Biology
Adult bean leaf beetles are approximately ¼ inch long
and are green, yellow, tan, or red, often with a distinct black
band around the outer edge of the wing covers. A black triangle
is always found behind the head on the wing covers. Usually
there are two black spots on each wing cover, but they are not
always present. Males are distinguished from females by the
black coloration between their eyes. |
Figure
1. Variation Found in Adult Bean Leaf Beetles
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Larvae
are white segmented grubs, with brown heads and a brown hardened
area at the posterior end of the body. They may reach 1/3 inch long.
Pupae are white and approximately 3/16 inch long. The lemon-shaped
eggs are red-orange in color and measure 3/100 inch.
Adult
bean leaf beetles overwinter beneath plant debris at the edge of
woodlots or fence rows near bean fields. They become active in the
spring when temperatures reach 50 F, and migrate to alfalfa, clover,
and occasionally to various weeds. Bean leaf beetles are strong
fliers and move to soybean fields when the plants emerge. Females
lay an average of 40 clusters of 10-30 eggs per cluster. The eggs
are laid over a period of 3 to 4 weeks in the top 2 inches of soil
under bean plants. Temperature impacts bean leaf beetle development.
Eggs hatch in 5 to 7 days at 75 F, but may take 3 weeks to hatch
at lower temperatures. The time from egg hatch to pupation is 15
to 16 days at 75 F, and 50 days at 65 F. Larvae remain in the soil,
first feeding on root hairs, then nodules, as they progress through
three instars. Larval mortality can be attributed to inadequate
soil moisture, poor nodule development, and natural enemies. Pupation
takes place in earthen cells below the soil surface.
Damage
Bean leaf beetle damage of economic importance is usually caused
by the adult, although the larvae feed on bean roots. The bean leaf
beetle has two generations in Illinois and the adults of both generations
feed on soybean leaves. Early in the summer the overwintering adults
may be found on the newly emerging soybean plants, chewing holes
in the leaves. In the summer their progeny will again be found in
the plants, but usually by this time there is sufficient foliage
that little damage is caused. Occasionally they feed on blossoms
or pods late in the season.
Scouting
Procedures
Several methods can be used to assess damage by the bean leaf beetle.
Early in the season it is easiest to examine the plants by hand
and count the number of beetles. Examine five feet of row in each
of five locations and divide the number of beetles found by 25 to
arrive at the number per foot of row. In addition to counting the
number of beetles, estimate the percent of defoliation. After plants
become larger it will be easier and more accurate to use a beat
cloth. Use the beat cloth in five locations and divide by the total
number of feet examined to arrive at a per foot of row average.
Remember with the beat cloth you are examining rows on both sides
of the cloth.
Threshold
Guide
Seedlings: Treat if 20 percent of the plants are cut and
the stand has gaps of 1 foot or more; or if at least 1 seedling
per foot of row is destroyed.
Before
Bloom: Treat when defoliation reaches 30 percent and there are
5 or more beetles per foot of row.
Bloom
to Pod Fill: Treat when defoliation reaches 20 percent and there
are 16 or more beetles per foot row.
Seed
Maturation: Treat when 5 to 10 percent of the pods are damaged,
the leaves are green, and there are 10 or more beetles per foot
of row.
Authors:
Susan T. Ratcliffe (sratclif@uiuc.edu)
Michael E. Gray (m-gray4@uiuc.edu)
Kevin L. Steffey (ksteffey@uiuc.edu)
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