Listroderes difficilis Germar Coleoptera Curculionidae

Natural History

Distribution. First found in North America in 1922, vegetable weevil quickly spread from Mississippi, its point of introduction, to other southern states from South Carolina to California, and to Hawaii. Its distribution is limited to warm climates, and it is most common in Alabama, Louisiana, and Mississippi. However, vegetable weevil can withstand moderately cold weather, including brief periods of below-freezing temperature. Originally described from Brazil, vegetable weevil has also invaded Australia, New Zealand, and South Africa.

Host Plants. Vegetable weevil has a very wide host range, and is known to attack such vegetables as artichoke, bean, beet, broccoli, Brussels sprouts, cabbage, carrot, cauliflower, celery, Chinese cabbage, collards, eggplant, endive, garlic, kale, kohlrabi, leek, lettuce, mustard, onion, parsley, parsnip, pepper, potato, radish, rutabaga, salsify, southern pea, spinach, sweet potato, Swiss chard, tomato, and turnip. Despite its wide host range, it is generally known as a pest of potato and tomato (Solanaceae); cabbage, col-lards, and turnip (Cruciferae); and carrot (Umbelli-ferae). Not surprisingly, vegetable weevil is known to feed on a long list of weeds, including common chickweed, Stellaria media; lambsquarters, Chenopo-dium spp.; thistle, Cirsium spp.; dogfennel, Eupatorium capillifolium; pepperweed, Lepidium spp.; mallow, Malva spp.; sorrel, Oralis spp.; and plantain, Plantago spp. In Georgia, turnip is the preferred vegetable crop, and chickweed is the most common wild host (Beckham 1953).

Natural Enemies. This insect seems relatively free of important parasitic insects, but is preyed upon by general predators, especially ants. Fire ants, Solenopsis spp., for example, are known to feed on both eggs and larvae of vegetable weevil. Steinernematid nematodes are known to parasitize vegetable weevil experimen tally (Poinar, 1979), and diplogasterid nematodes have caused natural infestations (High, 1939). Hyalomyodes triangulifera (Loew), a common parasitoid of Coleoptera and Lepidoptera, and Epiplagiops littoralis Blanchard, a little-known parasitoid (both Diptera: Tachinidae) are reported from vegetable weevil, but their effects are undetermined.

Life Cycle and Description. There is only a single generation per year, with the adults aestivating during the summer months. Typical oversummering locations for the inactive adults include beneath loose bark of trees, and under grass, weeds, and other organic debris, often at the edges of crop ields. In September, activity is resumed, and egg production begins. This beetle is parthenogenetic; only females are known, and all are capable of producing fertile eggs. Under ideal conditions, a life cycle can be completed in about 45-110 days.

  1. Eggs are deposited over an extended period of time, usually from September until March, if weather allows adults to be active. This results in protracted periods of activity for all other life stages, also. The egg is slightly elliptical, and about 0.64 mm long and 0.54 mm wide. Initially, it is white, but turns yellow and then gray, and eventually black, as it reaches maturity. The eggs are deposited near the crown of the plant, but sometimes on leaf petioles or on the soil adjacent to the plant. Estimates of egg production are from 1-30 per day and 300-1500 per female. Duration of the egg stage is 11-33 days, but generally is 15-20 days.
  2. Upon hatching, larvae begin feeding at the plant crown, often destroying new foliar tissue before it can develop fully. However, no part of the plant is immune to attack, and leaves, stems, and roots may be consumed. The period of larval abundance corresponds roughly to that of egg production, late autumn until early spring. The larvae initially measure about 1.7 mm long, and are creamy-white with a black head. As foliage is consumed, however, larvae acquire a yellowish or green color and attain a length of 14 mm. In larger larvae the head is yellowish-brown with dark spots. Larvae lack true legs, but use ventral and lateral ridges or protrusions on their body for locomotion. The lateral protrusions, which are pyramidal, are apparent when the larvae are viewed from above. Also present on the larvae are dark rings around the abdominal spiracles, which results in a line of dark spots along each side. There are four larval instars. Lovell (1932) gave mean head capsule widths of 0.21, 0.30, 0.44, and 0.70 mm, respectively, for the larval instars. In contrast, Beckham (1953) provided corre-

Vegetable weevil larva.

Vegetable weevil larva.

sponding measurements of 0.32, 0.48, 0.79, and 1.18 mm. The basis for this fairly significant difference in head capsule measurements is unexplained. Larval growth is completed in 23-45 days, averaging 38 days. Mature larvae drop to the ground to pupate. (See color figure 128.)

  1. Larvae prepare a small cell in the soil at a depth of 2.5-5.0 cm, and pupate within. The pupa measures about 7.5 mm long, is yellow-green, and resembles the adult in form. Pupation occurs in the winter and spring, with the adults not emerging until April or May. However, in the laboratory pupation may be completed in 14-16 days.
  2. The adult emerges from the soil several days after completing the transformation from the pupal stage. It measures about 8 mm long. The adult is grayish-brown, though generally there is a light-colored "V" on the elytra. The body is also covered with tan or gray scales and scattered hairs. The snout is short and stout. The antennae are slightly clubbed. Adults are somewhat gregarious, often collecting in small groups when they seek shelter or feed. They are nocturnal, hiding during the daylight hours under plant debris or clods of soil. Despite having fully formed wings, adults rarely fly, almost always walking from place to place. Adults are active from late autumn until early summer, and then aestivate through the summer months. Adults often feign death if disturbed.

The biology of vegetable weevil was provided by

m

í|ítí

mi'

'sffe

■t«;

i»8|

Adult vegetable weevil.

Damage

Both adults and larvae may feed upon foliage and roots, and inflict serious damage. The principal form of damage results when larvae feed on the developing tissue of plants, stunting them. Later, larvae may move onto mature foliage, consuming everything except large veins. Initially, larval feeding consists of small, round holes, but as larvae mature the holes get quite large and are irregular in shape. In crops with well-developed roots such as carrot and turnip, larvae may also move to the roots to feed, where they tunnel through the plant tissue. Adults can feed on stem tissue, sometimes cutting off the stems of young plants at the soil surface in a manner similar to cutworm damage.

Management

Insecticides. Although formerly considered an important pest, modern foliar insecticides now keep vegetable weevil under control. Botanical insecticides can be effective for suppression of this insect. Fruit-based poison bait formulations formerly were used for adult control with considerable success.

Cultural Practices. Cultural controls can be beneficial, but are rarely practiced, because insecticides applied for this, or other insects, effectively suppress vegetable weevil. Cultivation during the winter months, especially repeated cultivation, can destroy the pupal stage, which is found in the soil at this time of the year. Crop rotation can have some benefit, despite the wide host range of this insect. Repeated culture of highly preferred plants such as turnip, carrot, potato, and tomato should be avoided. Because adults almost never fly, susceptible vegetables following nonsusceptible crops such as corn are not damaged. Sanitation should be practiced, because summer-time aestivation takes place commonly under organic debris. If the soil is free of such shelter, summer survival may be poor.

0 0

Post a comment