Pod-stage soybeans statewide should be inspected for bean leaf beetles as soon as possible, a South Dakota State University Extension specialist said. Scouting must be continued until the full seed or "green bean" stage of soybean development.
SDSU Extension Entomologist Mike Catangui said bean leaf beetles can be very destructive pests of soybeans because they feed directly on the pods and developing seeds. Whole pods may also fall to the ground as a result of pod clipping by the beetles.
Bean leaf beetles are about one-quarter inch in length, yellowish in color with four dark spots on the back, and have chewing mouth parts.
Most of the bean leaf beetles on the field right now arose from the bean leaf beetles seen earlier on seedling stage soybeans. Bean leaf beetles are more damaging on pod fill soybeans than on seedling soybeans because while soybeans can regenerate leaf tissues injured by bean leaf beetles early in the season, they will not be able to compensate for damaged pods and seeds later in the season.
Field inspections of soybean fields in Brown County on July 25 by SDSU Extension personnel Gary Erickson, Dale Curtis, and Mike Catangui revealed significant numbers of bean leaf beetles. Growers and crop consultants have also reported increasing numbers of bean leaf beetles in most soybean-growing areas in the state.
The economic thresholds of bean leaf beetles on pod through fill stage soybeans can be calculated using a formula that takes into account the chemical-plus-application cost, predicted yield potential, and the expected market value of soybeans.
For example, a soybean field planted in 30" rows should have an average of six bean leaf beetles per foot of row, or four beetles per sweep of an insect net, for spraying to be cost-effective. This scenario assumes a yield of 40 bu/acre, expected soybean market value of $5/bu, and a chemical-plus-application cost of $8/acre.
In drought-stressed areas where the yield potential may only be 25 bu/acre, and assuming all other variables mentioned above to be the same, then the economic threshold would be 10 bean leaf beetles per foot of row, or six beetles per sweep of an insect net.
Details on how to calculate economic thresholds, current list of labeled insecticides, and other biological information can be found in SDSU Fact Sheet 905, Grasshopper and Bean Leaf Beetle Economic Thresholds in Soybeans, available in county Extension offices.
Information is also available online at the Internet address: http://www.abs.sdstate.edu/plantsci/ext/ent/entpubs/blb_mgmt_sd.htm
Bean leaf beetles will continue to feed on soybeans until the pods turn yellow. Scouting must be continued accordingly. Beetles on the field at harvest time will overwinter in soil litter and shelterbelts and will feed again on next year's soybean seedlings.
Growers considering treatment later in the season need to be aware of pre-harvest interval (PHI) of whatever product they intend to use. Read the label carefully because some products have a PHI of up to 60 days.
Insecticides labeled for use against bean leaf beetles on soybeans in SD include: Asana XL (5.8-9.6 fluid oz/acre, 21-day PHI); Dimethoate 4EC (1 pint/acre, 21-day PHI); Lorsban 4E (1-2 pints/acre, 28-day PHI); Mustang (3.0-4.3 fluid oz/acre, 21-day PHI); Pounce 3.2EC (2-4 fluid oz/acre, 60-day PHI); Sevin XLR PLUS (1-2 pints/acre, 21-day PHI); and Warrior (3.2-3.84 fluid oz/ acre, 45-day PHI). Always read and follow label directions. Most insecticides will only give a maximum of three weeks residual control. Heavily infested areas may need to be re-treated. Always use the recommended minimum gallonage of finished spray.
Entomologists assume that there are two broods or generations of bean leaf beetles in South Dakota. However, since they are long-lived and hardy, bean leaf beetles can be found on the field throughout the soybean-growing season. They are most numerous starting in August when soybeans are in their full pod through full seed stages of development.
Bean leaf beetles are native to the United States. Their host plants include soybeans, edible beans, clover, corn, peanut, and several leguminous weeds.
In the Midwest, bean leaf beetles were first reported clipping and feeding on pods in the 1970s. It was soon learned that this type of injury, along with defoliation early in the season, could make bean leaf beetles important pests of soybeans.