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Insecticide Use

Most insecticide application in North Carolina soybeans is for corn earworm control. Therefore this discussion focuses on this insect. Application methodology used for corn earworm is applicable to the other insects found in soybean.

Research has shown that the optimum timing for treating corn earworm in soybean is when most caterpillars are mid-size (1/2 to 3/4 inch). In this case almost all earworms will be killed if an effective insecticide is applied correctly and at a suitable rate. When caterpillars are very small (less than 3/8 inch) they are often found within flowers or flower clusters, or between the folds of new leaflets. Treatment when most earworms are quite small will often result in poor control. Large caterpillars are easily killed in soybeans, however, most damage (approx. 96%) is done during the last stages of growth and waiting to treat until most caterpillars are large may lead to damage before spraying is done. Since the objective of insect control is to prevent economic damage or expensive treatment, either treating too early or when caterpillars are large is unacceptable and treatment of corn earworm should be directed against mid-size caterpillars.

Treatment of soybeans can be successfully done by ground sprayer or aircraft. If by ground, a minimum volume of six gallons per acre applied through hollow cone nozzles (on row or broadcast spacings) should be used; pressures of about 60-70 psi or higher are preferred. Ground sprayers – high clearance or tractor mounted – can be operated in wide row soybeans without much equipment damage to the crop. Research has shown approximately 2% yield loss within the wheel lanes attributed to plant injury in large soybean plants from a high clearance sprayer and a slightly greater loss for a tractor sprayer. Equipment damage potential is highly influenced by soybean plant height and erectness, and the sprayer boom width.

Damage from ground sprayers in narrow row width soybeans can be significant. As shown in the table below, predicted yield loss per acre can be over four (4) bushels of soybeans per acre for both the high clearance and tractor mounted sprayers. Yield loss would be expected to be proportionally greater or less as yield increased or decreased from 35 bushel per acre used in the table. Also, yield loss would be greater if soybean plants were lodged and tangled. Although damage from ground sprayers may be undesirable, such equipment should be used to avoid spraying in drift sensitive areas and when waiting for an aircraft will allow much greater damage from the insects.

Predicted yield loss in 35 bu/A narrow row (7 inches) soybean after insecticide spray simulation with high-clearance or tractor mounted ground sprayer. *

Yield Reduction in Bushels Per Acre
Spray Boom Width % Area in
Traffic Lane
High Clearance Tractor
12′ 75 4.73 5.51
18′ 50 3.15 3.68
24′ 38 2.39 2.79
30′ 30 1.89 2.21
36′ 25 1.58 1.84
42′ 21 1.32 1.53

* average of three tests; C. D. Heim, J. W. Van Duyn, and J. R. Bradley, Jr.

Aircraft are often used to apply insecticide to soybean. Airplanes and helicopters normally apply from one to three gallons of water based spray per acre. If approximately one gallon per acre is used, the carrier often contains water plus an emulsified vegetable oil to reduce evaporation. All of these techniques may perform well if field size and shape, obstacles (e.g. power lines), and weather conditions are satisfactory. However, aircraft spraying may be undesirable if fields are small or close to houses or businesses, have significant road frontage, or are in areas adjacent to waterways (especially with restricted pyrethroid insecticides). Aircraft can be especially important in periods of corn earworm outbreak, when significant acreage must be treated quickly, or when wet field conditions make ground spraying impossible.

Insecticide Selection: Treatment with insecticide should only be made when insect numbers or damage exceed the economic threshold, as determined by scouting. Be sure that the target insects are present when treatment is applied, since several pests (e.g. corn earworm) can develop and leave the plants in a relatively short period in hot summer weather. Lower rates can be used with ground application and on lighter infestations. Mid rates to high rates may be warranted by air, when infestation is high, and/or in periods of rainy weather. Consult a current copy of the North Carolina Agricultural Chemicals Manual for specific insecticide suggestions to use against insect pests occurring on soybean.

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