As distributors begin to stock up insecticide inventory for the season, it is a good time to lay out a management strategy for corn earworm control in soybeans. This has been our most consistent soybean insect pest, sprayed on the majority of our acres in most years. I am predicting that this year’s pressure will be similar to last year, which is to say relatively light. Below are my suggestions going forward:
1. Although soybeans should be scouted throughout the season (think kudzu bug), begin scouting for corn earworms on soybeans that are flowering. These are very attractive places for moths to lay eggs. Scout at several places in the field, using a drop cloth rows above 30 inches and a sweep net for narrow row or drilled soybeans.
2. There are very few cases where corn earworms impact flowering soybeans. Recent research was conducted by a student working under me, Rachel Suits. This work, supported by the North Carolina Soybean Producers Association, proved that flowering soybeans can tolerate at least 2-3x the corn earworm threshold levels of podding soybeans. In fact, we were not able to create a situation in the field where corn earworm could cause a yield loss in flowering soybeans. Therefore, the threshold for flowering soybeans is likely much much higher than in podding soybeans.
I have heard that some folks would like to eliminate corn earworm while plants flower to protect pods from larger worms that developed in the flowering stages. This is not a good idea since the worms tend to be very small (first and second instar) during flowering and will hide in the blooms, making control difficult. Also, sprays impact natural enemies that can come in and reduce developing populations of corn earworm. The worst infestations of corn earworm, beet armyworm, and loopers that I saw last year were in fields where pyrethroids were sprayed automatically at flowering.
Scout during flowering to see how populations are developing. You can then be ready to spray during podding stages (R4 and greater), when soybeans are susceptible to yield loss.
3. The threshold for corn earworm is online and can be accessed using a recently updated threshold calculator (click here). This threshold is specifically for podding soybeans (R4-R7). It also is accessible from mobile devices and can be used in the field. This threshold is already conservative and there should be no need to adjust it. This is set below the break-even point where the cost of control is equal to the damage the worms are doing.
4. If you have reached threshold, you should spray. Keep an eye on blog posts for reports of pyrethroid-resistant moths in the system. Populations of these are spotty and not widespread across the state. If significant resistance is found, you should not spray a pyrethroid (common chemical names include beta-cyfluthrin, bifenthrin, lamda-cyhalothrin, zeta-cypermethrin). Furthermore, if you have a control failure with a pyrethroid, do not spray with a pyrethroid again or with a pre-mixed product that does not contain a chemical effective against corn earworm. Don’t forget that sometimes what we think is “corn earworm” might actually be tobacco budworm, a similar looking species that is difficult to control with pyrethroids.
Other chemicals you should consider are the diamides, like Belt and Prevathon. Syngenta has a new registration for Besiege, which is a pre-mixed product containing the active ingredient of Prevathon plus a pyrethroid (lambda-cyhalothrin, aka Karate). Blackhawk (Tracer) offers a unique chemistry class, the spinosyns, and is highly effective against corn earworm. Finally, in every trial that I’ve had it and at every rate (as low as 6.7 oz/A), Steward, which is also a unique type of chemistry, has been the most effective or among the most effective chemical for corn earworm, even at lower rates.
Each product has advantages or disadvantages. For example, the residual of the diamides is very good, but may not be a concern since only one generation of corn earworm a year generally develops in soybeans. However, in some years (like 2010) we can have multiple infestations, as well as late-season pests, like soybean loopers and fall armyworms. These products are excellent choices in these situations. Above all, rotate chemistry and only spray at threshold.