Nine answers to bean aphid questions
Kelley Tilmon, South Dakota State University Extension entomologist, answers questions about soybean aphids:
What’s the outlook for aphids this year?
I’m afraid that’s crystal-ball territory. I don’t put too much stock in the every-other-year pattern that some people cite. There’s too much variability in it to take that to the bank.
What do you think of seed treatments for aphid control?
I don’t recommend insecticidal seed treatments for soybean aphid management in spite of certain marketing pushes in this direction. The treatment has usually worn off by the time aphids are becoming a problem in mid to late July, and most university research does not show a yield benefit to using them for aphid control.
Do resistant varieties work?
In our trials we’ve seen good results with varieties containing the Rag1 resistance gene — we often see several hundred fewer aphids per plant compared to susceptible beans. But an important thing to keep in mind is that you can still get economic populations of aphid even on resistant varieties in a heavy aphid year, so you still need to scout and stay aware of what’s going on in your fields.
Resistance is a good tool that can prevent outbreaks under many circumstances when you’d have a bigger problem in susceptible beans, but it’s not a silver bullet.
When should you scout for aphids?
In South Dakota, the first aphids usually appear in soybeans around the first week of June, though these early populations are small and only occur in isolated pockets during most of June. Aphids are much more likely to reach the economic threshold in July or August, and this is the most important time to scout regularly.
Start actively scouting fields around the first week of July, and continue weekly through August. Once soybeans have reached the R6 growth stage (full seed, when a pod on one of the four top nodes has green seeds that fill the pod to capacity), research has not shown a reliable yield gain from insecticide treatment, regardless of aphid density.
Any scouting tips?
Walk a broad U or X pattern through the field and examine 20 to 30 plants total, spread out over the field. Aphids can occur in “hot spots,” so it’s important not to make your decision based on just one spot that doesn’t represent the field as a whole.
How do you count aphids?
Pick a plant at random and count aphids, starting at the bottom and working your way up. Soybean aphids don’t drop off easily, so you can pull the plant up for a closer look if your back gets tired. Pay special attention to the undersides of the leaves and the newest vegetation.
Early in the season, aphids tend to occur on the newest vegetation at the top of the plant. Late in the season, they’re often concentrated in the lower canopy. If it’s early in the season and you want to do a quick cruise-through to see what’s going on, you can usually spot new aphid colonies on the top trifoliate pretty quickly.
Sometimes they have ants with them, and you see the ants running around before you see anything else. But if you’re contemplating a management decision, you should take the time to count thoroughly and stick with the threshold.
What’s the threshold for spraying?
An average of 250 aphids per plant throughout the field (not just in isolated hot spots) is the threshold, provided treatment can be made within seven days. This threshold is supported through the R5 growth stage (beginning seed).
Though some producers feel like they should treat before aphids reach an average of 250 per plant to avoid damage, the needed safety factor is actually already built into the economic threshold. The economic injury level, where yield loss justifies the cost of treatment, is around 675 aphids per plant, on average.
The economic threshold of 250 aphids per plant is the decision point to plan for treatment, in order to keep aphids from reaching (higher) damaging levels. The economic threshold has a time-to-treatment buffer of seven days incorporated into it, based on the population growth rate of the insects under field conditions.
Various environmental factors can influence soybean aphid populations. For example, populations tend to not increase as quickly in hot weather (consistent daytime highs above 90 degrees F), though when temperatures cool, they can pick up fast growth again. Also, very heavy rains can sometimes knock aphid populations down, though this effect is usually temporary. Natural enemies — predators, parasites and diseases — also play a large role in general background suppression of aphids.
How about spraying aphids early, just to make sure they don’t reach threshold levels?
Even when the products are inexpensive or a free re-application is guaranteed, insurance spraying is not only an unnecessary cost, but actually has the potential to cause outbreaks of other pests such as spider mites, and may even worsen aphid problems.
This is due in large part to the pesticide killing natural enemies that help provide significant background control. Insurance-type application of fungicides is not recommended either, because these products also kill the beneficial insect-killing fungi that are shown to help keep soybean aphids and spider mites in check.
What’s going on with biological control?
The parasite we introduced a couple of years ago hasn’t overwintered successfully. Researchers at the University of Minnesota are preparing permit applications for a new parasite. When that’s green-lighted, we hope to conduct some releases in South Dakota.
This article published in the February, 2011 edition of DAKOTA FARMER.