Early-Season Bean Leaf Beetle in Soybean

Soybean bean leaf beetles (BLB) may be a threat to soybeans due to the potential for large, overwintered adult populations favored by the mild winter conditions. Early planted fields are at risk from BLB.

Bean Leaf Beetle with and without spots

​Adult bean leaf beetles overwinter on the edges of wooded areas, in fence-rows, and ditch banks near soybean fields. They become active and migrate to emerging soybean fields for feeding and propagating when springtime temperatures reach 50° F. The earliest-planted soybean fields have the greatest risk of economic feeding injury1.

 

Description

The adult BLB is about 1/4 inch long with variable coloration. The most common BLB color is light yellow or tan; however, some are orange or red. All BLBs, regardless of coloration, have a black triangle just behind the head (Figure 1). Additionally, they usually have four black spots with stripes along the body edges; however, these markings may be absent (Figure 1)1.

 

 

Mild Winter Survival

Warm winters favor BLB adult survival in leaf litter in wooded areas or soybean debris. Entomologists from Iowa State have developed a predictive model for BLB survival. BLB adults are susceptible to low temperatures in winter. Typically, winter mortality ranges from 60-99% in Iowa. However, this year mortality is estimated at only 30-53% for various regions within Iowa2. This is the lowest mortality since the predictive model was first used in 2000. Early-planted fields should be monitored closely this year given the potential overwintering survival of BLB adult beetles is significant. Food-grade soybean and seed fields may be at significant risk for reductions in yield and seed quality.

 

Injury to Soybeans

The most critical time for early-season soybean damage, is from emergence through the first trifoliate1. If growing points are destroyed or cotyledons are damaged before unifoliate leaves emerge, there is potential for soybean yield reduction3. Although feeding injury can be important, the transmission of bean pod mottle virus (BPMV) can have a significant impact on yield. BLB is a common vector of bean pod mottle virus.

 

 
Economic Thresholds in Beetles per Plant

​BLB feed on infected plants and transmit the virus to the next plant on which it feeds. Virus transmission can occur at any growth stage, however early infection poses the greatest risk of reduced seed size and pod set resulting in potential yield loss. BPMV causes the discoloration of soybeans at harvest as well as the crinkling and mottling of leaves during the growing season. BPMV is also a cause of the “green stem” phenomenon, which can hinder harvesting operations. BPMV can be difficult to distinguish from soybean mosaic virus. Soybean varieties differ in susceptibility to BPMV.

 
Soybean Treatment

Scouting and Thresholds

Scouting for BLB when soybeans are in the VC to V2 stage of growth can be challenging. When scouting for BLB be careful not to disturb the plants since these beetles will hide quickly in the soil, beneath clods or debris. Sample beetles in ten feet of row in at least five locations in the field. Determine the number of beetles per plant from the sampling sites and consult Table 1 to determine the treatment threshold. The current market value of soybeans may exceed $12 per bushel. Note that prices above $12 lower the threshold below 1 to 3 beetles/plant (Table 1)3. Thresholds early in soybean development are very dynamic and may need to be adjusted for local conditions. Scouting techniques, treatment thresholds and treatment recommendations may vary by state. Consult local University Extension personnel for the latest state recommendations for BLB. Thresholds will also be different later in the season for treating BLB at the R5 to 6 stages of growth.

Management Options

Although the research base for bean leaf beetle injury to newly emerging soybean is limited, research does indicate that early loss of both cotyledons can result in about a 5% yield loss3. If the beetles appear to be injuring or clipping the c​otyledons and growing points, an insecticide treatment may be warranted.​
 
  • Later planted soybeans generally have less early-season damage because more beans are emerging for the BLBs to feed upon.
  • Various insecticide seed treatments, such as those offered as Acceleron® Seed Treatment Products, can help protect seedlings from early bean leaf beetle feeding (Figure 2).
  • Several foliar insecticides (pyrethroid, carbamate, organophosphate) are labeled for bean leaf beetle management.
  • To prevent BPMV transmission, an insecticide application should be made early at VC-V2 stage soybeans. ​