Bean Leaf Beetle and Economic Injury on Soybeans

The bean leaf beetle (BLB) is found in almost every soybean field. The pest feeds on a range of legumes such as alfalfa, green beans and clover. Though soybeans are the primary host, BLB also consumes non-leguminous species, including stinging nettle, pumpkin and cucumber.

BLB usually doesn’t occur enough in soybean fields to warrant management, but economic populations can develop within a couple years if environmental conditions and cultural practices are right. Significant BLB populations have been reported in several fields, and it is very likely many more economic infestations have gone undetected due to lack of awareness.



Identification and Life Cycle
The BLB adult is the most visible and potentially damaging stage. Similar in appearance and size to an adult corn rootworm, about 1/5-inch long, it ranges from yellow, orange or red, usually with 2-4 black rectangular spots on its back or are absent. A black triangle near the middle of its back distinguishes it from other beetles. The photo at right shows the various markings of BLB.

Adults lay eggs in the soil near the base of the soybean plant. Larvae emerge from these eggs, live underground and feed on soybean roots. They resemble corn rootworm larvae and rarely cause economic injury. Larvae pupate in the soil, from which adults later emerge and start another round of above-ground feeding. The bean leaf beetle has 2 generations per year throughout most of the Corn Belt with only 1 generation in northern areas and 3 generations in the southern states.

Adults overwinter locally in wooded or grassy areas and emerge from overwintering in early spring to feed on available hosts, such as alfalfa or clover. They immediately move to soybean fields once plants emerge, a preferred host, and feed on cotyledons, stems and leaves. Though early soybean planting often results in a yield advantage, the first fields planted are the most susceptible to earlier BLB feeding and large population development later in the season.

Overwintering adult females lay eggs mid- to late May in soybeans. Larvae hatch from May through mid-June and pupate underground. First generation (F1) adults emerge during late June through July to feed primarily on soybean leaves. F1 feeding appears as round holes eaten through the leaf and rarely causes economic injury.
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In the north, F1 beetles are present late enough in the season to move to pod feeding and increase risk of economic injury. In central and southern areas, a second generation (F2) of adults will emerge during August and early September. As leaves mature, the adults move to the pods and feed on the outer green tissue, exposing the seeds to the weather and disease. F2 adults will overwinter locally in central regions. In the south, a third generation of beetles (F3) will often develop and overwinter.

Dispersal
When a significant BLB population establishes in a field, it typically remains for future generations. If BLB populations were high in the spring, there’s a good chance later generations will be significantly present further in the season. BLB can fly short distances, so neighboring fields may also experience economic damage later in the season.

Impact on Soybeans
BLB may cause economic damage throughout the season, but it’s most likely during spring on young seedlings or in late summer on soybean pods. BLB adults are known to transmit bean pod mottle virus, which may result in economic injury. This virus may cause mottling and puckering in soybean leaves, and mottling of pods and discoloration of seeds. Bean pod mottle causes “green stem” or the delayed dry down of stems and leaves, potentially causing yield loss from reduced seed size and pod set. Virus pressure is often correlated to BLB population size.

Bean Leaf Beetle Management
Areas where BLB populations have been historically moderate to high are at greatest risk for future yield loss. Prioritize monitoring these fields for BLB adults in the season ahead, especially if your field is one of the first to emerge locally. Scout for BLB adults twice during the soybean growing season: once at the seedling stage and again at the first appearance of F1 beetles in early July.

1. Seedling Stage Monitoring: Controlling the initial overwintering BLB generation with foliar or seed insecticides in the spring may greatly reduce yield loss if populations are high. Especially monitor soybean seedlings if seed insecticide wasn’t used and apply a foliar insecticide if there is an average of 3 or more beetles per plant. Managing BLB populations early avoids economic injury on seedlings and helps reduce the size of later beetle generations.
2. F1 Generation Monitoring: Economic injury from BLB is rare for F1 populations, but it’s still a good idea to scout for F1 beetles. High counts signal your field is at great risk for an economic F2 population in late summer. Start sampling for BLB with an insect sweep net at the first appearance of F1 adults in July. Sample 3 different dates, approximately each week after the first occurrence of F1 beetles. If BLB counts exceed those in Table 1 below, stop sampling and treat F2 beetles when they first appear in August. If counts don’t exceed the threshold, no F2 insecticide treatment is needed.

Table 1. Economic thresholds for first-generation bean leaf beetles (average number of beetles per 20 sweeps taken on length of the row, not across the row). Adapted from Iowa State University.



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