Bean Leaf Beetle Causing Damage in Some Early Planted Soybean Fields By Wayne Bailey

Although bean leaf beetle numbers have been low in recent years, some early planted soybean fields have required insecticide applications during the past two weeks. Adult beetles emerging from overwintering sites often accumulate in soybean fields first planted in an area. Early season damage from these beetles consists of foliar feeding on leaf tissue, which is characterized by oval to round holes located between main leaf veins. Heavy feeding may totally defoliate the soybean seedling, or in some cases, cause plant mortality. Beetle numbers are expected to increase during the next two weeks as additional overwintering beetles move from wooded overwintering sites to soybean fields.

The bean leaf beetle has two generations per year with both adults and larvae possessing chewing mouthparts and causing feeding damage on soybean plants. Adult beetles will initially leaf-feed on soybean seedling in early spring, with female beetles laying between 130-200 eggs into the soil near soybean stems. Adult beetles will die within a few weeks of emerging from overwintering sites, but eggs will hatch within 5-14 days of being laid and larvae will feed on soybean seedling roots, root hairs and especially on root nodules. Depending on soil temperatures, three larval instars or worm stages will feed from three to six weeks before pupating in the soil and emerging about one week later as first generation adult beetles, usually in early to mid-July. These beetles will feed on leaf tissues and again reproduce a second generation of larvae, which will emerge as adult beetles in late July into September. Second generation beetles will feed on soybean leaf tissues and pods prior to moving into wooded areas to overwinter.

Bean leaf beetles are about ¼ -inch in length, and may vary in color from tan to pale green to crimson. All bean leaf beetle adults have a small black triangle located at the base of their forewings, and often exhibit two black spots and a black line on each wing cover. The spots and lines may be less pronounced or even missing on some beetles. Larvae are white in color with three pairs of legs and a brown head and anal plate. Larvae grow through three instars and often reach 3/8-inch in length prior to pupating in soil cells and eventually emerging as adult beetles. Although dependent on soil temperatures, total time for development from egg through larval stages to adult emergence usually requires 25 to 40 days.

Early season damage thresholds are based on number of beetles present and number of plants destroyed per foot of row. In most instances, early season feeding by overwintering bean leaf beetle adults does not justify the use of control measures. However, if beetle numbers reach or exceed five beetles per foot of row and one or more soybean seedlings are destroyed per foot of row, then treatment may be justified. Another threshold states that treatment is justified if five or more beetles are present per row foot, defoliation reaches or exceeds 30 percent and plant growth is slowed due to environmental stresses.

Rapidly growing soybean seedlings can often compensate for defoliation levels up to 50 percent, whereas seedling under stresses such as drought, cold or wet conditions may require an insecticide rescue treatment to prevent yield loss. In general, defoliation and pod feeding by bean leaf beetle is most damaging during reproductive stages of soybean plant growth.

If seedling defoliation is severe and loss of stand reaches unacceptable levels, then use one of the following insecticides to manage bean leaf beetle adults on seedling soybean.

Insecticides Recommended For Control Of Bean Leaf Beetle on Soybean

Chemical Name	Product Name	Rate of Product/Acre	Preharvest Interval
permethrin	*Ambush	3.2 to 6.4 fl oz	60-days
esfenvalerate	*Asana XL	5.8 to 9.6 fl oz	21-days
cyfluthrin	*Baythroid 2	0.8 to 1.6 fl oz	45-days
deltamethrin	*Decis 1.5 EC	1.5 to 1.9 fl oz	21-days
dimethoate	Dimethoate	see product label	21-days
methomyl	*Lannate	1/4 to 1/2 lb.	14-days
methomyl	*Lannate LV	3/4 to 11/2 lb.	14-days
thiodicarb	*Larvin 3EC	18 to 30 fl oz	28-days
chlorpyrifos	*Lorsban 4E	1 to 2 pt.	28-days
zeta-cypermethrin	*Mustang Max	2.8 to 4.0 fl oz	21-days
chlorpyrifos	*Nufos 4E	1 to 2 pt.	28-days
methyl parathion	*Penncap-M	2 to 3 pt	20-days
permethrin	*Pounce 3.2EC	2 to 4 fl oz	60-days
gamma-cyhalothrin	*Proaxis	1.92 to 3.20 fl oz	45-days
carbaryl	Sevin XLR	1 to 2 pt.	14-days
lambda-cyhalothrin	Warrior	1.92 to 3.20 fl oz	45-days

Wayne Bailey, Entomology
573-882-2838 or 573-864-9905