STATE PLANT BOARD
Jun.e 1943 Z-596
UNITED STATES DEPARTMENT OF AGRICULTURZ
Agricultural Research Administration
Bureau, of Entomology and Plant Qgxarantine
THE 3EUROPEAN COBRN BOER AND ITS CONTROL IN DAHLIAS
By C. H. Batchelder, D. D. Questel, and A. V. Cosenza,
Division of Cereal and Forage Insect Investigations
Introduction .. ....................................... .* e 1
Dahlia borers, native and introduced ..................... 2
How to recognize European corn borer injuries in dahlias .. 2
Source of corn borers in dahlias ......................... 3
How corn borers get into dahlia plants ..................... 3
Control of the corn borer with insecticides ...............
Insecticidal materials ....................... ...... 4 5
Equipment for applying insecticides ................... 5
When to apply insecticides ................................ 6
Supplementary control measures ............................ 6
Disbudding and disbranching .............. ........... .... 6
Use of a tent *.... ...... ............................... 6
Trimming root clumps ......... .......................... 7
The European corn borer (Pyrausta nubilalis (Hbn.)) is a
serious pest of dahlias in the northeastern part of the United
As its name implies, the insect is most widely known as a
pest of corn, although it also infests other cultivated crops,
some weeds, and several ornamental garden plants.
The European corn borer infests all types and varieties of
dahlias. Certain varieties may sometimes show more infestation
than others, owing to differences in the size and growth stage of
the plants when the eggs of the insect were being laid.
I/ The European corn borer in 1942 was known to occur in
Wisconsin, Missouri, Iowa, and eastward to Maine and North Carolina,
as well as in Ontario, Quebec, and the Maritime Provinces of Canada.
JUN 2 1 1943
DAHLIA BORS, XATIVX AMD INTODUCBD
In addition to the European corn borer, a native insect known
as the stalk borer (Papaipema nebris nitela (Guen.)) bores in the
steas of dahlias in June and July. It has a light-tan head, a trans-
verse, dark-brown band near the middle of its body, and brown, inter-
rupted, longitudinal stripes on its sides. This native stalk borer
is rarely as numerous as the Zuropean corn borer. The latter is &
grayish-pink caterpillar with a dark-brown head, and when fully grew
is about 1 inch in length. It attacks various parts of the plant and.
occurs on dahlias in greatest numbers during August and September*
The treatments discussed in this circular are not recommended for
control of the native stalk borer.
NOW TO EEOOGNIZZ S7PEAN CORN BO3R IJTURIIS IN DAHLIAS
Signs of infestation usually begin to appear late in July and
early in August and continuao to develop until frost. The occurrence
of egg masses of the corn borer on the under side of lower leaves, as
shown in figures 1 and 2, is a timely warning of impending infestation
of dahlias. Later, one ay observe that some of the foliage and
flower buds have wilted. Infested buds turn black, and beside them
one may find small masses of black, granular castings, called frass,
which have been pushed out by burrowing larvae. This condition.
illustrated in figure 3, is followed by further wilting of other
leaves and buds on the affected branch or shoot. A borer, one-fourth
to one-half inch in length. may be found in a small, discolored cavity
beneath the black frass. As a result of further excavation by these
larvae, normal terminal bad growth is prevented, blooms are small and
ill-shaped, and various parts of the plant become broken.
Parts of the branches or stalks break off where the borer cuts
a small hole from the inside, pushing out particles of sawdustlike
material, as illustrated in figure 3, which cling to the edge of the
opening and drop to parts of the plant below. In cases of severe in-
festation, progressive wilting and breakage result in collapse of the
plant before propagating roots are fully formed.
When either roots or green plants are set out in advance of
normal planting time, and show five or six sets of leaves by mid-June.
dahlias are sometimes infested by the uropean corn borer in June*.
The injuries caused at this time are similar to those described above.
SOURCE 0F CORN BOMERS IN DAHLIAS
The seasonal history of the European corn borer, previous to
and including the appearance of the second generation in dahlias,
is illustrated in figure 1. It is important for the dahlia grower
to note in this diagram that there are two generations of the
hr-opean corn borer each year and that the larvae of the first gen-
eration usually live in early sweet corn and field corn during June
and July. These corn-inhabiting larvae change to moths in July and
August, and some of the moths fly into dahlia gardens and deposit
egg masses upon the under surfaces of dahlia leaves. Agg laying on
dahlias may continue for a period of 4 to 6 weeks during late July,
all of August, and early September. These egg masses (see figures 1
and 2) are about one-eighth of an inch across and usually consist ef
from 10 to 25 flattened, overlapping, whitish, disklike eggs. The
eggs hatch within a week and the young caterpillars disperse over
the dahlia plant.
NOW CORN BOBIRS GET INTO DAHLIA PLANTS
When one of these tiny caterpillars (less than 1/16 inch in
length) crawls into a space between closely oppressed foliage at
leaf and flower buds, or into an axial shoot between a branch and
the stalk, it begins to feed upon the soft tissues of the unexposed
parts. Sheltered in these spaces, it continues to excavate, driving
deeper into the bud or into the pith of the branch or stalk until
eventually it occupies a tunnel large enough to accommodate a full-
fed larva that is about 1 inch in length. The type of injury caused
by the corn borer in dahlias is determined by the kind of bud in
which the young larva becomes established. Establishment of young
larvae in terminal and lateral buds, as described above, results
later in destruction of foliage and flower buds and in breakage of
branches and the stalk.
OONTROL OF THE CORN BOHIR WITH ISEOCTICII3S
Several facts of importance in control measures should be noted
in connection with the manner in which the corn borer infests dahlias:
(1) entrance is made at points of new growth by very small, newly
hatched larvae. (2) This initial infestation usually takes place
during late July or in August (depending upon latitude and season).
(3) Oonspicuous damage, such as breakage in branches, does not occur
until the caterpillar has reached a large size and is inside the
plant. (4) Unless dahlias are carefully examined for signs of infes-
tation or injury, such as egg masses or wilted leaves, black frass,
or castings, and stunted flower buds, the infestation may not be dis-
covered until it is too late to protect the plants from further damage.
Dahlias can be protected froa serious corn borer injury if cer-
tain insecticidal sprays or dusts are applied with suitable equipment
at the right time. This method is effective when the spray or dust
is directed at terminal and lateral shoots while the young larvae are
feeding upoa accessible surfaces in the buds of these shoots.
The most satisfactory insecticides for use against the Zuropean
corn borer are made with derris, cube (pronounced koobay), or nicotine.
Owing to shortage of derris and cube resulting from war conditions,
the use of these insecticides is temporarily restricted by law to
certain food crops vital to the war program. Restrictions of use
have not been applied to nicotine insecticides. Nicotine, when com-
bined with chemical materials which prevent its lose by evaporation,
is a very useful corn borer insecticide. One of these mixtures,
nicotine bentonite, can be readily prepared by the dahlia grower who
prefers to purchase the separate ingredients for mixing a spray.
Required are (1) a container of spray water in which are mixed (2) a
spreading or wetting agent, (3) nicotine sulphate (containing 40 per-
cent of nicotine), which is stirred into the spray solution, and (4)
dry Wyoming bentonite clay, which should be slowly poured into the
spray mixture while it is being vigorously stirred. Chemical compounds
called spreading agents are useful in corn borer sprays because they
promote penetration of the insecticide into spaces inhabited by young
larvae. Ordinary soaps are not suitable for this purpose. The most
effective spreading agents for use in corn borer sprays are highly
complex chemical products which are sold under various commercial
names, as Areskap, In-181, In-438, Santomerse D, and Ultrawet.
Proportions of the materials used in home preparation of nico-
tine bentonite spray are given in the following formulas:
1. For 50 gallons of spray-
Spreading agent -----------------.
Nicotine sulphate (40 percent nicotine) -
Wyoming bentonite clay ------------
2. For 6 1/4 gallons of spray-
Spreading agent v---------------- ----
Nicotine sulphate (40 percent nicotine) -
Wyoming bentonite clay -----------.
3 ounces (dry weight) /
8 ounces (fluid)
2 pounds (dry weight)
6 11/4 gallons
1/3 ounce (dry weight) 1/
1 ounce (fluid)
4 ounces (dry weight)
I/ To facilitate accuracy in measuring mail quantities of spreading
agent, a stock solution may7 be prepared by dissolving 3 ounces of spread-
ing agent powder in 1 pint (16 oz.) of water. For example, 8 fluid
ounces of this solution will then contain the correct amount of spreader
for 25 gallons of spray, and 2 fluid ounces will be enough for 6 1/4
gallons of spray.
The amount of protection obtained from the application of
insecticidal spray is also dependent upon the precision with which
it is directed at the dahlia plants. Axial and terminal bud growth
should be wet thoroughly without washing away residues already de-
posited in them. These residues are caused by the use of the ben-
tonite clay. Best results are obtained when sufficient pressure is
used to force the insecticide deeply into bud growth. It is essen-
tial to stir the spray thoroughly when it is mixed and to agitate
it frequently during application. Choice blooms intended for exhi-
bition should not be wet by the spray after petals show color in
the flower buds. The use of overhead irrigation after sprays or
dusts have been applied reduces the effectiveness of insecticidal
residues because they are readily diluted and washed from the plants.
Tor growers who prefer to apply insecticides in dry powder or
dust form, several preparations containing nicotine bentonite are
available in packages. These are sold under various commercial
names and are mixed and ready for use as purchased. To be effec-
tive, they should contain not less than 4 percent of nicotine.
iquipaent for Applying Insecticides
Standard sprayers having capacities of from 21 to 30 gallons
have been found satisfactory for use in dahlia gardens. Excellent
results may be obtained with 2j- or 3-gallon, hand-operated, portable
sprayers, as illustrated in figure 4. if the pressure is maintained
by frequent pumping. The small quart-size atomizers operated by a
plunger of the bicycle-pump type are not always dependable for ob-
taining satisfactory control* In gardens of 500 or more plants
greater convenience is afforded by a wheelbarrow type of sprayer
equipped with a 12- to 2-gallon supply tank and 15 feet of hose.
The working radius of a wheelbarrow sprayer is increased when the
first few feet of hoseline have been elevated sufficiently to permit
extension of the hose in any direction. This can be accomplished
readily by attaching to the wheelbarrow frame a 3- or 4-foot length
of iron rod or strap which has been bent into a loop at its top for
holding the hose above the sprayer.
To facilitate direction of the spray to various plant parts,
the sprayer hose should be fitted with a lever-operated valve for
controlling the spray discharge, and an 15- to 24-inch brass exten-
sion pipe, as shown in figure 4. The outer end of the extension
pipe should be equipped with a suitable nozzle for breaking the
stream of insecticide into a fine miesty spray.
Any of the various types of hand-operated dusters is capable of
depositing a sufficient quantity of insecticide on the dahlia plant.
The hand-operated knapsack type of duster is most suitable for use
in both small and large plantings of dahlias. The round terminal
opening on the 19- to 24-inoh extension tube of most dusters
provides a satisfactory discharge of dust; no special nozzle is
When to Apply Insecticides
Dahlias require insecticidal protection from the uropean corn
borer during a period of 30 days or more, beginning late in July or
early in August, A spray or a dust treatment should be applied im-
mediately when signs of infestation have been found, such as hatching
of corn borer egg masses or injuries in terminal bids as described on
pages 2 and 3- Applications must be repeated thereafter at 5-day
intervals, owing to the constant production of new and unprotected
bud growth, the continuous hatching of corn borer eggs, and the dilu-
tion of insecticidal residues caused by rainfall. For example, if
hatching or infestation was first observed and an insecticide applied
on July 25, the succeeding applications would be scheduled for July 30,
and August 4, 9, and 14, and continued into the first week of September
according to the occurrence of corn borer hatching. Soomevhat less
satisfactory control nay be expected when treatments are applied at
weekly intervals. If insecticide applications are delayed until a
week or two after initial infestation, these treatments should provide
protection for the late blooms, but earlier infestation of stalks and
branches is likely to cause breakage. On the other hand, blooms
become infested when insecticide applications are discontinued too
SUPPLOM1TART CORTHOL M!ASURS
Disbudding and Disbranching
An understanding of how borers got into dahlias suggests several
methods that nay be adopted for keeping them out. Signs of infestation
in shoots and buds, for example, indicate where pruning will prevent
further trouble, and removal of infested shoots and buds is sometimes
sufficient in garden that are very lightly infested. However, the
corn borer does not always enter those lateral shoots and flower buds
that would normally be removed by good pruning practice. Where
heavier infestation prevails, disbudding and disbranching cannot be
lapended upon to protect the plants from serious injuries or the less
of quality blooms.
Use of a Tent
Choice varieties are sometimes grown under complete tents coo-
structed of tobacco shade-cloth, and this practice has been found to
reduce infestation greatly, Occasional injury is caused by larvae
hatching on nearby plants and gaining access te those grown under the
tent, or by larvae hatching from egg masses laid upon the tent, from
which the larvae penetrate the screen and drop or crawl to plants
within the enclosure. In localities where moisture held inside such a
tent does not promote tenderness in the blooms or fungous disease in
the plants, this has been found to be a very satisfactory method ef
keeping borers out of dahlias.
Trimming Root Clumps
Properly trimmed root clumps are not infested. After root
clumps have been harvested, the practice of burning dead stalks
and other infested debris remaining in the garden will reduce the
number of borers that otherwise would become egg-laying moths dxur-
ing the following spring. In preparing root clumps for winter
storage, it is considered good practice to remove the stalk near
the crown of the plant and below the pith cavity of the stem.
This leaves the roots free of borers, as larvae of the European
corn borer have not been found to excavate below the crown. Since
it is also standard practice among commercial growers to cut off
dahlia stalks at the crown, there is no reason to assume that
borers will be present in newly purchased root clumps.
Figure 1.-Seasonal history and developmental stages
of the European corn borer in early corn and in
dahlias where two generations occur each year.
Figure 2.- A, Egg masses of the Eropean corn borer
deposited on the under side of fully formed leaves.
The egg mass on the leaflet at right is 1 to 4 days
old, and the eggs on the terminal leaflet are about
to hatch after 5 to 7 days of incubation. B, Egg
mass 1 to 4 days old (5 times natural size). C, Eggs
5 to 7 days old, with black heads of embryo larvae
showing through eggshells (5 times natural size).
STATE PLANT BOARD
Figure 3.--An advanced stage in the infestation of terminal
sections of a branch, resulting in stunting, stem weakness,
granular frass, and wilted leaves.
Figure 4.-Spraying dahlias with a small portable sprayer
fitted with an extension pipe for directing- the spray.
UNIVERSITY OF FLORIDA
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