Coleoptera Curculionidae

Natural History

Distribution. The status of whitefringed beetle has long been in dispute because of disagreement over one or more species of "whitefringed beetle." Even the genus designation is disputed. There is consensus, however, that whitefringed beetle invaded the southeastern United States from South America about 1936, and was first recovered from western Florida. Alabama and Mississippi are the states most infested with this insect, but it is now distributed from Virginia to Florida in the east, and to Missouri and Texas in the west. This beetle has also been found in New Mexico and California, but its distribution is limited there. The potential range of this species is believed to include most of the southern United States, from Virginia and Kentucky to southern Colorado and nearly all of California. Whitefringed beetle is considered a pest in Argentina, Brazil, Chile, and Uruguay. Australia and New Zealand also have been invaded. The species occurring in the United States apparently are N. leuco-loma Boheman, N. peregrinus (Buchanan), and N. minor (Buchanan); the first two species are of greatest economic significance in the United States.

Host Plants. Whitefringed beetle is considered to have a very wide host range, with the number of plants eaten estimated at 385 species. Grasses, however, are considered to be relatively poor hosts. Vegetable crops attacked include bean, carrot, corn, cowpea, cucumber, lima bean, mustard, okra, potato, pea, squash, sweet potato, and watermelon. White-fringed beetle perhaps is better known as a field crop pest, attacking alfalfa, cotton, peanut, soybean, tobacco, and velvetbean. Numerous broadleaf weeds are fed upon by adults and larvae, whereas shrubs and trees are fed upon principally by adults. Damage to young pine trees planted on converted croplands containing residual beetle populations has become a problem in recent years.

Natural Enemies. No insect parasitoids are known from the United States or South America. Nematodes, particularly steinernematids, have been observed as significant mortality factors in some locations, but apparently they are limited to heavier soil types. Several pathogens, including the fungus Metar-hizium anisopliae, a microsporidian Nosema sp., and undetermined bacteria have been observed on numerous occasions to infect larvae. Also, general predators such as ground beetles (Coleoptera: Carabidae), stink bugs (Hemiptera: Pentatomidae), ants (Hymenoptera: Pentatomidae), and birds prey on adults.

Life Cycle and Description. In North America, there is generally one generation per year. In Alabama, eggs occur in mid- to late-summer, early instars tend to be found in late summer, intermediate instars in autumn and winter, and late instars in the spring months (Zehnder, 1997). In the colder regions of South America, a complete life cycle requires two years, so prolonged development may also occur among some populations in the United States, and this has been reported even among some southern populations. In all locations, the larval stage generally overwinters, but overwintering eggs have also been observed. Reproduction is parthenogenetic.

  1. Females deposit eggs in clusters, usually numbering from 10 to 15 per egg mass. Eggs measure about 0.9 mm long and 0.6 mm wide, and are elliptical. The eggs initially are white, but after 4-5 days they turn yellow. The eggs are covered with a sticky gelatinous material that hardens, causing the eggs to adhere to the substrate. They are deposited in various locations, including on the foliage and stems of plants, and below-ground, but most commonly they are deposited at the surface of the soil adjacent to the host plant. The egg production varies only a few (15-60 per female) when larvae feed on less suitable host plants such as grasses, to numerous (1500 or more per female) when favorable host plants such as peanut or velvetbean are available. Eggs deposited during the warmer seasons hatch in 10-30 days, with an average of about 15-17 days during the summer months. During the winter months eggs may require 100 days to hatch; viability is greatly reduced during the winter. The eggs reportedly require moisture in order for hatch to occur.
  2. The larva is legless, and yellowish-white, except for the head, which is brown. The larva lives within the soil and feeds principally on roots, though it frequently burrows into below-ground parts of plant, such as tubers. The summer, autumn, and winter months normally are spent in the larval stage, followed by pupation in the spring, but some apparently pass a second year in the larval stage.
  3. At maturity, the larva secretes adhesive which hardens and forms a pupal chamber. Larvae normally pupate in May-July. Pupation normally occurs within the top 5-15 cm of soil, but sometimes occurs at greater depths. The pupa measures about 12 mm long and is initially whitish but darkens as maturity approaches. Duration of the pupal stage is about 13 days, ranging from 8-15 days. Emergence is
Whitefringed beetle larva.

stimulated by moist conditions—neither dry nor wet soil is favorable.

Adult. The adult is brownish-gray, with a lighter band along the outer margins of the elytra. The thorax and the sides of the head also bear light-colored bands. The body is densely covered with short hairs. The elytra are fused together and the wings reduced; thus, the beetles are unable to fly. Only females are known. Adult emergence is typically protracted, with beetles emerging from the soil in May-August. Emergence is particularly apparent after rainfall. (See color igure 132.)

The biology of whitefringed beetle was provided by Henderson and Padget (1949), Young et al. (1950), Warner (1975), and Zehnder (1997). Descriptions and keys to the species were found in Lanteri and Marvaldi (1995). Observations from South America were given by Berry (1947).

Damage

These insects are able to develop on a wide range of host plants, and are relatively unchecked by natural enemies. Thus, they build up to very high levels. All the larvae are long-lived; they may destroy replanted ields unless they are killed. They seem to thrive on ield crops such as alfalfa, peanuts, and velvetbean, so vegetables following these crops are especially prone to injury. Lawns, woods, swamps, and old-stand vegetation are not considered as suitable habitats. Rather, well-drained and disturbed environments seem attractive.

The larvae feed on the roots, sometimes completely severing small roots. However, the principal damage results from burrowing into fleshy tissue such as the

Adult whitefringed beetle.

main tap root, tubers, and below-ground stems. Often damage is limited to surface scars or channels. Large sweet potato tubers are especially damaged (Zehnder, 1997). On fibrous roots, such as those found on trees, feeding may be limited to removal of the cortical tissues. Early symptoms of feeding injury include discoloration and wilting, but this may be followed by plant death.

The adults may feed on the foliage, particularly of broadleaf plants. They make irregular feeding sites, or notches, along the margins of leaves. Although they may occasionally defoliate a plant, the adult injury is not usually considered serious.

Management

  1. Damage potential is sometimes determined by sampling soil and sieving for larvae. Soil beneath favored plants, particularly weeds, is often selected for sampling. The frequency of leaf notching is directly related to the abundance of adults, and serves as a convenient index of damage potential.
  2. Persistent soil insecticides were formerly used extensively for larval suppression, but their use has been discontinued owing to environmental concerns. Less persistent insecticides are often applied at spring planting, and kill many overwintering larvae. However, larvae that survive may produce another generation that causes damage in the autumn after insecticide dissipation. Adult control is sometimes practiced. Foliar insecticides targeted against adults have been shown to reduce grub damage to sweet potato roots (Zehnder et al., 1998).

Cultural Practices. Although whitefringed beetle has a wide host range, rotation to grains and grasses is recommended because the ibrous root systems of grasses are not easily damaged, and larval growth is suboptimal. Also, planting of legume crops is discouraged, except during the winter months, because they are favored hosts.

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