March 28, 2024

Insects to watch for in 2020

CHAMPAIGN, Ill. — Management tactics for protecting crops from potential insect damage were the focus of a recent Illinois Soybean Association-hosted webinar.

Nick Seiter, University of Illinois entomologist, covered the insects that farmers need to keep an eye out during the soybean growing season.

Dectes Stem Borer

Issues with the insect were noted in southern Illinois the last couple years, particularly in the fall of 2018 when there were lodging issues caused by the pest.

The insect overwinters and adults begin to emerge in late June and into July, and the adults then begin laying their eggs in soybean petioles. The egg will hatch and bore its way up and down the petioles, typically causing the petiole to turn brown or yellow and ultimately break off at the main stem.

Dead and dying petioles are the first sign in the field of a Dectes stem borer infestation. The larvae create a tunnel in the pith of the plant. The tunnel will extend as the season progresses and is lined with dark brown excrement from the insect larvae.

Another indication of a Dectes stem borer infestation can be found near the end of the growing season when piles of sawdust forms at the base of the plant after the larvae have girdled the stem from the inside out.

Seiter said while the tunneling doesn’t cause much yield loss, the concern comes later in the season with the potential of substantial lodging.

Chemical control of Dectes stem borer is not recommended as it could require multiple applications because the insects lay eggs over an extended period of time — six to eight weeks.

Management efforts should focus on mitigating lodging with a timely harvest. Beginning in late summer, scout for wilted petioles, larvae tunneling and sawdust piles and target fields with infestations for harvest first.

Brown, Green Stink Bugs

Reports of green and brown stink bugs damage have been noted in southern Illinois and up into central Illinois. The pests feed on soybean pods and seeds, so populations build at R5 and R6.

The earlier the stink bug feeds on the soybean seeds, the greater the yield loss. Signs of damage include wrinkled and deformed seeds that reduce quality and create the potential for pathogens later in the season.

The brown stink bug is a bit more difficult to control than the green stink bug.

Seiter said heavy brown sting bug populations can be controlled with bifenthrin or organophosphate. With green stink bug, almost any pyrethroid insecticide is going to provide effective control.

The damage that the brown and green sink bugs do in soybeans is essentially the same. When scouting for them there often is a mixed population of these two insects.

For scouting purposes and for threshold purposes, count both of these the same and the immatures and adults the same as they are going to do the same kind of damage.

Brown Marmorated Stink Bug

The insect has been in Illinois for several years. Its population has been increasing and has numerous different host plants. It has been problematic for specialty crop production — orchard crops, fruits and vegetables.

One of the issues with soybeans is it is a little more difficult to scout for than brown and green stink bugs. They’re better at avoiding the sweep net and avoiding detection.

While the brown marmorated stink bug has not been a major issue in Illinois to day, farmers should be on the lookout for the pest.

Redbanded Stink Bug

This pest has been a major issue in the mid-south soybean production areas the past several years, particularly in Louisiana and coastal Texas, and causes more damage than other stink bug species in soybeans due to their ability to ingest more material.

Redbanded stink bugs also prefer soybeans and other legumes whereas other stink bugs have multiple hosts. Because of that, they tend to remain in soybeans fields through harvest.

While this insect has been a major issue with this in Illinois, Seiter said it is something that is being monitored, especially in southern Illinois after a mild winter.

“It’s something we want to identify and potentially mitigate very quickly because these can be a major issue for production because it can cause very severe yield loss and quality reductions when they build up,” Seiter said.

He recommends using a sweep net for scouting stink bugs. The economic threshold for brown, green and brown marmorated stink bugs is 9 in 25 sweeps or 1 per row using a drop cloth. Don’t apply an insecticide until the economic threshold has been reached.

The redbanded stink bug is more damaging than the green, brown and brown marmorated stink bugs. Therefore, the recommended threshold is 4 in 15 sweeps.

Most infestations occur late in the season, late R5 to R6. They don’t do any damage until there’s a pod on the plant.

“For something like a stink bug, applying an insecticide at R3 in almost every case is going to be about as good as nothing. It’s not that the insecticide won’t kill the insect. It’s that insecticide is not going to be there when the insect arrives in the field,” Seiter said.

Defoliating Insects

Common pests in this category in Illinois include Japanese beetles, bean leaf beetles, green clover worms and soybean loopers. Early season issues have been noted from armyworms, variegated cutworm and a variety of other caterpillars that feed on soybean tissue.

“Most of the defoliation we see in Illinois is more cosmetic than anything else. Soybeans tolerate a lot of defoliation. It especially tolerates a lot of defoliation early in the season in the vegetative stages. Our economic threshold for soybean defoliation in vegetative soybean is 30% of the canopy eaten away. Soybeans are able to recover from that damage very effectively,” Seiter noted.

Soybeans become more susceptible to damage after bloom and the threshold is 20% defoliation.

For scouting, identify the level of defoliation, identify the insect causing the problem and then select a control option that’s effective for that particular pest.

“We do have a reasonably high armyworm population in some parts of Illinois right now and those can in some cases in seedling soybeans actually tear the canopy to the point that you have some stand loss, and that is a different situation from just worrying about defoliation,” Seiter said.

“Where we have the worse problems with armyworms is typically a situation where we had a grassy weed infestation and for whatever reason our herbicide control was either late or only moderately effective. You have those grassy weeds dying at a time when soybeans are popping up out of the ground and the armyworms need something to feed on and they move over to soybeans. It’s fairly uncommon to get severe armyworm damage in a clean stand of soybeans.

“We have seen some issues with armyworm following a rye cover crop, especially when there’s the termination of the cover crop as the soybean is coming up. If there are armyworms in there, they will move to soybeans. You want to be vigilant.”