Field reports and observations are finding that soybean aphids are present in soybean fields, but those populations are small. Occasionally, a larger colony can be found on isolated plants, but widespread infestations just are not being found yet. This is quite different from what was experienced last year in this region.

Remember, last year by this time, we had fields that had reached the treatable threshold of an average of 250 aphids per plant with 80% of the plants with aphids present. Last year’s earlier than normal infestations in the northwest were the result of large numbers of eggs successfully overwintering in the region, successful hatch and survival of aphids on buckthorn in the spring, and successful migration to healthy soybeans in mid May. This year, our overwintering eggs were minimal, aphids on local buckthorn were few and far between and that left few aphids to move to local fields.

So, why the rush to do some treating for aphids ?
Even here in NW MN, there are reports of insecticide applications going on soybeans. These treatments are generally going out as a supplement to herbicide treatments. This strategy of herbicide-insecticide mixtures has been discussed and analyzed by many individuals involved in research on soybean aphid biology and management.

Unfortunately, there are some individuals who have encouraged this action through promotion of unrealistically low soybean aphid thresholds. This author is aware of a recent television program promoting a threshold of 5 aphids per plant at the R1 growth stage. These individuals are directly involved in ag chemical sales. Though they are entitled to their opinions, they are mischaracterizing the current research information regarding soybean aphid treatment thresholds, and in the process are misleading their audience. Let's see their research, though.

Should the treatment threshold be lowered for
small plants ?
Soybeans have a tremendous capacity to compensate for early season stress. There is no data to suggest that threshold should be lower for young plants as the hosts of the television show imply. It's not uncommon to see isolated young plants or hot spots where aphid populations exceed threshold, but these populations need to be prevalent throughout the field and persistent to justify insecticide use.

What about insurance treatments against
low-level infestations ?
Insurance treatments for soybean aphids appeal to growers for several reasons: previous losses with tardy aphid control, anxiety about waiting for the inevitable (which really isn’t inevitable!), avoidance of later-season infestations, protection of plant health, or convenience of tankmixing with a post-emergence herbicides, such as glyphosate.

We see little utility for insurance treatments against soybean aphid. Aphid populations vary widely from field to field, as does the timing of infestations. Applying insecticides to unscouted or low-level infestations is a recipe for problems. First, fields may not need treatment so growers waste their money. Second, early applications do not prevent subsequent colonization since any residual toxicity or repellency is gone in a week. There is only a temporary reprieve in scouting effort. Don't walk away from any sprayed field for the rest of the season. Third, early applications eliminate natural enemies and may actually increase the chances of needing to treat later in the season. Fourth, unnecessary insecticide use enhances the chance that insecticide resistance will develop in aphids or other soybean pests, such as two-spotted spider mites. Remember spider mite discussions from last year ?

Can I mix an insecticide with herbicide ?
Insecticide herbicide tank mixes can work if the timing is correct and soybean canopies are still open. Remember that the optimum water volume for aphid control is higher and pressure higher (= smaller droplets) than for drift free herbicide applications. Last year this approach may have been justifiable in NW MN; this year it is NOT!

University of Minnesota Extension