Janet Knodel

NDSU Extension Entomologist Janet Knodel

CARRINGTON, N.D. – Those attending the Central Dakota Ag Day on Dec. 5 in Carrington, N.D., were told about a new soybean insect that growers may have to start worrying about.

North Dakota State University Extension entomologist Janet Knodel says the soybean gall midge (SGM) is a growing problem in the surrounding states of Minnesota, Nebraska and South Dakota, but none have been detected in North Dakota as of the end of the 2019 growing season.

“I did a survey for the soybean gall midge last year, focusing on the counties that border Minnesota and South Dakota,” Knodel said. “Fortunately, we have not found it, but it is moving our way, so it is probably just a matter of time.”

The SGM is about a quarter-inch long with black and white banding on its legs and a reddish abdomen. The insect has a complete metamorphosis – egg, larva, pupa and adult stages. Neighboring states have noticed the adult has a long emergence window and that fact could make control by insecticide difficult. In Minnesota, larvae have been observed in soybean stems from late June through July.

Damage to the soybean plant is caused by the larval feeding inside the plant stem. This causes brittle stems and significant yield losses when populations are high due to the plants wilting and dying. The larvae feed under the epidermis of the stem, which can weaken the stem and cause lodging, further adding to yield losses.

“When you are out there, pay attention to darkened stems at the base and if you take your fingernail, scrape that away. If you see bright reddish-orange larvae, it is most likely soybean gall midge,” Knodel explained.  

As part of her presentation, Knodel reviewed the various foliage-feeding caterpillars that affect soybeans. She discussed how to scout for these insects and how to determine the amount of damage they are causing. This information can help growers determine an economic threshold for control measures and making use of natural predators.

Some of the insects she mentioned included the thistle caterpillar/painted lady butterfly, green cloverworm, velvetbean caterpillar, soybean looper, cabbage looper and alfalfa webworm. 

Estimating damage by defoliating insects

According to Knodel, there are two methods that can be used to estimate damage – determining the percent of defoliation, as well as the number of insects per foot of row space.

Scouting for insect defoliation should be done from the late vegetative to the R6 soybean crop stage. Samples should be taken at least 10 rows into the field and taken in a “W” pattern within the field. Ten plants should be sampled per location over four different locations. Leaves should be removed from the top, middle and bottom of randomly selected plants. An estimate of leaf defoliation can then be made by comparing the illustrations on page 82 of the 2019 NDSU North Dakota Field Crop Insect Management Guide Extension publication.

Insects per foot of row is determined by shaking plants over the inter-row space, where a white drop cloth has been laid. The total number of insect pests per foot of row that fall on the cloth are then counted. An average infestation of 4-8 larvae per foot of row typically causes 20-30 percent defoliation.

Knodel noted the growth stage the plants are in is important, since under most conditions, moderate defoliation early in the season has little effect on the final soybean yield. However, as plants reach the flowering and pod filling stages, defoliation poses a greater threat to yield. A 30 percent defoliation level or higher is indicated for plants in the vegetative growth stages, but that figure drops down to 20 percent once the plants are in the reproductive stages.

Natural control for defoliating insects

There are many forms of natural control of these insect pests that growers should also consider. There are fungal and viral diseases that are enhanced by high humidity and warm temperatures. Parasitic wasps can also play a role in controlling harmful insects as well as predators such as ground beetles, predaceous stink bugs, birds, frogs and rodents.

Finally, there are several insecticides that can be used as well and there is a complete listing in the soybean section of the NDSU Field Crop Insect Management Guide.

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