Cerotoma trifurcata Frster Coleoptera Chrysomelidae

Natural History

Distribution. Bean leaf beetle is a native species, one of the large complex of legume-feeding species found in the American tropics and subtropics. Among the Cerotoma species, however, only C. trifurcata is abundant in the United States. It occurs widely east of the Great Plains, and is known from all eastern states and the southern portions of Ontario and Quebec.

Host Plants. This species feeds on plants in the family Leguminosae. Vegetable crops attacked include cowpea, lima bean, and snap bean. Other crops attacked are alfalfa, clover, and sweet clover, but bean leaf beetle is best known as a pest of soybean. Wild legumes known to support bean leaf beetle include bush clover, Lespedeza striata; hog peanut, Amphicar-paea bracteata; tick trefoil, Desmodium cuspidatum; and wild pea, Strophostyles helvola. In the spring, before legumes are available, adults may feed for a time on plants other than legumes (Helm et al., 1983), though this is rarely observed. Although the host range of adults is well-known, larval feeding is poorly documented. Larval and adult hosts are assumed to be the same.

Natural Enemies. An important parasitoid of bean leaf beetle and several other chrysomelids is Cela-toria diabroticae (Shimer) (Diptera: Tachinidae). This species attacks mostly irst-generation adult bean leaf beetles, with incidence of parasitism peaking in midsummer. Parasitized adult beetles produce few eggs prior to death (Herzog, 1977). Hyalomyodes triangulifer (Loew) (Diptera: Tachinidae), also a parasitoid of several beetles, occurs occasionally in bean leaf beetle. In Minnesota, Medina sp. (Diptera: Tachinidae) caused high levels of parasitism, but this species has not been reported elsewhere (Loughran and Ragsdale, 1986b). Two species of ectoparasitic mites, Trombidium hypri Vercammen-Grandjean, Van Driesche and Gyrisco, and T. newelli Welbourn and Flessel (Acari: Trombidii-dae) are found feeding beneath the adult elytra, but incidence of infestation is low (Peterson et al., 1992).

Life Cycle and Description. There are three generations in southern states, two in northern areas such as Nebraska, Illinois, and North Carolina, and only one generation in Minnesota, the northern limit of bean leaf beetle's range. A complete generation requires about 30 days. In southern states adults become active and begin oviposition in April or May, with a new generation of adults present in June-July, August, and September. In more northern states the overwintering adults become active in May and June, producing new adults in July-August and August-September. In Minnesota, the phenology is similar to other northern states, except that the second generation does not develop. The adult is the overwintering stage, and surviving adults from all generations may overwinter.

  1. The eggs of bean leaf beetle are deposited in the soil around the base of adult host plants to a depth of up to 3.8 cm, usually in clusters of about twelve. The eggs are oval and taper to a pronounced point at each end. They are reddish orange. Eggs measure about 0.8 mm long and 0.35 mm wide. Mean duration of the egg stage is 9.6 days at 25°C (ranging from 15 days at 20°C to 7 days at 30°C). Females generally produce about 250-350 eggs, with individuals producing as few as 100 to as many as 1,400.
  2. The larval stage has three instars. The larva is elongate, with well-developed thoracic legs and a fleshy anal proleg on the terminal abdominal segment. The larva is white, with a blackish head and anal plate. The thoracic plate may also be darkened. The larva grows from 1 mm at hatching to 7-10 mm at maturity. Mean duration of the larval stage is 17.6 days at 25°C,

Bean leaf beetle larva.

and ranges from 23.9 days at 21°C to 12.8 days at 30°C. Development time of the three instars is reported to be 4.5, 3.4, and 7.9 days, respectively, at 26°C. The head capsule widths of the larvae are 0.17-0.19, 0.27-0.30, and 0.40-0.42 mm for instars 1-3, respectively. Only about two-thirds of the larval period is devoted to feeding; during the remainder of the time the larva is immobile in preparation for the molt to the pupal stage.

  1. Pupation occurs in the soil, in a small cell formed by the larva. The pupa is whitish and resembles the adult in form, though the wings are not fully developed. It measures 3.0-4.5 mm long. Mean duration of the pupal stage is 7.1 days at 25°C, and ranges from 10 days at 20°C to 5 days at 30°C. After successful molting to the adult stage, the beetle remains immobile for a few hours to a few days, depending on weather, and then digs to the soil surface.
  2. The adult is a relatively small beetle, measuring 3.5-5.0 mm long. The beetle is yellowish to dull red and is marked with variable amounts of black. The head is black and the antennae pale. The legs are yellowish and black. In many individuals, the leading edge of the elytra bear a band of black, and there are 2-3 black spots centrally on each elytron. On other individuals, however, most of the black pigment is faded or absent. In nearly all specimens, there is a triangular spot of black on the elytra immediately behind the prothorax. The beetle feeds voraciously for a few days after emerging from the soil. During this time it mates and susequently commences oviposition. The pre-oviposition period is reported to average nine days, but this varies considerably. Availability of

Adult bean leaf beetle.

young seedling host plants is critical for egg development. Ovipositing females live 2-4 weeks, whereas those entering diapause persist for several months. Diapausing adults overwinter in clumps of grass, under leaf litter, and in soil. Forest litter is a favorite diapausing site (Boiteau et al., 1980; Jeffords et al., 1983) with emergence in the spring occurring when mean daily temperature exceeds 26°C and day length exceeds 13 h (Boiteau et al., 1979).

The information on biology was provided by Isely (1930), Eddy and Nettles (1930), and Kogan et al. (1980). Phenology was given by Boiteau et al. (1980), Loughran and Ragsdale (1986a), and Smelser and Pedigo (1991). Laboratory rearing was outlined by Herzog et al. (1974).

Damage

The adults prefer young tissue and are most damaging to young plants. They feed on young leaves, if these are available, but also attack young pods. Typical above-ground injury involves consumption of holes of varying sizes in leaf tissues. The larger veins are usually spared. The consumption rate of foliage by adults is estimated at about 2 sq cm per day. The holes are too large to be confused with damage by flea beetles, and bean beetles eat completely through the smallest veins and the leaf blade, so they are not readily confused with Mexican bean beetle, Epilachna vari-vestris Mulsant. Damage could be mistaken for spotted cucumber beetle, Diabrotica undecimpunctata Mannerheim, however. The larvae of bean leaf beetle feed below-ground around the base of the seedling, causing direct injury and making it easier for soil-borne plant pathogens to gain entry into the young plant. Roots and nodules containing nitrogen-fixing bacteria also are consumed.

The adults transmit plant viruses, including pod mottle, cowpea mosaic, and southern bean mosaic viruses. Beetles are mechanical vectors of viruses, and not very efficient transmitters. They are likely the most import vectors, however, due to the abundance of the beetles (Jansen and Staples, 1970b; Patel and Pitre, 1976; Mueller and Haddox, 1980; Pitre, 1989).

Management

  1. Most studies of bean leaf beetle sampling have involved soybean, and the applicability of these studies to other crops is not known. Adult populations tend to be uniformly distributed to slightly aggregated. Sweep-net sampling is useful for adults in all crops, but visual observation is probably most appropriate early in the season when plants are small. Eggs are sampled from soil cores taken close to host plants, and separated from soil by washing through a series of screens. The larvae and pupae are separated from soil by brine flotation. Kogan et al. (1980) thoroughly discussed sampling.
  2. Planting-time application of granular systemic insecticide to the seed furrow or over the row has been common to protect crops, because it prevents both root injury and foliar damage to young plants (Lentz et al., 1983). Foliar insecticide applications are also effective, but do not protect against injury by larvae. A study in Alabama showed that insecticide incorporated into citrus pulp bait and applied to foliage was particularly effective for bean leaf beetle suppression (Harper, 1981), but this approach has not come into general practice.

Cultural Practices. Early plantings are highly attractive to overwintering beetles; later plantings sometimes escape injury (Witkowski and Echtenkamp, 1996). Zeiss and Pedigo (1996) examined the potential benefits of late planting and observed that beetles denied legumes early in life and forced to feed on grasses failed to reproduce, even if later supplied with soybean. Beetles provided only with alfalfa early in life initially had low fecundity, but regained a high rate of egg production once provided with soybean. Thus, area-wide delay in annual legume-crop planting has considerable benefit in reducing population increase, but it is reduced by availability of alternative legume food sources such as alfalfa.

Soil moisture and texture affect egg and larval survival, and development. The eggs are laid in moist organic soil in preference to dry and sandy soil (Mar-rone and Stinner, 1983a). The eggs absorb water and swell as part of their normal embryonic development. Lack of water causes egg development to cease, but both high and low amounts of water can be deleterious to eggs (Marrone and Stinner, 1983b). First instars are the larval stage that is most susceptible to mortality induced by unfavorable physical conditions in the soil. Pupae also are relatively susceptible to injury. Larvae can move several centimeters to areas of soil with higher organic matter or moisture content (Marrone and Stinner, 1983c). Young larvae are likely to burrow into root nodules under dry soil conditions. Larval survival is highest in organic and wet soils, and lowest in sandy and dry soils. Thus, fields with high organic matter content are more likely to experience damage from bean leaf beetle larvae (Marrone and Stinner, 1984).

Proximity of woodlots is implicated in outbreaks of bean leaf beetle. Availability of such suitable overwintering habitat, especially when combined with early season host availability, leads to higher likelihood of crop damage.

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