Agronomy Talk

AGRONOMY TALK: EARLY-SEASON SOYBEAN PESTS

Published on Tuesday, May 12, 2020

Young soybean plants are battling to bust through the crust in many areas. Before they even emerge, the plants are under attack from insect pests. Below, you’ll find a refresher on three of the creatures that “bug” our beans. 

BEAN LEAF BEETLE
Cerotoma trifurcate

Bean leaf beetles are often seen throughout the growing season because they are capable of over-wintering in the Midwest and they complete two life cycles per year. The survival rate of overwintering bean leaf beetles is linked to winter conditions. A long, cold winter leads to fewer beetles, while a mild winter and an early spring green-up results in a higher number. Bean leaf beetles are usually 0.25 in. long and can be several colors, ranging from yellow and green to tan and occasionally red. The distinguishing feature of a bean leaf beetle is the characteristic black triangle on the wing covers behind the head.

Overwintering adults may feed on newly emerging cotyledons, stems, and leaf tissue. Bean leaf beetles do the most damage by chewing small, round holes between the veins of soybean leaves. The second generation also feeds on blossoms and pods later in the season. These pests can transmit the pathogen that causes the bean pod mottle virus.

Bean leaf beetle presence does not always warrant control measures. The control threshold, based on number of beetles and extent of defoliation, depends on the growth stage of the crop. The cotyledon stage is highly vulnerable and has a control threshold of two beetles per plant. At the V2 to V5 growth stages, a foliar insecticide is warranted when defoliation reaches 30%, and during the reproductive stages (R1 to R6), when defoliation reaches 20%.

SEEDCORN MAGGOT
Delia platura

Seedcorn maggots strike in a very narrow time frame as corn and soybean seed are only susceptible between planting and emergence. Seedcorn maggots are white, about 0.25 in. long, and cause damage by burrowing into seeds. The pupae (the stage before adulthood in an insect’s life cycle) overwinter in Midwest soils. The emergence of seedcorn maggots can be estimated using Growing Degree Units (GDUs); adults emerge at around 200 GDUs after soil temperatures reach 39°F. Seedcorn maggots have multiple generations per year. Second-generation adults emerge when 600 GDUs have accumulated and another at approximately 1,000 GDUs. They attack germinating seed, often destroying the embryo, which results in poor emergence from the dead or damaged seed.

Fields with recently incorporated green vegetation or manure are more likely to be infested with seedcorn maggots. Planting into cold, wet soils leaves seeds exposed to seedcorn maggots because the interval between planting and germination is extended. Seed treatments (like Beck’s Escalate® yield enhancement system) that contain insecticides with activity on seedcorn maggot can provide very effective control. There are no known rescue treatment options. 

THISTLE CATERPILLAR
Vanessa cardui

The thistle caterpillar is the larval stage of the painted lady butterfly. The caterpillar does not overwinter in the Midwest, but adult butterflies migrate north from the southern U.S. and Mexico to lay eggs in the spring. Thistle caterpillars are 1 to 1.75 in. long and are highly variable in body color. They can be identified easily by the multipronged spines along their backs. There are two generations of thistle caterpillar per year, and they damage soybeans by consuming leaf tissue in the upper canopy. They feed for about two to six weeks after they hatch. In most years, crop damage is limited to outer rows or those rows near thistles. However, if the first generation lays eggs in a soybean field, the second generation can spread across an entire field.

Thresholds for treatment with a foliar insecticide are not required until defoliation reaches 30% for soybean plants in the V2 to V5 growth stage. The threshold is only 20% during the more yield sensitive reproductive growth stages of soybean (R1 to R6). Spot treatment may be the best option due to the irregular distribution of the pest. 

LEAF DEFOLIATION ESTIMATION TOOL

Beck’s precision farming tool FARMserver® can help you to estimate the extent of insect feeding on soybean leaves. It can also help you estimate the yield damage based on the defoliation and growth stage of the crop. Use the Leaf Health Analysis section of the soybean scout note. If you encounter one of these pests causing damage to your fields, we recommend you contact your local Beck’s representative for further information and specific recommendations. 

Keep a careful eye out on areas where pests have been problematic in the past, and where factors like residue, weeds, or fencerows increase the likelihood of insect pests getting a foothold in your soybean fields. Continue scouting throughout the spring to ensure a successful soybean crop.

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Author: Greg Shepherd

Categories: Agronomy, Agronomy Talk

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