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Strategic Farming: Let's Talk Crops Focused on What Pays for Soybean Pest Management

By Phyllis Bongard and Bob Koch

Which soybean pest management practices will pay off in 2024? Dr. Bob Koch, Extension soybean entomologist, described the latest developments in soybean pests and their management in the February 21 Strategic Farming: Let’s talk crops session.

Soybean aphid

Insecticide management

For over 20 years, insecticides have been used to control soybean aphids (SBA). Keeping track of the available products has been something of a roller coaster ride recently, particularly as it pertains to chlorpyrifos, one of the organophosphate insecticides (Group 1B). In 2022, the EPA revoked the tolerances and chlorpyrifos could no longer be applied to Minnesota crops. Then in November, the 8th Circuit Court vacated that order, which allowed the Minnesota Department of Agriculture to begin conditionally registering products containing chlorpyrifos for crop use in the 2024 growing season.

Where does chlorpyrifos fit in this season? While it is an effective tool, it’s also a toxic chemical with associated health concerns. Koch warns against immediate, widespread use, but instead suggests using it in rotation with other effective insecticides.

Pyrethroid (Group 3) insecticides have also been used heavily to control soybean aphid. That reliance or over-reliance has resulted in an increasing number of performance failures. Lab assays and molecular work have documented resistant soybean aphid populations, but that data couldn’t readily predict how products are performing in the field.
Practical pyrethroid resistance

To get at that, Koch dug into efficacy studies conducted in Rosemount and Lamberton from 2005 to 2020 and compiled the data for the pyrethroid, lambda-cyhalothrin. What he found was that the efficacy of field applications for this pyrethroid worked very well against soybean aphid from 2005 to 2014. After 2014, however, control in the field deteriorated rapidly and this coincided with a significant increase in SBA pyrethroid resistance in lab studies, as well.

As a result, Koch shies away from recommending a pyrethroid as the initial treatment to control soybean aphid. While it may be tempting to apply a pyrethroid because it’s inexpensive, it may do more harm than good if aphids are resistant to it. The application wouldn’t provide much for aphid control, but it would be very effective in killing the predators and natural enemies that suppress SBA populations. The result may be even more aphids and the potential for developing even stronger resistance.

Can pyrethroid resistance be reversed?

If insecticide resistance management practices were implemented, could pyrethroid resistance be reversed? To study this, Koch asked whether aphid populations display a fitness cost when developing resistance. If they did, it might be possible for susceptible populations to outnumber resistant ones in the absence of pyrethroids.

What he found was that the resistant populations did better than the susceptible ones when not exposed to insecticides, so the result was the opposite of a fitness cost. The evidence suggested that even if pyrethroid inputs into the system were reduced, it’s unlikely that the resistant populations would revert to being susceptible again.

Protect insecticide effectiveness

The resistance issues with pyrethroids reinforce how important it is to manage the other available insecticides to maintain their efficacy. Koch’s research is showing that the newer insecticides – sulfloxaflor (e.g. Transform), flupyradifurone (Sivanto) and afidopyropen (Sefina) – and mixtures with neonicotinoids are still effective against soybean aphids, including pyrethroid-resistant aphids. Some of these insecticides, like afidopyropen, sulfoxaflor, flupyradifurone, are also less toxic to SBA’s natural enemies.

Scouting pays!

By the time soybean aphids colonize many soybean fields, seed treatment concentrations in the plant have generally decreased to ineffective levels. In a regional economic study comparing prophylactic seed treatments to scouting and threshold-based applications, scouting and using the 250 aphids per plant economic threshold (ET) provided a much larger return on investment. Preventative or insurance applications are not only costly, they kill beneficial insects and put the field at risk for aphid recolonization and increased insecticide resistance.

For more information, see Soybean aphid in Minnesota soybeanScouting for soybean aphid, and Fact-based insecticide recommendations.

Soybean gall midge

Distribution

Soybean gall midge (SGM) is a newer pest in Minnesota. It’s been found throughout much of the southwestern part of the state up to Stevens and Swift Counties and as far east as Rice County. Females lay eggs in the fissures in soybean stems. Once hatched, the bright orange larvae feed under the outer layers of the stem tissue, causing dark lesions at the base of the plant. Feeding damage can lead to plants breaking off, wilting, and dying.

Entomologists in Nebraska have documented yield losses of 100% on field edges and 17 to 31% losses in the field interior in infested fields. Most Minnesota fields are nowhere near this heavily infested, but it is a pest we need to watch for and be aware of.

Potential management strategies

Because the larvae feed under the stem tissue and adult emergence occurs over an extended period of time, spraying insecticides is largely ineffective. Seed treatments haven’t shown much efficacy, but research from the University of Nebraska-Lincoln has shown that the soil-applied insecticide Thimet (phorate) has some efficacy; however, specialized equipment is often needed for application of such treatments. With chemical control not standing out as effective as it is for other pests, true integrated pest management incorporating non-chemical tactics will have to be used for SGM.

Cultural control

The University of Nebraska-Lincoln has been conducting research on SGM for several years with some promising results. Through a lot of screening work, they’ve identified soybean lines or varieties that appear to have resistance to SGM. Those could be fed into different breeding programs to increase the availability of SGM-resistant varieties.

They have also done interesting research into hilling soybeans, which protects the stem from SGM adult egg-laying. While the results are positive, the logistics of hilling soybeans would be a challenge and require special equipment.

Biological control

Koch and colleagues have been studying biological control with SGM predators and parasitic wasps. They wondered if ground beetles, those abundant little beetles that scurry around on the soil surface, might include SGM in their diets. In a lab setting, they found that ground beetles did indeed eat soybean gall midge larvae. Then through gut analysis, they documented that ground beetles will also eat SGM larvae in the field.

Parasitism is the other aspect of biological control that they are studying. They discovered an entirely new species of parasitic wasp that’s associated with SGM. Through molecular and other methods, this new wasp has also been confirmed in multiple locations across Minnesota, South Dakota, Iowa, and Nebraska. The female wasp lays eggs directly in the SGM larva, which eventually kills it. Koch estimates that 5 to 10% of the SGM population is being killed by these wasps.

For more information, see Soybean gall midge in Minnesota soybean.

Soybean tentiform leafminer – a new pest

Distribution

The soybean tentiform leafminer was first detected in Minnesota soybean in 2021. The heaviest infestations to date have occurred in pockets in the Minnesota River Valley and seem to be most severe on field edges near trees. This developing pest situation is being examined closely.

Damage

Larvae hatch from eggs that are laid on the leaf surface. These tiny caterpillars move into the leaf where they live and feed. To begin with, the feeding pattern seen on the leaf is linear or serpentine. As they grow, the larvae hollow out the leaf leaving a leaf blotch and eventually they create a tentiform mine – where the upper leaf surface buckles up. These blotches reduce photosynthetic area, so larvae can be considered defoliators. Standard defoliation thresholds will likely apply to this pest: 30% prior to flowering and 20% after flowering. However, research to examine yield loss specifically from this pest will begin this summer.

Control studies

Insecticide

Koch’s lab started evaluating insecticides in case this pest becomes an economic issue. In a lab study, insecticides with translaminar properties were applied at different larval development stages: serpentine mine stage, blotch stage, or tentiform stage. What they discovered was that the if insecticides were applied at the serpentine or blotch stages, control was good. However, there was almost no control at the tentiform stage.

Choice studies – what do adults prefer for laying eggs?

Several soybean genotypes – both susceptible and resistant to defoliators or aphids - were included in a choice study to evaluate which were preferred by females for laying their eggs. They found that susceptible plants had the most eggs while varieties known to be resistant some other pests had fewer eggs. These promising results suggest that there is some soybean germplasm that could potentially provide resistance to this pest.

They also conducted a choice study with several legume species. Soybean was the most preferred host species followed by vegetable soybean (edamame). None to few eggs were found on lima bean, mung bean, cowpea, red kidney bean, chickpea, fava bean or pea. Alfalfa was not included.

For more information, see Soybean tentiform leafminer in Minnesota soybean.

Green cloverworm

The green cloverworm is a sporadic pest in soybean. Typically, infestations are usually not large enough to reach treatable levels alone, but when combined with other defoliators may reach threshold. To date, no insecticide performance issues have been noted.

For more information, see Foliage feeding caterpillars nearing threshold in some Minnesota soybeans

Other pests

There are increasing reports of the defoliating insects, Japanese beetles and bean leaf beetles, in Minnesota. While typically not major pests, there may be pockets that approach thresholds. To get a good defoliation estimate, examine the whole canopy – not just the top where most of the Japanese beetle feeding occurs - at several field locations. Use the same threshold mentioned above: 30% defoliation prior to flowering and 20% after flowering.

Weather and soybean insect pests

Winter survival outlook

If we were experiencing normal winter temperatures, we’d expect higher insect mortality since there is no insulating blanket of snow to protect them. However, the above normal temperatures – even without protection from snow – may lead to good survival for overwintering insects.

Drought and the soybean insect pests

Two-spotted spider mites, an arthropod, is one species that benefits from drought. Scouting for this pest and assessing damage will be important if the dry pattern continues. For any infestations that reach the economic threshold this season, chlorpyrifos’ conditional registration reinstates an effective management tool. Otherwise, the only pyrethroid effective against spider mites is bifenthrin. Dimethoate, one of the organophosphates, does an okay job and traditional miticides are also an option.

Dry weather is also conducive to grasshoppers. Populations can increase over consecutive droughty years because the dry conditions suppress pathogens that help control populations. For more information, see Grasshoppers in Minnesota soybean.

Source : umn.edu

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