Soybean Aphids Found in North Missouri

Published: August 4, 2009

Low numbers of soybean aphids (Aphis glycines Matsamura) can be found in some north Missouri soybean fields. Numbers remain very low in many fields with an average of 1 to 5 aphids or less on up to 50% of the soybean plants. A much lower number of fields have higher numbers of aphids averaging in the range of 60 to 100 aphids per plant. In most fields, beneficial insects such as 12-spotted, Asian, and pink ladybird beetles, insidious flour bugs, and damsel bugs populations are actively feeding on the soybean aphids. Beneficial insects can have major detrimental impact on soybean aphids with low to moderate aphid populations often quickly eliminated from infested fields. Warm weather predicted for the first couple weeks of August also should help slow aphid reproduction. Soybean aphid reproduction is most rapid when temperatures of 72-77 degrees F and relative humidity below 78% occur during late July and early August. A limited number of fields have received insecticide applications for this pest at this time in the growing season.

Populations of winged soybean aphid most likely migrated into Missouri from more northern infestations during late July. Migrating aphids are usually all females which are capable of producing 3-8 female offspring per day for a 1 month period. These offspring are all female, born pregnant, and give live birth instead of laying eggs. This allows soybean aphids to rapidly reproduce with populations often doubling every 2-3 days when conditions are favorable. A total of up to 18 generations of soybean aphids may be produced during a single crop season in more northern states with few generations produced in Missouri after migrating aphids arrive during July. Spring infestations of soybean aphid occasionally occur in Missouri from overwintering soybean aphid eggs laid on several species of buckthorn (Rhamnus spp.). Whether spring infestations of this pest occur in Missouri are often determined by the rate of egg mortality caused by several beneficial insects while eggs are on the overwintering hosts. Ladybird beetles (ladybugs) typically cause the greatest amounts of egg mortality.

Figure 1. Soybean Aphid (Photo by Claudio Gratton)

Producers are encourage to treat soybean aphid populations only after they have reached or exceeded the economic threshold of 250 aphids or more per plant during plant growth stages of R1 (flowering) through R5 (seed fill). At present, higher populations of aphids can be found on late planted soybean and on fields of soybean growing under potassium deficient conditions. Older plants past the R5 stage of growth may support soybean aphid populations, but data from more northern states suggest they have little impact on yield once the soybean plants move past the R5 growth stage. Although the entire plant should be inspected to determine total aphid numbers, most aphids will be found on the stems and underside of soybean leaves. Newer plant tissue is often preferred as feeding sites.

Several insecticides are labeled for control of soybean aphid on soybean. The following table provides a list of recommended insecticides and use rates. In trials in more northern states where soybean aphids are a very severe problem, it has been observed that insecticides in the pyrethroid class (Asana XL, Baythroid 2, Mustang Max, Proaxis, and Warrior) provide from 2-4 weeks of residual activity, but may require from 24 to 72 hours to initially kill the aphid. In contrast, organophosphate insecticides (Lorsban, Nufos,) and carbamate insecticides (Furadan) provide for quick knockdown of the aphid population, but have a lesser period of residual control which may vary from 7 – 21 days postapplication depending on the rate of insecticide applied and specific field conditions. In some areas of the Midwest "cocktail" mixes are sometimes used for soybean aphid control. A "cocktail" mix will contain both a pyrethroid class of insecticide for long residual combined with an organophosphate or carbamate insecticide for quicker knockdown of the pest. Under Missouri conditions any of the labeled insecticides should provide good control for soybean aphid.