Biology of the
Bean Leaf Beetle
in South Dakota

Michael A. Catangui, Ph.D. - Associate Professor
 South Dakota State University

Bean leaf beetles overwinter as adults on the field in shelterbelts, under soil clods, and plant debris.  The fact the they can survive SD winters indicates that they are a very hardy insect.  Bean leaf  beetles become active again in April feeding first on alfalfa or other early-growing plants in the field then moving into soybeans as soon as seedlings become available.

Injuries to soybean seedlings include numerous holes on the leaves and chewing marks on the cotyledons.  These injuries do not usually result to yield reductions because the soybean plant can tolerate substantial leaf feeding during its vegetative stage of development.  However, very high beetle populations can cause stand losses especially along field margins.

The overwintered beetles lay their eggs on the soil close to plant roots starting in late-May.  The beetles eventually die off in late-June.  Larvae of the bean leaf beetle feed on the roots of soybeans and perhaps other plants.  Root feeding by the larvae is currently not considered economically important.  Development from eggs to larvae to pupae then adults may take about a month depending on soil temperatures.

In SD, first generation bean leaf beetles start appearing on the field in mid-July as soybeans start blooming in the southern counties.  These beetles presumably lay eggs again to give rise to the second brood or generation beetles in late-August.  Since the beetles are long-lived, beetles are found on the field from soybean bloom through harvest.  Distinguishing first and second brood bean leaf beetles may be quite hard, and perhaps not necessary in terms of managing bean leaf beetles.  Second brood bean leaf beetles are known to be the overwintering brood.

Bean leaf beetles are most damaging from R5 (beginning seed) through R6 ("green bean" or full seed) stages of soybean development.  Injuries produced by the beetles at this time may be pod clipping, feeding on pod surface tissues (resulting to moldy and shrunken seeds), as well as leaf defoliation.  Stems of the plant may also be scraped as leaves and pods mature late in the season.

Bean leaf beetles are also capable of transmitting certain viral diseases of legumes such as bean pod mottle, cowpea mosaic, and southern bean mosaic.  In the Midwest, bean leaf beetles are managed due to their leaf and pod feeding and not due to the diseases that they may transmit.

Reference:  

Higley, G. L. and D. J. Boethel.  1994.  Handbook of soybean  insect pests.  Entomological
              Society of America.  Annapolis, MD.

            (Dr. Leon Higley is Professor of Entomology at the University of Nebraska)