Chaetocnema denticulata Illiger Coleoptera Chrysomelidae

Natural History

Distribution. Corn flea beetle and toothed flea beetle are found primarily in the northeastern and midwestern United States, but their range extends as far west as the Rocky Mountains. The northern limit of these native species seems to be Massachusetts, New York, and southern Ontario. Survival is affected by winter weather, and damage resulting from flea beetle feeding is noticed most commonly following mild winters.

Host Plants. The adults and larvae of corn flea beetle and toothed flea beetle feed and develop on a wide variety of cultivated and wild grasses and sedges. In addition to corn, other crops attacked include barley, chufa, oats, orchard grass, wheat, and timothy grass. Examples of weeds suitable for these flea beetles are crabgrass, Digitaria spp.; barnyardgrass, Echinochloa crusgalli; witchgrass, Panicum capillare; straw-colored sedge, Cyperus strigosus; and yellow bristlegrass, Setaria glauca.

Natural Enemies. The natural enemies of corn flea beetle and toothed flea beetle are poorly known. Several investigators of corn flea beetle biology noted an absence of parasitoids. A wasp was found attacking toothed flea beetle eggs in Virginia, and identified as Patasson pullicrura (Girault) (Hymenoptera: Mymari-dae) (Poos, 1955).

Life Cycle and Description. Corn flea beetle and toothed flea beetle are very similar in appearance and biology. Toothed flea beetle is much less common in corn, and so is less well studied. Except where noted below, their biology is believed to be virtually the same.

Two generations of these flea beetles are reported annually on corn in Connecticut and Virginia, with adults from the spring generation occurring in late June and early July, and those from the second brood appearing in mid-August. Poos (1955) suggested that in Virginia, corn flea beetle might complete a generation on wild grasses and sedges before moving to corn, leaving some doubt about the actual number of generations. The time required for completion of a generation (egg to adult) is estimated to average 30 days.

Overwintering of corn flea beetle and toothed flea beetle occurs in the adult stage in soil. Beetles are most commonly recovered from the upper 2-3 cm of grass sod planted in the vicinity of corn. Bluegrass, Poa sp., and other grasses have been found to be suitable habitats. The beetles become active when the soil surface warms to about 15-20°C.

  1. Poos (1955) speculated that eggs were deposited at the base of corn or wild grasses in the manner of other flea beetles. However, he was unable to locate eggs in the field, and obtained them by confining beetles in the laboratory. The eggs of corn flea beetle are oval and white in color. They measure about 0.2 mm wide and 0.4 mm long. Development time of the egg stage averages 4-6 days, with a range of 2-10 days, depending principally on temperature. The eggs of toothed flea beetle are slightly larger, measuring about 0.6 mm long, and are pale yellow.
  2. Poos found that larvae were also difficult to locate, but a few could be found in the soil surrounding corn and other grasses. Larval development requires about 16 days (range 10-23 days), not including an additional two days (range 1-5 days) as a pre-pupa. Larvae attain a maximum length of about 4.5 mm before pupation. The pupal stage requires about five days (range 3-7 days).
  3. The adult corn flea beetle is shining black with a slight greenish, bluish or bronze luster. They measure about 1.5-1.8 mm long. Elytra are marked with rows of closely spaced punctures.

The adult toothed flea beetle is bronze in color, and measures 2.3-2.5 mm long. The elytra of this beetle also are marked with rows of punctures, but the punctures are not spaced closely. The head also bears punctures, a feature lacking from corn flea beetle. Although these two flea beetle species are not especially difficult to distinguish, there are several other flea beetle species that may occur in corn, usually in small numbers.

The biology of corn flea beetle and toothed flea beetle were given by Poos and Elliott (1936), Elliott and Poos (1940), and Poos (1955).

Chaetocnema Denticulata
Adult corn flea beetle.

Damage

The adults skeletonize leaves of seedling corn in the spring, sometimes completely defoliating fields. They feed on the lower surface of foliage in narrow linear strips, usually restricting their feeding to the first three leaves.

The principal form of injury, however, is through transmission of Stewart's bacterial wilt, a disease caused by the bacterium Erwinia stewartii. Stewart's wilt is transmitted when beetles feed on corn, and defecate in or near the feeding site. The bacterium, which is harbored in the digestive tract of the beetles, thus gains entry to the plant through the wound. Infected beetles remain infected and are capable of transmitting the disease for the duration of their life. The disease is harbored by the beetle during the winter months, but the bacteria also occur in wild grasses, including many species that do not express symptoms of infection (Poos, 1939). The studies conducted in Connecticut demonstrated that the incidence of beetles containing bacteria generally increased from about 40% to 70% during the season, although there was some evidence of a mid-season reduction in disease levels (Heichel et al., 1977). The incidence of disease in beetles is quite variable. Although both species may exhibit high levels of infection, corn flea beetle is much more abundant, and therefore is much more important as a vector. Elliott and Poos (1940) provided a long list of insects that were found contaminated with E. stewartii. Although the list is lengthy, incidence of infection was slight except for the aforementioned flea beetles.

The symptoms of Stewart's wilt infection in sweet corn includes wilting, pale green linear streaking, stunting, and death. Surviving plants may tassel prematurely and produce deformed ears. About three weeks are required before symptoms of infection are evident. Early season infection, or wilt phase of the disease, is most damaging to sweet corn. Field corn varieties are generally resistant to early infection, but suffer from late season infection. Stewart's wilt is a limiting factor in sweet corn production in the northeastern United States in some years.

Management

  1. A system to aid prediction of the severity of Stewart's wilt, based on winter temperature, was developed in Pennsylvania (Castor et al., 1975). Mean monthly temperatures during December, January, and February, exceeding 3°C, favor adult overwintering survival. Beetle numbers can also be monitored during the growing season. Adams and Los (1986) found that yellow sticky traps hung close to the soil were most efficient in capturing corn flea beetles. Beetles can be counted on plants, and a threshold of six beetles per 100 plants is sometimes used to initiate control (Adams and Los, 1986). Hoffmann et al. (1995) studied the distribution of beetles between and within plants. Beetles tend to aggregate; about 50% are found on the uppermost of fully emerged leaf.
  2. Systemic insecticides are commonly applied in-furrow at planting, and after emergence, to protect the young corn from feeding injury and transmission of Stewart's wilt (Ayers et al., 1979; Munkvold et al., 1996). For example, Ayers et al. (1979) reported that Stewart's wilt infection was reduced from about 34% in untreated corn plants to only 2-4% when effective insecticide was used. The insecticide carbofuran seems to interfere directly with the bacterium, imparting resistance to the plant (Sands et al., 1979).

Cultural Practices. Sweet corn varieties differ in their susceptibility to Stewart's wilt. Some varieties are quite resistant, but if high, overwintering populations of corn flea beetles are forecast, insecticide treatment is also advised (Ayers et al., 1979). Early maturing varieties tend to be more susceptible to injury.

0 0

Post a comment