A number of insects can attack soybeans early in the season, either at aphid or during the early vegetative stages. It pays to be able to identify these insects and understand a bit about their lifecycle, treatment thresholds and methods of control.

Seedcorn Maggots: Seedcorn maggots are approximately ¼” in length and are pale, yellowish white maggots that lack defined heads and legs.  The Seedcorn Maggot damages soybean plant stands by burrowing into seeds and destroying the germ.  Seedcorn Maggots are usually found in fields with manure or fields with high residue.  Scouting usually occurs after reduced stands are noticed.  A seed treatment is needed, especially if planting into fields with residue or manure.

Seedcorn Maggots

White Grub: The White Grub is a white, C-shaped larvae, about ¼” to a little larger than 1”, with a brown head.  To identify the species of white grub look at the pattern of the rasters, hairs on the tip of the abdomen.  True white grubs and masked chafer grubs can damage soybean seedlings by feeding on plant roots, especially when the soybean crop follows sod or a cover crop.  Sampling is difficult due to the patchy distribution within a field and the depth at which the grub has burrowed.  You can dig a soil sample 2’ long by 1’ wide by 6” deep and inspect the soil for grubs.  Also, look for grubs during spring tillage.  If you discover two or more live, non-Japanese beetle grubs per cubic foot of soil, you have enough grubs to potentially cause damage and reduce stands.  Control is by seed treatment.

White Grub

Wireworm: Wireworms are slender, hard-bodied brown larvae that grow to about an inch in length.  They feed on seeds and young seedlings below the soil surface, causing the soybean plant to wilt or die.  Wireworms are especially an issue if the soybean field was planted in sod or small grains the previous growing season.  To sample for wireworms either soil sample a 1’ wide by 2’ long by 6” deep volume of soil or set bait stations throughout the field two to three weeks prior to planting. To establish a bait station, bury ½ cup of untreated wheat and ½ cup of untreated corn seeds about six inches in the soil.  The goal is to have the bait station seed germinate; to increase soil temperatures and speed germination cover the trap with black plastic, which will need to be covered with a large piece of clear plastic.  If the bait station has one or more wireworms, a seed treatment is necessary.

Wireworm

Bean Leaf Beetle: The Bean Leaf Beetle (BLB) is usually dark yellow but can also be varying colors of yellow, orange or red.  The most distinguishing marking is the small black triangle located between the wing covers behind the head.  You will also notice most BLB have four black spots on the wings.  The overwintered adult BLB feeds on the cotyledons, leaves and stems of young soybean plants.  Scouting for BLB should occur on a weekly basis once the cotyledons have emerged.  To scout inspect several plants throughout the field and determine the percent defoliation caused by the BLB.  If you find more than 5 BLB per plant, or you have a stand loss of 30%, or defoliation is greater than 30% during any stage of vegetative growth, you have reached economic threshold.  Once you have achieved economic threshold, an insecticide application is warranted.  Seed treatment products that contain Imidacloprid or Clothianidin are a good source for preventative measure and provide above ground protection from early season BLB for up to 30 days post planting.

Bean Leaf Beetle

Soybean Aphid: The Soybean Aphid is a very small, less than a 1/16”, ranging in color from yellow, green, or white, with black cornicles on the back of the abdomen.  The soybean aphid is very slow moving and is typically found on the underside of the newest growth on the soybean plant.  Aphids feed on the leaves, sucking the plant juices.  The lower leaves of infested plants can be sticky and turn gray or black as a result of the sooty mold that grows on aphid exudates, reducing the photosynthetic ability of the plant.  The feeding leads to stunted plants with fewer nodes which results in fewer pods. The most critical damage is during the reproductive life cycle of the soybean plant.  During the reproduction stage, there are many factors in determining if a treatment is warranted.  First, if soybean aphid counts average 250 aphids or more per plant from beginning bloom to full pod or if aphids are increasing beyond the 250 per plant during seed fill.  Secondly, determine if the plants are under moisture or other stresses.  Last, determine if there are natural predators present at sufficient levels.   Considering all of these factors will help determine if an insecticide control is warranted.  For early season soybean aphid control, a soybean seed treatment containing Imidacloprid and Clothianidin can provide soybean aphid control for up to 30 days after planting.

Soybean Aphid and Aphids Feeding on Soybean Leaves

Lynda Anderson
District 1