May 13, 2021

Soybean Summit: Managing emerging insect populations

CHAMPAIGN, Ill. — Dectes stem borer and bean leaf beetle were troublesome for some soybean growers in parts of Illinois last growing season.

Nick Seiter, University of Illinois field crop entomologist, noted bean leaf beetle was an emerging problem in northern Illinois in 2020 and dectes stem borer has been gaining attention for several years in southern Illinois. He focused on management strategies for the pests at the recent Illinois Soybean Association’s Soybean Summit.

“The bean leaf beetle has been with us for many years, probably as along as we’ve been growing soybeans in Illinois, and probably before that in green beans and other legumes,” Seiter opened.

“Dectes stem borer is an insect we’ve been paying a little bit more attention to in southern Illinois in recent years. It’s not one that we’ve thought of historically as a major pest in Illinois, but one that we’re seeing more and more. This is one that we’d like everybody to learn a little bit in terms of the biology and management to be ready to deal with it in soybeans in southern Illinois in particular.”

Dectes stem borer adults are a bluish gray long-horned beetle that chews holes into the plant, typically into the soybean petiole, and lays eggs in the pitch. Upon hatching, the larva tunnels throughout the stem and feeds on the pith.

“The adult isn’t really going to damage soybeans. However, the larva feed on pith within the soybean stem and the damage can be fairly noticeable if you open a stem up and find feeding,” Seiter said.

Feeding in the petioles often results in individual petioles wilting or falling completely off of the plant, which is usually the first sign of an infestation.

“They’re going to feed on the pith, especially the structural part of that pith within the xylem and the phloem. So, the actual impact of vascular flow within the main stem is relatively minor,” Seiter said.

“They’ll line the tunnels with frass. It’s not just a hallowing out of the stem that you’ll see sometimes for a variety reasons, but they’ll have a tunnel through there that’s going to be lined with their excrement.

“The damage may look pretty bad and I think most would say it looks pretty bad and looks like it’s going to have a pretty serious impact on yield. It actually doesn’t. The actual yield reduction we see from the tunneling in and of itself from this feeding on the pith is minimal, if any.”

The economic losses from dectes stem borer occur if the damage leads to lodging.

As the plant nears maturity, larva moves to the base of the stem where it over-winters. This can cause the plant to break off and lead to harvest problems and reduced yield.

“We don’t see this every time that a field is infested. In fact, we don’t see it all that often. Where we’re more likely to see this lodging is when, for whatever reason, typically the weather, those plants have had an opportunity to sit out there in the field for some time after they’ve matured. So, it does take some time for this to occur,” Seiter said.

“Where we especially saw some lodging in spots was when we had fairly wet conditions throughout the fall of 2018 when we had trouble getting into the field. We did see some lodging from this insect and of course that can have serious impact on yield and not just on yield, but also on harvest efficiency. That potential for lodging is something we want to avoid if at all possible.”

Management

Insecticides are generally not recommended for control because the larvae are protected within the stem and the adults lay eggs over a long period of time in the summer — approximately mid-July through August in Illinois.

There are the following management options available:

Monitoring: There is no economic threshold established, but finding adults at higher than average numbers will be the first indication of a problem. Look for wilting or broken-off petioles and split soybean stems toward the end of the season to gauge the infestation levels. Also, examine soybean stems in the lodged areas to determine if dectes stem borer was part of the problem.

Timely harvest: Note fields that are infested with dectes stem borer and harvest those fields as early as possible to reduce lodging potential.

Soybean stubble: Tillage will have “fairly mediocre success” in reducing the larva survival rate in a particular field, but adults move readily from their over-wintering sites to surrounding fields. Areas with a lot of no-till production are likely to have more issues with dectes stem borer.

Bean Leaf Beetle

“You could argue that bean leaf beetle is probably the most common soybean pest we have in Illinois and you would be hard-pressed to go out and scout a soybean field throughout a season and not find at least a few bean leaf beetles,” Seiter said.

“However, it’s somewhat rare for us to see the kinds of bean leaf beetle numbers that it takes to cause significant yield loss. We did see that in a few cases this last year, especially in central, northern and northeastern Illinois with instances where we suffered some yield loss from bean leaf beetles. It was sort of an unusual outbreak of this insect.”

Bean leaf beetle is in the same family as the corn rootworm beetle. It looks very similar to an adult western, northern or southern corn rootworm beetle. There are some similarities in its behavior and life cycle.

“The one key difference is it’s the adults that we’re worried about in this pest as opposed to larva,” Seiter said.

There are two generations of the bean leaf beetle per year. The adults overwinter and typically emerge before soybeans are planted to feed on alfalfa, clover or other legumes. They tend to concentrate in fields where soybeans first emerge.

The overwinter adults lay eggs in the soil within bean fields in the spring and another flush of adults emerge from larvae in July and lay their eggs. The second generation adults then emerge later in the summer and seek over-wintering sites including soybean residue when temperatures cool.

“We usually don’t see a lot of impact from the second generation, but they’ll develop and you’ll get a final flush of adults in the fall,” Seiter continued.

Defoliation from bean leaf beetle feeding is common, but it does not necessarily cause economic damage.

“We saw in some cases of fairly significant defoliation last year. It is rare that soybean defoliation causes yield losses, but that doesn’t necessarily mean it never happens. But what we do want to remember about defoliation is it takes a lot of damage to really see a reduction in yield,” Seiter said.

“How much defoliation is a lot? Thirty percent defoliation before bloom is the economic threshold and I’ve never seen 30% since I’ve been an entomologist. That doesn’t mean it doesn’t happen, but it doesn’t happen terrible often. Soybeans are pretty incredible in overcoming defoliation prior to bloom. The damage threshold after boom is 20%.

“It’s something we have to vigilant for, but often what we think is a damaging level of defoliation, but we’re probably actually looking at maybe 5% to 10% defoliation. It’s going to be more cosmetic than having an impact on yield.”

More concerning is the potential for bean leaf beetles to transmit virus, especially bean pod mottle virus that can reduce soybean yields, particularly when plants are infected early in the season. Early control of bean leaf beetles can reduce bean pod mottle virus infection.

During seed fill, bean leaf beetle adults feed on and scar soybean pods. The feeding not only can cause direct damage to seeds, but also allow an entry point for the virus.

Control

Soybeans are fairly tolerant of defoliation and the economic thresholds to control defoliating insects are quite high. Consider a rescue treatment if defoliation exceeds 30% prior to bloom or 20% after bloom and target the pest still present in the field.

Management decisions should not only be based on populations, but also scarring.

“We want to target the beetles while the pods are still green and while we’re seeing 8% to 10% of the pods scarred. That’s the economic threshold and we’ll want to apply an insecticide to control bean leaf beetles populations,” Seiter said.

“You may want to lean on that 8% or maybe even a little lower than that if this is occurring early. If you have pods that have a lot of maturing left to go and you’re still seeing a lot of bean leaf beetle numbers out there, you’ll want to be a little bit more aggressive. The closer those pods are to turning yellow, the more of that kind of damage you can tolerate.

“Once the majority of the pods are yellow or even brown, then you can stop worrying about this because it’s the scarring that occurs in the earlier stages that leads to the holes as the pod matures.”

If a damaging population occurs, bean leaf beetles can be effectively controlled with insecticides. Insecticide seed treatments will provide effective protection of seedling plants, but will not persist beyond the early vegetative stages. A broadcast application of a pyrethroid will generally provide effective control.

“Unfortunately insects come at us at different times and that’s one of the many reasons why monitoring and spraying at economic thresholds is so critical to get effective control. We want to make sure they’re there when we spray that material. So, that if we have to spray an insecticide we get what we’re hoping for out of that material in terms of insect control and protecting yield,” Seiter concluded.

Tom Doran

Field Editor