Bean leaf beetle

Bean leaf beetle (Cerotoma trifurcata (Chrysomelidae: Coleoptera)) is one of the most economically important insect pests of soybean in the United States. Damage is primarily caused by foliage- and pod-feeding adults, which can significantly reduce seed quality and yield. Larvae feed on soybean nodules and roots, but economic impact is not well documented. In addition to physical damage from feeding, the bean leaf beetle is known to transmit bean pod mottle virus disease.

The adult is a 1/8 to 1/4 inch long beetle. The body is slightly convex and longer than wide. BLB color varies from light brown to dark red. Black spots and/or stripes may be present on the wing covers. All bean leaf beetles will have a backwards pointing black triangle behind the head. These insects look very much like spotted cucumber beetles but are smaller. The small rootworm-like larval stage lives below ground and feeds on soybean roots. They are rarely seen and cause no apparent damage.

Egg: The egg is lemon-shaped, orange, and about 0.85 mm long. Larvae: Larvae are whitish with dark brown plates at both ends and segmented with three pairs of small legs near the head. They grow up to 10 mm long and are often mistaken for corn rootworm larvae. Pupa: The pupa is soft-bodied, white, and about 5 mm long.

Both adults and larvae feed on cured meats, dried fish, cheeses, raw skins, hides, furs, feathers, hair, fish meal, dry dog and cat food, stored tobacco, stuffed animals, dead insects in wall voids, museum specimens, abandoned bird nests, dead rodents in wall partitions or chimneys, dead beehives, and even rat or mouse poison baits. Infestations are often hard to locate because beetles and larvae can migrate far from the original food source.

Habitat: Snap beans, soybeans, and other legumes. Eggs, larvae, and pupae of the bean leaf beetle occur in the soil and are typically not encountered.

Life Cycle : Adults over winter in soybean stubble and leaf litter in wooded areas. Beetles begin emerging from over wintering sites in early April at which time they mate and disperse to weedy and brushy areas, especially along roads and ditches. They may move into spring legumes such as alfalfa and sweet clover.

Beetles move into soybeans as soon as plants have emerged. These colonizers feed on developing leaves and cotyledons and begin laying eggs. Eggs are laid in the upper two inches of soil, usually within three inches of the plant stem. A female normally lives about 40 days and lays 125 to 250 eggs. Eggs hatch in 4-14 days, depending on soil temperature.

Larvae live in the soil where they feed on plant roots and have three instars. Larvae develop to pupae in about 23 days. Warmer soil temperatures can shorten larval development time. Pupation is completed in about a week and adults emerge from the soil. Total developmental time from egg to adult normally ranges from 25 to 40 days. There are two generations per year in Nebraska.

Damage :Both bean leaf beetle larvae and adults possess chewing mouthparts. Larvae may feed on roots and root hairs but show a preference for root nodules. Even though nodule damage can diminish the plant’s ability to fix atmospheric nitrogen, it is the adult feeding damage to foliage and particularly pods that is economically important.

Defoliation by adult bean leaf beetle is identifiable by the small round holes between the major leaflet veins. This damage differs from the larger, irregular holes or jagged leaflet margins caused by caterpillars and grasshoppers. Even though soybean plants may sustain more than 50 percent foliage damage, plants can generally compensate unless damage occurs during the reproductive growth stages.

Based on current research, pod damage by adult bean leaf beetles is the most important type of injury. This damage can cause complete pod loss when adults feed at the base of the pod. This type of injury is referred to as “pod clipping.” Adult damage to the outer pod wall also leads to the formation of pod lesions. Moisture can then enter through these pod lesions, and this increased moisture level permits the entry of secondary pathogens.

Seeds damaged by these pathogens become shrunken, discolored and moldy. Several primary pathogens (e.g., bean pod mottle, cowpea mosaic and southern bean mosaic viruses) also can be transmitted by adult beetles. The impact of bean leaf beetles on yield has been quantified in several different studies. Overall, a reduction of 0.6 pound per acre can occur when beetles number one or more per row foot.

Management: Decisions to apply an insecticide treatment against bean leaf beetles should be based on economic thresholds as determined by information from regular scouting and monitoring programs. Economic thresholds for bean leaf beetles are based upon damage to aboveground parts of the plant. Severe cotyledon feeding, threatened destruction of the growing point, or populations of seven beetles per rowfoot on soybeans with four or fewer nodes and 25 percent defoliation may justify treatment.

Late season defoliation does not reduce yields unless beetle populations meet or exceed 50 per row-foot. Treatments may be economically justified if pod feeding results in the loss of three or more seeds per plant, pods are not completely dry, or beetles are not yet leaving the fields.

This insect is known to transmit bean pod mottle virus disease. Research is ongoing to determine whether reducing the treatment threshold reduce the likelihood of infection.

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