Insect review: The bean leaf beetle
One of the first soybean insects to plague the newly planted bean crop each spring is the bean leaf beetle. Also known by the scientific name Ceratoma trifurcata, bean leaf beetles present a potential problem for "early bird" bean fields but they only rarely create latter season problems.
Bean leaf beetles come in an assortment of colors (red, orange, yellow, and green) and an assortment of spotting. Some even think they resemble ladybugs. However, all can be identified by a small, triangle-shaped marking located just behind the head (actually just behind the thorax).
Two generations of the bean leaf beetle occur in Illinois each season. The first bean leaf beetles of the season emerge from overwintering under woodland or field edge debris and migrate to alfalfa, clover, and various weed species around the time that ambient temperatures hit 50 degrees. Once soybean fields are planted and begin to emerge, beetles migrate. Because the species prefers to feed upon soybean plants, beetles in an area can flock to early emerging bean fields (what we have here called "early bird" bean fields). When this happens, the rather small seedlings can be decimated. Occasionally this will result in enough stand reduction to require management. Populations often become diluted as other bean fields emerge and this typically means that latter season bean leaf beetle problems tend to be "rare" rather than "perennial."
Shortly after entering soybean fields, females – females that actually mated while in alfalfa, clover, and weeds - proceed to deposit several dozen egg clusters in the soil near soybean plants. Eggs are slightly orange in color and are lemon-shaped. Each clutch contains about ten to thirty eggs and the eggs are deposited over the course of a month in the top couple inches of soil. Eggs hatch over the span of about one to three weeks. The resulting larvae somewhat resemble rootworm larvae. They feed on roots and nodules, but their feeding typically does not equate to measurable yield loss. Larvae pupate in an earthen cell and later emerge as the first "in-season" batch of adult beetles. This typically happens in July. Adult females of this "first generation" mate and deposit eggs once again. Larvae feed soon after and "second generation adults" emerge in August. These "second generation adults" are actually the third batch of adults observed during the growing season. Second generation adults eventually migrate to alfalfa and clover when beans mature. They later nestle beneath residue in wooded areas and field edges to seed the next season's bean leaf beetle population.
The latter first and second generation of adult beetles can occasionally present a problem for well-developed soybean fields. Adults can chew on leaf material and thus can reduce photosynthetic area or they can chew on pods allowing disease to enter the pod chamber. Before bloom, U of I recommends treatment when defoliation reaches 30 percent and five or more beetles are detected per foot of row. From bloom to pod fill, U of I recommends treatment when defoliation reaches 20 percent and there are 16 or more beetles per foot of row. A threshold of 5 to 10 percent pod damage with 10 or more beetles per foot of row (while plants are still green) is recommended for commercial soybean production. However, those involved in seed production will often manage beetles at much lower densities to avoid seed discoloration and other issues that may precipitate pod injury
Early season injury, as noted earlier, tends to present the most severe threat from the bean leaf beetle. While also exceptionally rare, a seedling treatment is recommended when 16 beetles exist per foot of row. After the first two leave are visible, about 39 beetles are needed per foot of row.