# 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**

** click image to zoom Chemical control. **Some pesticides labeled for soybean aphids in South Dakota^{a}

**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.

### Buyers Guide

Can you show us how you calculated your 356 aphid/plant economic injury level? Just the formula that way we can calculate injury levels ourselves. How was the damage boundary calculated?

Still waiting for the formula. You came up with a number, exactly how did you come up with this 356 aphid/plant economic injury level? How did you relate control cost, market value, yield potential, and soybean growth stage in your mathematical formula? Just show us the formula so that we can do our own calculation! You claim that your method resulted from many years of work by universities. Where is the formula for calculating your economic injury levels?!!!

Can you show us how you calculated your 356 aphid/plant economic injury level? Just the formula that way we can calculate injury levels ourselves. How was the damage boundary calculated?

I forgot to ask - can 250 aphids become 1000 aphids in 48 hours in August at R4? When do you treat for the aphids in this situation? Maybe in the 27th hour? Hope that speed scouting bcrap is speedy enough!

The original research that established the EIL is available at http://labs.russell.wisc.edu/cullenlab/files/2012/03/SBA-ET-JEE-v100-pp-1258.pdf

Thank you Dr. Wright! According to this study, the aphid doubling time can be as short as 2.7 days (Table 1). For the sake of argument, if we accept the damage boundary of 800 aphids/plant, and an ET of 250 aphids/plant, the damage boundary will be exceeded in 5.4 days. The paper does not provide a formula on how damage boundary can be calculated objectively. The linear yield loss function has a very low coefficient of determination only slightly better than a coin flip probability. Lastly, tell me which formula from the paper should I use to calculate an EIL for organic soybean with a market value of $60/bu; yield potential of 60 bu/ac; organic spray cost of $35/acre; plant stage at R4. There is no formula specified in the paper. It is full of pretty logistic curves but "Where's the beef?"

Good morning Doctor and Professor Wright! So, what do you think? What about conventional soybean with $12.12/bu market value; 70 bu/ac yield potential; $4/ac Asana or Endigo ZC (ad above) spray cost; at R4 stage? What would be the EIL and damage boundary? Show your calculations neatly on the "blackboard" please! I'm concerned that you may not actually have the mathematical procedure and basis for your 250 recommendation and yet you and your university folks keep on spewing this 250 farce every single year. Go big red?

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