Soybean aphid has developed resistance to pyrethroid (Group 3A) insecticides. Currently, any populations of soybean aphid should be considered potentially resistant to pyrethroids. Insecticide rotations are an important part of insecticide resistance management and may help reduce the chances of further development of insecticide resistance in this pest.
Rotate insecticide groups
Table 1 summarizes some suggested rotations of insecticide groups for management of pyrethroid-resistant soybean aphids. Whether an insecticide seed treatment was used (see first column) will affect the suggested insecticide groups available for use for the first foliar application (see second column) against soybean aphid. Some considerations about these different insecticide groups are given in the fourth column. The insecticide groups suggested for a second foliar application (see third column), depend on what insecticide group was used for the first foliar application.
Table 2 provides a summary of different insecticides available for soybean aphid management.
Table 1. Suggested insecticide rotations for pyrethroid resistant soybean aphids by insecticide treatment (insecticide groupa).
aInsecticide groups: 1A=Carbamates, 1B=Organophosphates, 3A=Pyrethroids, 4A=Neonicotinoids, 4C=Sulfoxamines, 4D=Butenoloides, 9D=Pyropenes.
Aphids resistant to Group 3A are widespread; all populations should be considered potentially resistant to Group 3A. Migration of aphids previously exposed to insecticides reduces ability to maintain effective insecticide rotation, particularly later in the season.
bIf a neonicotinoid seed treatment was applied, avoid any Group 4A insecticides in initial foliar application. It may be prudent to avoid 4C and 4D, as well
cAlthough there are no documented cases of cross-resistance among Group 4 insecticide subgroups (4A, 4C, 4D), it is suggested to avoid using them in sequential applications including seed treatments.
dOther labeled group 1B insecticides (dimethoate) are not recommended as a stand-alone insecticide, due to inconsistent aphid control.
eInsecticide mixtures are usually not desirable from an insecticide resistance management standpoint, but may be unavoidable with limited effective insecticide groups. Mixes with reduced rates of either insecticide may increase risk for development of insecticide resistance.
Table 2. Insecticide groups, active ingredients and example products for management of soybean aphid.
Scout for soybean aphids
Decisions to apply foliar insecticides for soybean aphid should be based on regular scouting of soybean fields. Through the R5 soybean growth stage, application of foliar insecticides is justified when aphid populations reach the economic threshold of an average of 250 aphids per plant (with 80% of plants infested with aphids and aphid populations increasing).
Continued scouting of soybean fields after insecticide application is necessary to determine if the insecticide application was effective and to determine if another insecticide application is necessary. Retreatment of a field may be necessary due factors such as poor performance of the insecticide or reinfestation of the field by winged aphids coming from elsewhere.