Adult. The bean leaf beetle, Cerotoma trifurcata, is a member of the leaf beetle family Chrysomelidae and order Coleoptera. This leaf beetle was first described in 1771. The adult is typically dark yellow; however, it ranges between yellow, orange and red with black markings (Fig. 1). Adults are about 5 mm (1/5 inch) in length and have four, large, quadrangular, black markings on the elytra (forewings). Occasionally, these four rectangular marks are reduced to two, or they may be completely absent. The most constant identifying character for this beetle is the presence of a black triangle at the “neck" region. The heads are black, although often males have brown faces. In addition, sex can also be determined by examining a male's forelegs. The base of a male beetle's first tarsal segment has a patch of dense hairs that are thought to help the male cling to the back of a female beetle during mating. Female beetles lack this character.
Egg. The egg is orange and spindle-shaped. It is not much more than a few millimeters long and a couple millimeters wide.
Larva. The larva is a white and cylindrical insect with a dark brown head and brown sclerite (plate) on the top side of the last abdominal segment (Fig. 2). As the larva matures, it passes through successive stages and the larvae develop more plates on more segments. This gives it a somewhat speckled appearance under high magnification.
Pupa. The pupa is white, immobile and slightly resembles the shape of the adult beetle.
Biology and Ecology
The bean leaf beetle is native to North America and may be common anywhere soybeans are grown. It has a large host range that mostly includes legumes such as soybean, green bean, and clover; however, it will occasionally feed on stinging nettle and cucurbits such as pumpkin and cucumber (Fig. 3).
The number of generations bean leaf beetle has every year is somewhat dependent on latitude. This beetle is multivoltine (three generations per year) in the southeastern U.S.; bivoltine (two generations per year) in Iowa and Illinois (Fig. 4); uni- or bivoltine in Wisconsin; and univoltine (one generation per year) in Ontario, Canada. The adults overwinter in leaf debris in woodlots (approximately 80%); however, some do overwinter is soybean field residue (approximately 20 percent). Survival for overwintering beetles is governed by the number of days accumulated below the temperature of 14oF (-10oC). Studies by Lam and Pedigo found that > 50 percent of bean leaf beetles can survive for hundreds of hours at 23oF (-5oC); however, most beetles died by 15 minutes at 14oF (-10oC). In southern latitudes, it is not clear what key factors are responsible for mortality during the winter; however, the weather is thought to be important.
In the spring, adult beetles emerge from overwintering habitat and migrate to available host plants. They are commonly found in alfalfa and other legumes such as tick trefoil and various clovers in late April or early May. As the season progresses, bean leaf beetles are found on more preferred hosts (e.g., soybean or green bean). Note that although these first beetles begin to appear well before soybean emergence, peak abundance can coincide with the emergence of soybean (depending on the date the soybean was planted).
Adult beetles that colonize crops such as soybean will also reproduce in those fields and deposit eggs in the soil near the bases of the plants. In about one week (at 82.4oF or 28oC), the eggs will hatch and the larvae will disperse into the soil to feed on soybean roots and root nodules. The larvae will develop through six instars before pupating in the soil. It takes beetles about three weeks to develop from egg hatch until eclosion (emergence from the pupal stage) of the adult at 82.4oF or 28oC. However, larval survival requires highly organic soils (approximately 66 percent organic matter) and is inversely proportional to clay content. During their time in the soil, bean leaf beetles are most susceptible to heavy rainfall. Once the adult beetles emerge from the soil, they remain soft and probably flightless (these beetles are termed "teneral" or "callow" adults) for a short time until their exoskeleton hardens. In Iowa, the time required for the development of these first-generation from egg laying to adult emergence is 1,212 degree days with a lower developmental threshold of 46oF or 7.8oC. Depending on the length of the growing season, these adults either produce another generation or move to overwintering sites when the host plants begin to senesce.
Dispersion and Dispersal
The bean leaf beetle is common throughout the northcentral United States and has been recorded from Illinois, Indiana, Iowa, Kansas, Kentucky, Michigan, Minnesota, Missouri, Nebraska, Ohio, South Dakota, Wisconsin, and the Canadian provinces of Manitoba and Ontario. On average, these beetles have the physiological capacity for flying short distances < 167 ft and populations at the field scale are highly aggregated. The spatial relationship between the first and second generations are highly correlated to each other with the second generation showing the greatest aggregation pattern in soybean fields.
Dispersion patterns and flight potential of bean leaf beetles are both related and relevant for understanding how these beetles affect soybean. They are related because short flights within each generation increase the likelihood of establishing aggregated populations. Because bean leaf beetles generally fly short distances and establish aggregated populations within fields there is potential for site-specific management. Additionally, if this same pattern and flight potential is true regardless of population size, local beetle populations should be predictable from year to year. Lastly, short flights and highly aggregated populations should result in localized infections of diseases that might be transmitted by bean leaf beetles (e.g., bean pod mottle virus). The distribution of some organisms tend to have random or uniform dispersion patterns as population abundance increases. If this were true for bean leaf beetles it may help explain the outbreak potential for bean pod mottle virus in Iowa.
Injury and Damage
Generally bean leaf beetles exist as either root-feeding larvae, or leaf- and pod-feeding adults. Depending on the plant stage, the amount of feeding activity or injury can have a profound impact on soybean growth and development. The relationship between differing levels of injury and the yield response is termed the damage curve (Fig. 5). A number of factors can change the relationship between injury and yield, including the nature of the insect injury. The type of injury suffered by plants can be classified as indirect, direct, or quantal injury. Indirect injury is the result of an insect feeding on any non-yield-bearing tissue of a plant (e.g., leaves or roots). Conversely, direct injury is the result of an insect feeding on the yield-bearing tissue of a plant (e.g., soybean pods). Quantal injury is where the quantity of injury is independent of yield (e.g., the transmission of disease agents). In terms of the damage curve, direct injury is often conceptualized within the region of linearity (see below). That is, for every unit of yield loss there is a unit of injury. However, indirect and quantal injury are more difficult to confidently describe.
Little is known about the damage curve as it pertains to bean leaf beetle larvae; however, there has been a long-held belief that bean leaf beetle larvae injure soybean. The relationship between adult injury of soybean and yield is better understood. The bean leaf beetle feeding period is about 21 days or less during which time they will consume 1 to 0.384 cm2 of leaf tissue per day. This kind of injury is known as indirect injury because it does not directly affect the yield-bearing tissues of the plant (Figs. 6-7). Direct injury results from bean leaf beetles feeding on soybean pods (Fig. 8). This direct injury occurs an average rate of 0.184 pods per bean leaf beetle feeding day. This means, on average, a bean leaf beetle will eat about two tenths of a pod per day. Bean leaf beetles also cause yield loss through quantal injury by the transmission of bean pod mottle virus or by the secondary invasion of fungi (e.g., Diaporthe spp. and Phomopsis spp.) into the pods (Figs. 9-11).
Bean Pod Mottle Virus
Bean leaf beetle is the primary vector for bean pod mottle virus. This virus is the most prevalent soybean viruses in the North Central states. Possible sources of primary inocula include soybean seed, overwintering bean leaf beetles, and perennial legumes. The bean pod mottle virus pathogen was first described from garden bean and later discovered in soybean. This virus is in the Comovirus group and is partly characterized by having two single-stranded RNAs (ribonucleic acid) encapsulated by coat proteins that form an icosahedral structure (i.e., a polyhedron with 20 faces). Two subgroups of bean pod mottle virus have been identified in nature. Although one subgroup (subgroup II) caused more severe symptoms, the two subgroups can reassort, recombine, or a single virus particle can contain RNA-1 from both subgroups (known as a partial
On soybeans, bean pod mottle virus may cause a severe systemic mottling with a puckering of leaflets and mottling of pods and seed coats; however, symptomatic response varies by soybean variety and soybean stage at inoculation and planting date. Additionally, the foliar symptoms of bean pod mottle virus are masked by cool temperatures.
There can be a reduction in yield in bean pod mottle virus-infected plants resulting from reduced seed size and pod set. This effect on soybean yield is most severe when soybeans are infected as seedlings. Seed-infection either does not occur or occurs at a very low infection-rate, with virus usually present in the seed coat. Additionally, when present, bean pod mottle virus-symptoms can be difficult to distinguish from other viral symptoms (e.g., soybean mosaic virus).
Management decisions. For food-grade soybeans, bean pod mottle virus can be an important management problem. Soybean grain is subjected to stringent standards of seed-coat quality when sold for food. However, for soybean grain where seed-coat quality is not a concern, this virus is of lesser concern because bean pod mottle virus does not affect the color of a soybean seed beneath the seed coat. However, for some soybean varieties, yield may also be affected by the virus and bean leaf beetles. In particular, soybean fields with a recent history of a large number of bean leaf beetles may be at risk for yield and quality reductions from both bean pod mottle virus and bean leaf beetle.
Predicting outbreaks for management decisions. Bean leaf beetle populations essentially have boom and bust cycles. Cold winters are thought to keep beetle abundance in check. Every spring, a prediction for overwintering mortality is published in ICM News. Alternatively, predicting outbreak years based on the actual densities the previous growing season could help growers make better-informed management decisions (e.g., planting date and seed treatments) further ahead of planting.
Chemical control. Currently no thresholds are available for managing bean pod mottle virus; however, insecticidal control has shown potential for managing this virus by reducing vector abundance. One concept is to target two pyrethroid sprays at overwintering beetles in late May or early June and at the presence of first-generation beetles in July. The idea behind this management tactic is to reduce the initial inoculation of the field and the mid-season inoculation and spread of the virus. An ISU study showed that under high vector abundance, seed treatments alone reduced bean pod mottle virus incidence; yield was only improved with the application of a mid-season insecticide application program.
Cultural control. Planting date, although it can protect pods from damage, may or may not have a consistent effect on bean pod mottle virus incidence. However, it seems likely that having the first soybean emergence date in a region may increase a crop’s risk of an economically important level of infection. Managing soybeans for pod damage can be economically important for any soybean grade. Even if a later planting date is employed, monitoring for bean leaf beetles is still important. For this monitoring a degree-day model has been developed. The degree days for the first-generation adults are estimated to be 1,212 degree days with a developmental base threshold at 46°F. Overwintered female beetles usually begin to lay their eggs after colonizing the bean fields. The degree-day estimation for the first-generation adults is calculated by accumulating the temperature at the week of soybean emergence. For adult beetles use the following:
- Determine what week your soybean plants emerged from the soil.
- Sample your soybean fields one week after the predicted peak emergence. If the number of beetles reaches or exceeds the threshold (Table 2), stop sampling. If the sample is below the threshold, sample the following week. If the sample remains below the threshold, sample a third and final week. If the threshold is not reached, an economic infestation of bean leaf beetles should not occur in your pod-stage soybeans.
- If the first-generation population is above the threshold, do not spray now, but scout the fields again in late August to monitor for the first-emerging beetles of the second generation. When the first beetles appear, spray the field with an insecticide (45-day preharvest interval or less). Based upon the population size of the first generation, it is expected that the second generation will exceed the economic threshold. Fields can be sampled for first-generation beetles by using either a drop cloth or a sweep net.
Originally prepared by Jeff Bradshaw and Marlin Rice. Updated by Erin Hodgson in 2017.