Soybean aphid scouting and management
Prophylactic or “insurance” treatments at low aphid levels do not provide good value, because treated fields often see pest resurgence, frequently to levels higher than untreated fields. This can occur because the insecticide has eliminated the beneficial predatory insects that help keep pests in check, or because winged migrant aphids have recolonized the field, or both factors working together. A single well-timed application based on scouting information and thresholds is more economical than two poorly timed applications; and in some years even a single application may not be needed. Another problem with insurance treatments in soybean is secondary pest outbreaks of twospotted spider mites. Spider mites are a particular problem when the weather is hot and dry. Spider mite outbreaks often happen in fields that have previously been treated for soybean aphid. This is because many products (particularly most pyrethroids), can actually flare spider mite populations.
Soybean aphid management
Resistant varieties. Aphid-resistant soybean varieties are available and are an under-utilized tool in aphid management. We have extensively tested lines containing the resistance genes Rag1 and Rag2 in South Dakota. Though resistant varieties will seldom be aphid-free, these genes typically provide a five-fold to ten-fold or greater reduction in aphid populations compared to susceptible checks. Many companies provide aphid resistant varieties in various maturity groups. Two such providers are Syngenta and, for organic producers, Blue River Hybrids.
Natural enemies. There is a diverse community of natural enemies in soybean, which help suppress soybean aphid colonization and population growth. These natural enemies include ladybeetles, lacewings, pirate bugs, and entomophagous (insect-killing) fungi. One of the most important predators of soybean aphid is the multicolored Asian ladybeetle, Harmonia axyridis. In the absence of these natural enemies, soybean aphid population growth is significantly faster (2–7 times). Pesticides often more thoroughly eliminate natural enemies than aphids, which allows the remaining aphids to rebound quickly without their natural checks.