Sweetpotato Weevil
Cylas formicarius elegantulus (Summers), Curculionidae, COLEOPTERA


DESCRIPTION

Adult - This ant-like snout beetle is about 6 mm long. The head and wing covers are metallic dark blue and the thorax and legs, bright red-orange.

Egg - Each white or pale yellow egg is inserted into a shallow hole in the vine. Broadly oval and about 0.6 mm long, the egg is slightly narrower at the attached end. The dark head of the larva becomes visible inside the egg, just before hatching.

Larva - The fat, legless, slightly crescent-shaped larva has a dirty white to gray body and a pale brown head. When fully grown it is about 9 mm long.

Pupa - When newly formed, the pupa is the same color as the larva and about 5 mm long. Before transformation to the adult, the eyes, wing pads, and legs turn dark brown and the rest of the body is pale yellow. The last abdominal segment has two outward and backward curved tubercles.


BIOLOGY

Distribution - Sweetpotato weevils are a serious problem in some coastal areas from North Carolina to Texas. Discovered in eastern North Carolina in 1967, sweetpotato weevils are now largely under control in this area. Rarely, weevils are found as far north as New Jersey.

Host Plants - Sweetpotato and related wild plants such as morning glory are the only hosts of sweetpotato weevils.

Damage - These weevils and their larvae are the most destructive sweetpotato pests. Infestations may reduce plant growth during the first month after planting, but other damage often is not evident until harvest. Larvae and adults feed on foliage but they prefer to attack stems and sweetpotatoes underground. Small holes scattered over the surface of infested sweet potatoes, particularly at the stem end, are the beetles' egg-laying and/or feeding punctures which cause sweet potatoes to turn bitter. Such sweet potatoes are unfit either for human consumption or stock feed.

Life History - Beetles become active in the field as soon as host plants are available. They first feed on leaves and stems. As plant stalks enlarge and become woody, adult females prepare to deposit eggs. They make holes in stems and fleshy roots near the soil surface. Eggs are placed in these holes and covered with a jelly-like secretion. Each female deposits an average of 120 eggs.

Larvae hatch less than a week after eggs are laid. They burrow deep into stems and fleshy roots for about 2 to 3 weeks. At the end of this period, third instar larvae return to the plant surface nearest the soil line to pupate. Pupae transform into adults in about a week, but another 4 days often elapse before the new beetles emerge from their pupal cells. Adults live about 2.5 to 3 months in summer and up to 8 months in winter.

Sweetpotato weevils continue to feed and breed throughout winter in stored sweet potatoes. Development and activity, however, are much slower at temperatures below 15 degrees C (60 F). As many as 6 to 8 generations may be produced each year.


CONTROL

Cultural practices such as crop rotation, use of weevil-free planting stock, and destruction of volunteer plants and crop residue are primary elements of weevil control. Planting sweet potatoes in the same fields year after year leads to increased weevil populations. If slips for planting cannot be obtained from a weevil-free area, each sweet potato chosen for seed should be examined carefully and destroyed if infested. Also, use of deep-rooted varieties such as Porto Rico over shallow-rooted varieties like Gold Rush is advisable.

Postharvest insecticide treatments can be applied to prevent development of weevils in storage. Treated sweet potatoes, however, will need to be washed thoroughly once they are removed from storage. For recommended insecticides and rates, consult the current North Carolina Agricultural Chemicals Manual.

Return to AG-295 Table of Contents