severe damage occurred
to the 47.50 percent of the flowers expected to set seed. Carlson 6/ has
investigated the effect of Lygus feeding upon alfalfa and has shown the
occurrence of many types of plant reaction.

Seed Classification

Seed examinations were made by taking one seed pod from each raceme
in a sample until 250 seeds were obtained. All the seeds in each pod were
examined under a binocular microscope to determine the number of good seeds,
and the numbers damaged by chalcids, pentatomids, and LYg-us bugs, respec-
tively. Lgs damage is shown as brown seeds. Experimental work has shown
that the feeding of the Lygus bugs is responsible for a very high percentage,
though not all, of the brown seeds. A complete classification of all seeds
examined was thus recorded. Detailed records of raceme and seed examinations
were completed for every field surveyed, as shown for field No. 5 (table 2).


5/ See footnote 2.
6/ Carlson, John W. Lygus Damage to Alfalfa in Relation to Seed
Production. Jour. Agr. Res. 61: 791-815, illus. 1940.




STAT P




-8-


Table 2.--Results of the examination of the raceme sample from field No. 5,
Mohawk area, collected July 4, 1940

Raceme examination for pod set and flower fall


Pod set
per raceme


Flower fall
per raceme


Pod set
per raceme


Flower fall
per raceme


Number
15
18
6
5
19
12
9
11
15
17
12
21
14
13
1
18
15
11
18
8
12
6
16
19
14


Number
16
11
21
17
9
3
26
15
8
4
17
7
15
17
15
9
6
8
13
16
22
16
10
6
3


Totals for 50 racemes 655
Percent 52.E

Seed examination of raceme sample


Classification
of seeds


Number of
seeds


Percent of seeds in
each classification


Good seeds
B. gibbus-infested
Pentatomid-damaged
Brown seeds


Number
8
11
19
12
13
19
17
14
12
6
9
9
23
17
15
3
14
12
14
19
12
14
13
7
18


Number
12
9
4
15
10
5
5
7
11
26
10
11
20
23
13
13
4
7
6
2
25
3
6
13
13
583
47.09


236
0
1
13
250


94.40
0.CO
0.40
5.20
100.00


1




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These records show that when the Lygjs populations were high a
greater flower fall and more brown seeds occurred, which fact is in agree-
ment with the positive correlations obtained in previous years. There was
a noticeable variation in the different samples. For example, the sample
from field No. 40, South Gila area, showed 84.89 percent flower fall, 26.4
percent brown seeds, and 70.0 percent good seeds with a Lygys population
record of 5,731, whereas the sample from field No. 36, in the Mohawk area,
under control, showed 56.71 percent flower fall, 2.4 percent brown seeds,
and 96.0 percent good seeds with a Lygus population record of 475. Indi-
vidual field records were summarized by years for the comparisons used in
the community program, and the data are presented in table 3.<


Table 3.--Populations of Lygus and percentages of brown seeds, good seeds,
and flower fall in control and noncontrol areas

Lygus Pedicel
per 100 bases Flower Seeds Brown Good
Area Sweeps sweeps examined fall examined seeds seeds

Mohawk Number Number Number Percent Number Percent Percent

1938 8,100 275.8 6,228 62.1 1,735 21.6 72.22
(6 fields)
1939 1/ 12,200 75.1 11,995 60.5 2,750 8.15 88.95
(11 fields)
1940 l/ 13,500 83.2 14,698 57.8 3,250 7.38 91.08
(13 fields)
South Gila 6,200 437.5 7,616 73.73 1,750 30.86 62 66
(7 fields)

I/ Community control program functioning.

Comparisons between the data for 1940 from the control area (Mohawk)
with those from the noncontrol area (South Gila) indicate that the LYus
populations were more than 400 percent higher in the South Gila than in
the Mohawk area; losses due to brown seed were more than 300 percent greater
in the South Gila than in the Mohawk area; the proportion of good seeds
was approximately 30 percent less in the South Gila than in the Mohawk
area; and the flower fall was more than 25 percent greater in the South
Gila than in the Mohawk area. It may also be seen from table 3 that high
Lygus populations occurred when no community program was functioning, and
resulted in a greater flower fall, less good seed, and more brown seed.
The summaries for the Mohawk area in 1938 and the South Gila area in 1940
show general similarity during a time when no community program was oper-
ating, and in the following discussion on seed yields mention is made of
the comparable seed production.




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Alfalfa Seed Crop Yields

The alfalfa seed growers appraise the results of the program by
increases in yields. In 1940 the yield of thresher-run alfalfa seed per
acre for the Mohawk experimental area was 314 pounds, whereas for the
South Gila area (with no control program) it was only 171 pounds. In the
Mohawk area the fiel s with good thick stands of alfalfa and free from
salt spots produced from 400 to 600 pounds of seed per acre. Thick stands
produced the maximum yields of seed this year in the Mohawk area where the
Lygus populations were low, whereas in the noncontrol area (South Gila)
in 1940 the thick stands had extremely high Ly&us populations and the seed
yields were practically failures. About 25 percent of the 2,895 acres of
alfalfa on which the production of seed was attempted in the South Gila
area was not threshed because the Lygus damage was so severe. This condi-
tion is similar to that found by J. W. Carlson 7/ in his investigations.
In 1938 the seed yield in the Mohawk area was 174 pounds per acre when no
uniform control program was followed. It is interesting to note the simi-
larity of the yields for the Mohawk area (174 pounds in 1938) and the South
Gila area (171 pounds in 1940) in years in which neither district conducted
a cultural control program.

The yields for 1938, 1939, and 1940 in the Mohawk area were 174,
186, and 314 pounds of seed per acre, respectively, which showed a, 12-pound
or 7-percent increase in lZ97) and a 140-pound or 80-percent increase in 1940
above the yield of 1933. These results certainly appear to indicate that
the cultural control program was effective in reducing Lygus damage and
increasing alfalfa seed production.

The estimate for Yuma County showed a 23-pound or 13-percent decrease
in seed yield per acre in 1940 from that of 1939, and a 63-pound or 29-
percent decrease in 1940 from the 1938 yield. The estimate for the Yuma
Valley area of Yuma County for 1940 was 105 pounds, or one-third as much
as that produced in the Mohawk area, The estimates of alfalfa seed produc-
tion for 1940 in the South Gila and Mohawk areas of Yuma County were ob-
tained from the seed warehouses. These estimates are in agreement with
those used by the agricultural statistician from whom the estimate for the
Arizona yield was obtained. The estimated yield for Arizona in 1940 was
18 pounds or approximately 9 percent less than the estimated yield in 1939.
The yields for the Mo awk area, other sections of Yuma County, and the State
of Arizona are presented in table 4. These figures show that the yields for
Arizona and Yuma County have been decreasing since 1938, whereas the yields
in the community cultural control area (Mohawk) have increased during the
two years in which the program has operated.

7/ Carlson, John W. Alfalfa-Seed Investigations in Utah. Utah Agr.
Expt. Sta. Bul. 258, 47 pp., illus. 1935.





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Table 4.--Alfalfa seed production and acreage for the Mohawk area, with
comparisons


Mohawk area
1938
1939 1/
1940 l/
South Gila area
1940


Total
area
(acres)


5,240
5,340
4,618

2,895


Total seed
Production
(thresher-run
pounds)


910,531
1,007,281
1,448,751

494,185


Yield
per
acre
(pounds)


174
186
314

171


Increase in
yields over
1938 for the
Mohawk area
Pounds Percent


12
140


Yields in pounds per acre?


Arizona 2/
Yuma County a/
Mohawk area 4/
South Gila 4/
Yuma Valley


1938

222
218
174


1939

204
178
186


1940

186 3/
156
314
171
105


Acres in 1940

43,000 3/
25,000
4,618
2,895
17,500


1/ Community control program operated.


2/ Estimates obtained from U. S. Dept. Agr. Agricultural Statis-
tician.

3/ Preliminary estimates of yield and acreage in Arizona for 1940
by U. S. Dept. Agr. Agricultural Statistician.

4/ Actual production and acreage records for the Mohawk and South
Gila areas were obtained with the cooperation of Mr. G. E. Blackledge,
Yuma County Agricultural Agent, and the seed dealers of Yuma County.

DISCUSSION

Field observations have shown that even when conditions are other-
wise optimum for seed production, high Lyus populations may cause a complete
failure of the alfalfa seed crop, while low populations normally cause
little damage. Fortunately, however, conditions favorable to seed produc-
tion also tend to reduce Lygus populations. The observations in Yuma County
indicate that early development of the seed crop secured by starting it




- 12 -


s ox-able for both Lygus control and seed production, for
(1 s ) The crop develops before Lygus populations have
I ijurious numbers. (2) Less excessive heat early
.!t ihe transpiration and thus reduces the water require-
s oi > salfa plant. (3) Advantage is taken of the known tendency
r igh poportDon of flowers to produce pods during the latter part of
C, he Ihe range between day and night temperatures is at a maximum
oe i.s and high temperatures occur. (4) The crop has an oppor-
u t uare U oeloe the advent of the violent storms of midsummer which
',ig of seed or other losses before the crop can be threshed.

The res.ts n the cultural control area and the noncontrol area
....~ ~s .eIiat the observations made in previous years, indicating
po'n c of a year-round program. Clean winter culture, with retarded
..... of he alfls, ts very important for full success of the program,
e t s timing of he April cultural and cutting practices is handi-
ca i large Lygus populations are allowed to develop during January and
february ierzuse of heavy irrigations and long periods of growth between
cuttib or pasturing of the alfalfa, The practice of cutting or pasturing
j0 c s. during the winter, is suggested as a means of reducing the
ubr A Lsz~: ae to complete a generation during the period prior to
.i rtig the s d erop. The potential life span of adults maturing in
February oul permit them to lay eggs in the crop which produces seed,
1u if iost of ih~ can be prevented from developing in January and February
the pote, l populations to infest the seed crop in May and June are
S edud Whe excellent clean-culture methods were followed in
:ry a.8 3e u, ry it as evident that the eggs and nymphs were largely
destroyed an] te mitgant adults were mostly spent.

The crop imivately preceding the seed crop should be harvested as
o pstured, because gradual harvesting of the crop by pasturing pro-
s VC uniformi cty in growth which is undesirable from the stand-
po~it o" L~i's control. It is important that this crop be completely
ca up in nl! fieds ;ithin the 10-day period, so that little available
oth Lgus bugs is left anywhere in the area. Cutting this crop
.or .ay also leaves t bugs less shelter from the heat than does pasturing.

The Soil 2oisture should be low when the last crop prior to the
,' i of the se crop is removed, so that new growth may be retarded
perod of the nymphs thus be prolonged. Irrigations can
o o- to bring about this condition, but Lh timing varies with the
soil ;I a .d mut v wo kd out by the individual grower.

On o prreuisites for a successful community control program
he ihole-herted willingness of the growers to cooperate
ani ca ryo fix plan providing for the more effective utilization in
ao~t ref a.. LMu o' the cultural practices prevailing in the area.
~'m or .small groups of farms probably can be con-
mii areus and shouo c n notieable benefits
1 :,otces so as to reduce I.Yo bu:s.





- 13-


Fundamental ecological knowledge of Lygus bugs is essential i
determining the cultural practices best suited for- their control in y
district. However, it is known that the habits of .the respective specie
are quite similar throughout large regions. The cultural methods of control
found effective in the Mohawk area of Arizona, as described herein, iway
therefore, with certain modifications, be found applicable to-other alfalfa,
seed-growing areas in the Western States.

SUMMARY

A community experiment for the cultural control of bugs of the genus
Lygus was conducted in 1939 and 1940 in the Mohawk area of Yuma County,
Ariz., in cooperation with the Agricultural Extension Service of the Uni-
versity of Arizona and the Agricultural Agent of Yuma County. Questionnaires
were distributed to the alfalfa seed growers to obtain information on the
prevailing cultural practices so that a plan of action could be formulated
by which the Lygus damage might be reduced without excessive expense to the
growers. At a meeting of the seed growers Lygus damage to the alfalfa seed
crop and beneficial cultural practices were discussed, a program was agreed
upon, and a committee of growers was selected to supervise its execution,
According to this program a clean-up of alfalfa and weeds was to be con-
ducted in January and repeated in March and April, seed crops in all fields
were to be started within a definitely restricted period, and other pre-
scribed cropping methods were to be followed.

The experiment for 1939 and 1940 gave favorable results in reducing
the populations of Lygis bugs, lowering the damage to the alfalfa seed
crop caused by Lygus and other seed insects, and increasing the seed yields,
whereas the noncontrol areas continued to have high LygEu populations, severe
damage to the alfalfa seed crop, and low seed yields. The average LygLs
populations per 100 sweeps during the development of the seed crop was
75.1 and 83.2 under control conditions, as compared with 275.8 in the Mohawk
area in 1938 and 437.9 in the South Gila area in 1940, under noncontrol
conditions. The losses due to brown seed for the control area were 8.15
and 7.38 percent, whereas in the noncontrol areas the losses were 21.6
and 30.86 percent. The comparisons for 1940 between the control area
(Mohawk) and the noncontrol area (South Gila) indicate that Lygu popula-
tions were more than 400 percent higher in the South Gila than in the
Mohawk area; losses due to brown seed were more than 300 percent greater
in the South Gila than in the Mohawk area; the percentage of good seeds
was approximately 30 percent lower in the South Gila than in the Mohawk
area; and the flower fall was more than 25 percent greater in the South
Gilasthan in the Mohawk area.

The alfalfa seed yields for 1938, 1939, and 1940 in the Mohawk area
were 174, 186, and 314 pounds per acre, respectively, the control program
being in effect during the last two of the three years. A 12-pound or 7-
percent increase occurred in 1939, and a 140-pound or 80-percent increase
resulted in 1940 above the yield of 1938. The estimates for Yuma County
showed a 23-pound or 13-percent decrease in seed yield in 1940 from that




UNIVERSITY OF FLORIDA
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of 1939, and a 63-pound or 29-percent decrease in 1940 from the 1938 yield.
The yields for the different seed areas of Arizona show that the yields
for the State of Arizona and Yuma County have been decreasing since 1938,
whereas the yields in the community cultural control area (Mohawk) have
increased since 1938 or for the two years during which the program has
been functioning.

Lygus damage has been reduced and the seed yields have been increased
in the area where the community program was in operation. These facts
strongly indicate that cultural practices known to be detrimental to the
bug's development can, if executed in an efficient manner throughout a
district, reduce the losses in seed production due to Lygus. This method
of control does not cause the seed grower a large added expense, which is
an important consideration in the control of insects attacking low-value
"cash-return" crops, such as alfalfa seed, and it may be found more or
less applicable in the other alfalfa seed-growing areas of the Western
States.