Alfalfa Weevil
Hypera postica (Gyllenhal), Curculionidae, COLEOPTERA


DESCRIPTION

Adult - Usually 5 to 6 mm long, alfalfa weevil adults vary widely in color and marking. The newly emerged weevil is light brown with a distinct, dark line down the center of its back. After a few days, it becomes entirely dark brown or black. Color plate.

Egg - The egg is small, oval, and pale yellow when laid. It darkens before hatching. The surface, although appearing smooth, is sculptured.

Larva - There are three or four larval instars. The young larva is legless, cream colored or yellowish-green and about 0.75 mm long. As it matures, it becomes green and has a black head. The mature larva has a distinct, white stripe down the center of its back and a faint, white line along each side. The entire body is wrinkled and about 9.5 mm long when mature. Color plate.

Pupa - About the size of the mature larva, the newly formed pupa is yellowish green, but later changes to light brown. Its loosely constructed, white, silken cocoon usually contains leaf fragments.



BIOLOGY

Distribution - Although this weevil now occurs throughout the 48 contiguous states, 2 different strains are believed to exist. The eastern strain occurs as far west as Kansas, the western strain as far east as Nebraska, North Dakota, and South Dakota. In North Carolina, heaviest infestations parallel the alfalfa- producing areas in the northern Piedmont and mountains.

Host Plants - In addition to alfalfa, host plants include white clover, red clover, bur clover, yellow sweet clover, white sweet clover, and a few other clovers. However, alfalfa is the preferred host and economic damage to other crops is very rare.

Damage - Larvae do the most damage, although adult alfalfa weevils are also injurious. Early season feeding in leaf buds and terminal growing areas may seriously retard normal plant growth. As the larvae grow, they feed on leaves, and eventually skeletonize them. The larvae can easily be seen curled around stems or leaves. As feeding persists, an infested field takes on a distinct grayish appearance.

Adult weevils notch main stems, side shoots, and leaf stems. They often sever the latter two from the plant. They also feed on the leaves, causing ragged and torn leaf margins. Females puncture the stem during oviposition. Feeding damage by both adults and larvae greatly reduces the quality and yield of the first cutting of hay, and may reduce the quality of subsequent cuttings.

Life History - Alfalfa weevils overwinter as adults, larvae, or even eggs. In spring, adults resume feeding and egg-laying activities; viable eggs hatch; and larvae begin feeding on new leaves. Small clusters of spring eggs are usually deposited in cavities of dead alfalfa stems and hatch within a couple of weeks. The larvae feed for 3 to 4 weeks, usually causing much damage in April and May. Mature larvae spin cocoons around themselves and pupate either on alfalfa leaves or among soil debris. Adult weevils emerge from the cocoons 8 to 16 days later. Alfalfa weevils estivate throughout most of the summer and resume activity in the fall. The weevils have been known to deposit eggs as early as mid- August and as late as November in the North Carolina Piedmont. Only one generation is completed each year.


CONTROL

Cultural control of alfalfa weevils includes the following: (1) cutting newly seeded alfalfa at one-tenth bloom; (2) cutting established, second year alfalfa at full bud for the first harvest in spring and at one-tenth bloom for succeeding cuts; and (3) removing plants as hay or allowing the field to be grazed after a freeze to reduce overwintering egg populations.

Parasite release programs by governmental agencies have established effective alfalfa weevil parasite populations in most alfalfa- growing areas. In some states, an economic threshold of 25 larvae per sweep is commonly recognized, although no such threshold has been established in North Carolina. In this state, weevil damage is consistent enough in all areas where the crop is grown to warrant one insecticide application per growing season. Research at N. C. State University indicates that application at a predetermined date prescribed for each areas is the most efficient approach to alfalfa weevil control. For specific chemical recommendations see the current North Carolina Agricultural Chemicals Manual.