Source: CropWatch

Another insecticidal seed treatment for soybeans is now available. Bayer CropSciences announced registration of Gaucho® seed-applied insecticide for use against early season insect pests of soybean. The other currently available seed-applied insecticide is Cruiser® from Syngenta. Both are neonicitinoid insecticides -- the active ingredient in Gaucho® is imidicloprid and in Cruiser®, thiamethoxam.

The question is; “Where do these insecticidal seed treatments fit in Nebraska soybean production?” Our major early season soybean insect pest is the bean leaf beetle. In early spring the over-wintered beetles become active and often enter soybean fields as soon as plants emerge. Although the defoliation the beetles cause can appear quite severe, research in Nebraska and elsewhere has shown that it usually does not result in economic damage. Soybean plants can compensate for a large amount of early tissue loss, so it takes a considerable amount of beetle feeding (generally over 50% defoliation) to impact yield.

A possible exception to this is in very early planted fields, when plants emerge well before those in surrounding fields. Because the beetles move to soybean fields during seedling emergence, early planted fields usually will have more beetles and suffer the most injury. For those who regularly plant before most of their neighbors and have a history of high beetle numbers, insecticidal seed treatments may be an option. Both products, Cruiser® and Gaucho®, have proved effective against early season bean leaf beetle in our insecticide trials.

The other soybean insect pest that some have suggested can be controlled with insecticidal seed treatments is the soybean aphid. Although early season aphid mortality and sub-lethal effects have been observed with insecticidal seed treatments, we do not recommend seed treatments for controlling soybean aphids in Nebraska.

Dr. David Ragsdale and Brian McCornack of the University of Minnesota, have conducted soybean aphid mortality and fecundity (number of offspring) studies to investigate the questions “How long can we expect systemic activity of thiamethoxam to last in the soybean plant?” and “How successful will female soybean aphids be at reproducing on Cruiser-treated soybean leaves?”

In lab bioassays, they exposed soybean aphids to soybean leaves pulled from thiamethoxam-treated plants (50g/100Kg seed) at successively later intervals after planting. They placed aphids on treated leaves and recorded percent mortality and the number of offspring produced per female at 24 hour and 48 hour exposure/feeding times. Ragsdale and McCornack found that soybean aphid mortality was significant up to about 35 days (V3/V4 growth stages), but beyond that point, aphid mortality decreased toward zero and was no longer significant. They also found that soybean aphid fecundity was significantly reduced up to about 40 days after planting. However, beyond 40 days they saw no difference in the number of nymphs deposited by aphids on treated and untreated leaves. The Minnesota researchers concluded that under normal planting dates they did not expect thiamethoxam seed treatment efficacy to hold, particularly under high and/or late season soybean aphid pressure.

In a field study, Dr. Michael Catangui and colleagues from South Dakota State University examined the effect of insecticidal seed treatments (thiamethoxam, imidicloprid, and clothianidin, another neonicitinoid) on soybean aphid populations during R5 soybeans in August. They found that none of the seed treatments significantly reduced aphid numbers or increased yield and concluded that insecticidal sprays would be necessary to control late season aphid infestations. Their findings are similar to our trials in Nebraska.

To date in Nebraska, the soybean aphid has not been an early season pest; it has been a late season pest. Most of our soybeans are planted in May, and we generally don’t begin to see aphids until mid to late July, well past the early vegetative stages. In most fields soybean aphid populations peak in August. This is too long a period of time to expect currently available seed treatments to have a significant effect. In addition, not all soybean fields in Nebraska will have economic populations of soybean aphids, even in the northeast part of the state where we have seen the most soybean aphid injury. This year is a perfect example Why spend money up front when the need may not arise? With the cost of a seed treatment at approximately $12-$14 an acre, it is an expensive “insurance” with a high probability of either not being needed or not being efficacious when used to manage soybean aphid. At current prices foliar insecticide treatments are cheaper than seed treatments and need only be applied when necessary.

In conclusion, we believe that insecticidal seed treatments have a role to play in early planted soybeans for early season bean leaf beetle management, but we do not recommend insecticidal seed treatments for soybean aphid management in Nebraska. Manage soybean aphids by scouting and using economic thresholds. In Nebraska, one well-timed foliar insecticide application, based on two or more field visits, will adequately control soybean aphids and protect yield.

Thomas Hunt
Extension Entomologist
Haskell Ag Lab, Northeast REC
Keith Jarvi
IPM Assistant
Northeast REC