It’s time to scout soybeans for insect problems

It’s time to scout soybeans for insect problems

• Insects of particular note to watch for are three-cornered alfalfa hopper and kudzu bug.

It’s probably a good idea to scout full-season soybeans sometime this week. 

Insects of particular note to watch for are three-cornered alfalfa hopper and kudzu bug. 

Stand reducers

Possible critters that are out in the system right now that can reduce stands or chew parts of plants in seedling soybeans, are slugs and grasshoppers. For pests that may reduce stand levels, try to ensure that the stand stays above Jim Dunphy’s recommended densities for a replanting decision. 

These are 50,000 plants per acre or more of May-planted soybeans or 75,000 plants per acre of June-planted soybeans. He also recommends replanting stands of lower densities only if the grower can get seed of an equally acceptable variety.

Slugs like to harbor in fields with lots of residue. They feed at night or on cloudy days and are favored by lots of moisture. 

During the day, their presence can be indirectly detected by shiny slime trails. These can be washed off by rain, so dig around in the residue to look for them. 

Insecticides are ineffective against these pests, so our best recommendation is a product called Deadline M-P’s. A trial in Mississippi showed this product could provide excellent control. 

Scott Stewart, University of Tennessee, recommends an evenly spread distribution at 10 lbs./A, as this product is relatively expensive. Also, he mentioned that it is not often in stock at the local dealer. 

Slugs are generally an early season phenomenon, as warm and dry weather is not favorable for their development. Under good growing conditions the plants should be able to outgrow this injury.

Grasshoppers can reduce stands by clipping seedling plants, but can also eat a lot of foliage. These are more of a problem later in the year, but by this time, plants should be large enough to tolerate the feeding injury. 

Problem near field edges

Grasshoppers are generally more of a problem near field edges and if a treatment is applied, sometimes a border treatment can be effective. Adult insects are difficult to kill and pyrethroids or acephate (Orthene) are equally effective. Nymphs can be killed with Dimilin.

Cutworms have not been an issue this year, but will feed at night and are difficult to find. You might suspect cutworms are a problem based on their feeding habit of cleanly clipping plants. They rarely are an economic problem, but are more of an issue in no-till fields.

Foliar feeders

Fortunately the threshold for foliar-feeding pests of soybeans is easy — 30 percent defoliation throughout the entire canopy up to two weeks before blooming. 

Pests to watch for this time of year are bean leaf beetle, southern corn rootworm, and various caterpillars. As of this time, I have not heard of a treatable situation for any foliar-feeding pest of soybeans in 2012.

Suckers and girdlers

Three-cornered alfalfa hopper has been a recent problem in Mississippi cotton and Mississippi and Tennessee soybeans. 

Last year, they caused lodging issues in the North Carolina Piedmont from girdling done during the early season (see previous blog post for description of injury). 

Unfortunately, once you notice the girdling on the plant — generally later in the season — the damage has been done. Now is the time to scout for these insects in soybeans. 

Although sampling beans with a sweep net is difficult early in the season, do your best to sweep the foliage. 

The treatment threshold for three-cornered alfalfa hopper is one per sweep in beans less than 10 inches tall. Pyrethroids do a fair job of managing this insect, with bifenthrin looking a little better than most.

Kudzu bug is widely distributed across the state, albeit generally in low abundances on soybeans.

There have been a few cases where treatments have been made. These were very early planted soybeans that were an early maturing variety (Group IVs). 

See this previous blog post for threshold recommendation levels in early season soybean and this blog post for information on the presence of kudzu bug nymphs in soybeans.

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