|Crop Knowledge Master|
Listroderes difficilis (Germar)
Ronald F.L. Mau, Extension Entomologist
Jayma L. Martin Kessing, Educational Specialist
Department of Entomology
Principal vegetable hosts include bean, beet, burdock (gobo), cabbage, carrot, cauliflower, celery, chard, Chinese cabbage, garlic, head cabbage, kale, lettuce, mustard, mustard cabbage, onion, pepper, peanut, potato, radish, rape, spinach, tomato, turnip and sweet potato. Cultivated flowers attacked include pansy, petunia, poppy, phlox and verbena. The principal weed hosts include dandelion, mallow, milk thistle, mustard, wild aster, wild radish and wild parsnip. High (1939) gives an extensive list of cultivated and wild host plants.
Originally from South America, the vegetable weevil has spread to many parts of the world including Australia (Queensland, New South Wales and Tasmania), South Africa and the United States (Clausen, 1978). This insect was found in the United States in Mississippi in 1922 and San Francisco in 1926. It is now widely found in the southern U.S. mainland states and Hawaii.
Adult weevils attack young tomato, potato and other vegetables. They may girdle or cut-off the entire plant like cutworms. Damage to young plants is often severe. Larval damage to fleshy root crops is characterized by stunted growth, roughened irregular shaped grooves on the outer walls and by tunnels being eaten into the roots of turnip, carrot, radish, etc. The first and perhaps greatest amount of larval damage occurs on lower stems and crowns of host plants. Only the larger stems may be left and the entire crown of plants may be destroyed. Larvae may be found inside tubular onion leaves.
When there are 1 to 25 larvae per plant the leaves are skeletonized (High, 1939).
Female vegetable weevils reproduce parthenogenetically (without fertilization). The development period from egg to adult may take as many as 111 days at 55û F and as few as 48 days at 76û F (High, 1939). There are continuous generations throughout the year in Hawaii.
The small (1/25 inch in diameter), spherical, creamy white to greenish eggs are laid on the soil or crowns of plants. They are placed singly or in small groups in the crown of plants, on leaf petioles, or in soil near the base of plants (Metcalf, 1962). Eggs turn black just prior to hatching. Eggs hatch in 33 days at 55û F, 17 days at 58û F, 16 days at 68û F and 15 days at 76û F (High, 1939).
Larvae are strongly convex, legless, slender grubs. The grubs are pale green or yellow depending on their host. The head is pale yellow to brown with darker dotted lines. A shield, just back of the head, is lighter in color, divided down the middle and has a pattern of dots forming two darker areas on the front part of the body. Mature grubs are about 1/2 inch long. There are four molts or stages. Larval development is completed in about 39 days 55û F, 43 days at 58û F, and 23 days at 68û F (High, 1939).
Pupation occurs in the soil 1/2 to 2 inches below the surface (High, 1939). Pupae are 3/8 inch long and over half as wide as long. They are yellowish green with a light snout and pale yellow legs and wing pads. Pupal development is completed in 39 days at 55û F, 18 days at 58û F, and 14 days at 68û F (High, 1939).
The grayish brown adults are about 1/4 to 1/3 inch long and about half as wide. Their snouts (beaks) are short and stout and curved beneath the body. The adult has two pairs of wings. The outer pair is hardened and covers the membranous pair. Each wing cover is marked with a pale, short band about two-thirds of the distance from the front of the wing. Each band begins on the inner margin and slants outward to form a V-shaped marking across the wing covers. In addition, the front of the thorax is formed in such a way as to give the appearance of shoulders.
Female weevils begin laying eggs six to eight months after emerging. They can lay from 300 to 1,500 eggs during their lifetime. Adults can live for 1 to 2 years (High, 1939).
Grubs are found in the soil during the day and emerge at night to feed on foliage, roots and stems. Young larvae feed on the buds of the host plant or on the under sides of the leaves close to the buds. Older grubs feed on all foliage parts and roots (High, 1939).
Adults are primarily active at night. While winged, they seldom ever fly. They can be found on any part of the host plant, but they prefer to feed on the crowns, the base of plants, and the roots. During the day they are often under leaves, clods of soil, or other objects on or close to the surface of the soil (High, 1939).
When disturbed, the adult weevils feign death by falling upon their back or side, leg either spread widely apart or drawn under the body and their antennae drawn along side the beak (High, 1939).
Adults are attracted to light at night and crawl rather rapidly.
Natural enemy searches conducted in Argentina and Uruguay in 1942-1945 found four parasites: two ichneumonid wasps Thersilochus argentinensis (Blanch.) and T. parkeri (Blanch.), a braconid wasp (Triapsis sp.), and a tachnid fly (Epiplagiops littoralis Blanch). Collected parasites were taken to California and reared for release. Larvae containing parasitic nematodes (Mermis sp.) were also included in the shipment from South America. Reared braconid wasps and tachnid flies were released during 1943-1946 (Clausen, 1978). Both of these parasites attack the vegetable weevil larvae. A different type of Thersilochus sp. was found attacking larvae in California, although its origin is unknown (Clancy, 1986). Ants, birds and spiders also attack grubs. Parker et al. (1950) gives extensive lists, descriptions and biologies of parasites of vegetable weevil larvae and adults.
Several attempts at releases of natural enemies have occurred in Australia, none of them have resulted in the establishment of these parasites (Clausen, 1978).
Crop rotation helps to keep populations down in localities where the vegetable weevil is a serious pest. The preferred host crops should not be planted in the same field or adjacent fields in successive years (High, 1939).
Susceptible crops should be thoroughly cultivated and borders of the field should be kept clean and free of wild hosts such as chickweed (High, 1939).
Because of their behavior, control can be achieved using residual insecticide sprays or granular insecticides. If this is not possible, a plan using non-chemical tactics should be used. A main tactic of this plan should be to avoid planting susceptible hosts in adjacent, sequential plots.
Clausen, C. P. 1978. Vegetable Weevil (Listroderes costirostris obliquus (Klug). pp. 272-273. In: Introduced Parasites and Predators of Arthropod Pests and weeds. U.S. Dept. Agric. Handbook No. 480. 545 pages.
Ferguson, A. M. 1978. Insecticidal Control of Vegetable Weevil Larvae. Proc. New Zeal. Weed & Pest Control Confer. pp. 108-112.
Hassan, E. 1977. Major Insect and Mite Pest of Australian Crops. Ento Press. Queensland.
High, M. M. 1939. The Vegetable Weevil. U.S. Dept. Agric. Circ. 530. 26 pages.
Lockwood, S. 1957. Vegetable Weevil, Listroderes costirostris obliquus. California Department of Agriculture, Bureau of Entomology, Loose-Leaf Manual of Insect Control.
McCarthy, T. 1924. The Brown Vegetable Weevil. Agric. Gaz. New South Whales. 35: 573-580.
Metcalf, C. L., W.P. Flint and R. L. Metcalf. 1962. 78A. Vegetable Weevil. pp. 608-609. In: Destructive and Useful Insects, 4th Ed. McGraw-Hill Book Company, New York, San Francisco, Toronto, London. 1087 pages.
Parker, H. C., P. A. Berry, and A. Silveira. 1950. Vegetable Weevils and Their Natural Enemies in Argentina and Uruguay. U.S. Dept. Agric. Tech. Bull. 1016. 28 pages.
MAR/1991 revised JUN/1992.