Authors: Buyung Hadi, Jeffrey Bradshaw, Tom Hunt, Jan Knodel, Ken Ostlie, Bruce Potter
Bean leaf beetle, Cerotoma trifurcata, is a member of family Chrysomelidae within the order Coleoptera. The beetle is a native fauna of North America and commonly found in the eastern United States of America.
The adult bean leaf beetle is commonly yellow or tan but red colored variants are often observed. A black triangle is always present on the front of the elytra (forewings). Typically, four large black spots and a black margin are present on the elytra, but variants lacking these markings occur.
Eggs are found in the soil, lemon-shaped and orange in color.
The cylindrical bean leaf beetle larva is white in color with dark brown plates on both ends of the body. The brown color results from sclerotization (hardening) of the larva's head and last abdominal segment. Six small legs are present near the head. The larvae are found in the soil and associated with soybean root systems. Mature larvae are 10 mm long. The larvae are sometimes mistaken for their relatives, corn rootworms species.
The bean leaf beetle pupae is white and enclosed within an earthen vessel.
Life Cycle and Seasonal History
Bean leaf beetle adults can survive winter within leaf debris in woodlots, vegetation near soybean fields and crop residue, particularly near the edges of in soybean fields. Overwintering adults are able withstand temperatures below freezing. A laboratory study confirmed that it takes about 12 minutes of continuous exposure to temperatures below 14˚F (-10˚C) or over 400 hours of continuous exposure to temperatures between 23 and 14˚F (-5 and -10˚C) to kill 50% of a given winter-acclimated beetle population. Survival rate of overwintering bean leaf beetle is significantly higher in sheltered habitats (e.g. woodlands) compared to exposed habitats (e.g. soybean fields) and some beetles survive even in very cold winters.
Surviving adult beetles emerge from their overwintering sites once temperatures reach 50 to 55˚F (10-12.7˚C) which normally occur in April to early May depending on latitude and growing season. The overwintering beetles disperse to various legumes such as alfalfa and sweet clover and volunteer soybean. In time, bean leaf beetle adults move to more preferred hosts such as soybean. Mated female beetles lay small clusters of eggs in the top two inches (5.08 cm) of soil near the base of the plant. At 82.4˚F (28˚C), the eggs take a week to hatch. The larvae lived underground, feeding on soybean roots,particularly nodules. Bean leaf beetle larvae develop through six instars before pupating in the soil. The beetles emerging from these pupae form the first generation of adults for the particular year. Bean leaf beetle commonly has two generations per year in Kansas, Nebraska, Iowa and southern Minnesota. A single generation may be produced in Minnesota, particularly in cold growing seasons or northern soybean growing areas.
Bean leaf beetle as Bean Pod Mottle Virus vector
Bean Pod Mottle Virus (BPMV) is potentially transmitted by several leaf feeding beetles, including bean leaf beetle, grape colaspis, banded cucumber beetle, spotted cucumber beetle, striped blister beetle and Mexican bean beetle. By virtue of its abundance, bean leaf beetle is the primary vector of BPMV in the northern plains.
The first confirmed incidence of BPMV in soybeans was reported from Arkansas in 1958. Since then, the incidence of BPMV on soybeans has been reported in other states. In northern plains, BPMV incidence has been reported in Kansas, Nebraska, Iowa, Minnesota and South Dakota.
Bean leaf beetle is a an efficient BPMV vector. Up to 80% of beetles feeding on BPMV-infected soybean for three days were able to transmit the virus to healthy plants. Overwintering beetles have been shown to retain BPMV from previous year although the importance of retained virus in transmission the next year is not clear. BPMV infects a wide host range including dry beans, alfalfa and some perennial legume weeds such as tick-trefoil. Bean leaf beetle adults emerging from overwintering habitats and feeding on infected perennial tick-trefoil may acquire the virus. When soybean seedlings emerge, the beetles move to these plants and the virus is transmitted to soybean. A small percentage of volunteer soybean from infected seed may carry the virus and also serve as a source of BPMV. Soybean that is infected by BPMV in early growth stages shows greater yield reduction compared to the those infected in later stages. The subsequent generations of bean leaf beetle may continue to spread the virus within and between soybean fields.
Plant Injury and Damage
Bean leaf beetle larvae feed on root nodules of soybean plant. Soybean compensates for larvae feeding by producing additional nodules. High larvae infestation (19 larvae per plant) may cause significantly smaller root nodules and reduce leaf and pod nitrogen content. The economic damage, if any, of larval feeding is not understood.
Bean leaf beetle adults feed on the above ground part of soybean such as cotyledon, leaves and pods. The beetle's feeding may commence at the leaf edge or in the 'middle' of the leaves producing round holes on leaf tissues between leaf veins. The uniform sized round holes produced by beetle feeding are quite distinct from more jagged-edged feeding holes produced by grasshoppers and caterpillars. In a season, an average of 0.06 inch2 (0.38 cm2) of foliage is consumed by an adult bean leaf beetle per day.
Adult beetle feeding on soybean leaves and cotyledons in vegetative stages is estimated to reduce up to 2.15 inch2 (13.93 cm2) of foliage. A study simulating foliage reduction on cotyledons and seedlings showed a delay in canopy development and, consequently, yield reduction as a result of cumulative foliage loss between VC and V3 stages. Nevertheless, it has been noted that a soybean plant's ability to tolerate defoliation, either by delayed leaf senescence or compensatory leaf regrowth, can offset the foliage loss due to bean leaf beetle feeding. The defoliation tolerance seems to depend on optimum growth environment, thus in drought years the tolerance may be limited.
A study where soybean leaves were removed manually between beginning bloom and pod set (R1-R4) showed that yield loss occurs at high levels of defoliation (above 50%). Adult bean leaf beetles feeding on later stage soybean plants may ultimately contribute high defoliation when they occur with other leaf feeding insects.
During pod fill, bean leaf beetles may feed on the surface of pods. While larval feeding on root and defoliation due to adult beetle feeding affect soybean yield indirectly, pod feeding inflicts direct injury on yield. The feeding injury on the pod surface facilitates an entry point of bacterial and fungal infection. For example, bean leaf beetle feeding on pods increases the risk of Phomopsis spp. infections of seed. Additionally, the feeding injury to soybean pods exposes the seeds to excess moisture. The seeds in scarred pods may become shrunken, discolored and moldy. Thus pod surface feeding by bean leaf beetles directly reduce yield weight and quality. Particularly under dry conditions, bean leaf beetles feed on the pedicles, clipping the pod in the process. It has been estimated that an adult beetle may cause as much as 0.125 pods lost per day.
Because bean leaf beetle acts as a vector of BPMV, a short discussion of plant response to BPMV is given here. Soybean infected by BPMV showed a variety of symptoms ranging from a mild chlorotic mottling to a severe mosaic. The most obvious symptoms are shown on younger leaves. BPMV infection delays maturity of soybean stem and produces a symptom known as 'green stem'. Soybean infected by BPMV produces 3-52% less yield depending on the time of infection. Early infection causes highest yield reduction. The seeds produced by BPMV-infected soybean may show mottled seed coats, reducing the quality of the seed. When co-infection between BPMV and Soybean Mosaic Virus (not transmitted by bean leaf beetle) occur, the percentage of mottled seeds increase.
The general content of this section describes management approaches of bean leaf beetle in the absence of BPMV. If a particular field history showed incidence of BPMV and BPMV-related loss of yield quantity and quality, a different management approach should be adopted. Bean leaf beetle management in the presence of BPMV history is discussed separately under the sub-section "Management of Bean Leaf Beetle in the Presence of BPMV".
Scouting and Threshold
Scouting for bean leaf beetle can be done by direct observation, sweep net and ground cloth. Some thresholds are based on the field estimation of percent defoliation or pods injured.
Direct observation accurately estimates bean leaf beetle population in early season (seedling to V4 stage). This count estimates the amount of overwintering beetles from previous year. Randomly select five 16-32 ft (5-10 m) row-length from across the entire field. At each sampling site, walk slowly and carefully count all beetles. Do not disturb the plants but make sure that beetles on the underside of the leaves are counted. Cool early-morning hours is probably the best time to conduct the observation as the beetles are not yet active at these hours. At the end of sampling, the average number of beetles per row-foot or, given a fixed inter-row spacing, number of beetles per plant can be calculated.
The ground cloth technique is reliable in moderate to high beetle population levels (defined as the number of beetle per row-meter greater than 10) while sweep net is reliable in all population levels. Both techniques are used when the soybean plants are too large for direct observation. Because of the optimum timing for these techniques, they are essential in estimating the first and second generation of bean leaf beetles in soybean field.
Five to eight sampling sites should be chosen randomly across the field for sweep net scouting. On each site, walk between two rows and swing a sweep net upward alternatively on each row with successive steps. Each stroke of the net count as a sweep and twenty sweeps per sampling site should be conducted. Count bean leaf beetle after each sampling unit. At the end of sampling, the average count of bean leaf beetles per twenty sweeps can be calculated.
The equipment for ground cloth sampling consists of an off-white cloth measuring 36 x 42 inch (0.91 x 1 m) with strips of wood, approximately 1/2 x 1 inch (1.27 x 2.54 cm) wide, stapled to each long side of the cloth. To conduct sampling, randomly chose at least five sampling sites across the field. In each site, roll the cloth beneath the canopy from one row over to the next row without disturbing the foliage. Vigorously shake the plants from both rows over the ground cloth using both hands and forearms. Count the beetles collected on the cloth. This way, two, 3-ft (0.91 m) row sections (6 ft, or 1.82 m, total) are sampled. If some insects can't be identified on the field, collect several specimens for later identification. By the end of sampling, the average count of beetle per row-foot can be calculated.
Economic threshold varies with regional ecology. Please consult each state's web page on bean leaf beetle provided in the end of this article.
Planting soybean as late as economically tolerable within the recommended planting period for a given variety may reduce beetle densities while maintaining yield potential. Isolated early emerging fields are most often colonized heavily by overwintering bean leaf beetles.
Management of Bean Leaf Beetle in the Presence of BPMV
Early BPMV infection is more critical than late infection in determining the resulting yield loss. BPMV infection after stage V6 in soybean development has little effect on yield, thus control of bean leaf beetles in early season is an option to limit yield loss due to BPMV.
Seed treatment using thimethoxam insecticide or foliar insecticide application between emergence and first trifoliate using λ-cyhalothrin has been shown to reduce total BPMV incidence, by protecting soybean from overwintering beetles. Additional application of foliar insecticide using λ-cyhalothrin mid-season (around blooming) was shown to further suppress BPMV incidence. A mid-season foliar insecticide application is aimed at controlling the first generation of bean leaf beetle. BPMV incidence peaks at mid-season after the emergence of first generation bean leaf beetle. However, due to changing plant response to BPMV infection with plant aging, mid-season application of foliar insecticide is only economically beneficial in years with large beetle population and heavy virus pressures.
Late planting date of soybean, compared to other fields in the area may result in low initial population of bean leaf beetle and thus contribute to lower total BPMV incidence.
Other Online Resources
For information regarding labels of chemical control options, please visit Agrian.com