Egg: Eggs are deposited in small cavities created by the female with her mouthparts in the sweet potato root or stem. The female deposits a single egg at a time, and seals the egg within the oviposition cavity with a plug of fecal material, making it difficult to observe the egg. Most eggs tend to be deposited near the juncture of the stem and root (tuber). Sometimes the adult will crawl down cracks in the soil to access tubers for oviposition, in preference to depositing eggs in stem tissue. The egg is oval in shape and creamy white in color. Its size is reported to be about 0.7 mm in length and 0.5 mm in width. Duration of the egg stage varies from about five to six days during the summer to about 11 to 12 days during colder weather. Females apparently produce two to four eggs per day, or 75 to 90 eggs during their life span of about 30 days. Under laboratory conditions, however, mean fecundity of 122 and 50 to 250 eggs per female has been reported.
Larva: When the egg hatches the larva usually burrows directly into the tuber or stem of the plant. Those hatching in the stem usually burrow down into the tuber. The larva is legless, white in color, and displays three instars. The mean head capsule widths of the instars are 0.29 to 0.32 mm, 0.43 to 0.49 mm, and 0.75 to 0.78 mm for instars one to three, respectively. Duration of each instar is eight to 16, 12 to 21, and 35 to 56 days, respectively. Temperature is the principal factor affecting larval development rate, with larval development (not including the prepupal period) occurring in about 10 and 35 days at 30° and 24°C, respectively. The larva creates winding tunnels packed with fecal material as it feeds and grows.
Pupa: The mature larva creates a small pupal chamber in the tuber or stem. The pupa is similar to the adult in appearance, although the head and elytra are bent ventrally. The pupa measures about 6.5 mm in length. Initially the pupa is white, but with time this stage becomes grayish in color with darker eyes and legs. Duration of the pupal stage averages seven to 10 days, but in cool weather it may be extended to up to 28 days.
Adult: Normally the adult emerges from the pupation site by chewing a hole through the exterior of the plant tissue, but sometimes it remains for a considerable period and feeds within the tuber. The adult is striking in form and color. The body, legs, and head are long and thin, giving it an ant-like appearance. The head is black, the antennae, thorax and legs orange to reddish brown, and the abdomen and elytra are metallic blue. The snout is slightly curved and about as long as the thorax; the antennae are attached at about the mid point on the snout. The beetle appears smooth and shiny, but close examination shows a layer of short hairs. The adult measures 5.5 to 8.0 mm in length. Under laboratory conditions at 15°C, adults can live over 200 days if provided with food and about 30 days if starved. In contrast, their longevity decreases to about three months if held at 30°C with food, and eight days without food. Adults are secretive, often feeding on the lower surface of leaves, and are not readily noticed. The adult is quick to feign death if disturbed. Adults can fly, but seem to do so rarely and in short, low flights. However, because they are active mostly at night, their dispersive abilities are probably underestimated. Females feed for a day or more before becoming sexually active, but commence oviposition shortly after mating; the average preoviposition period is seven days. A sex pheromone produced by females has been identified and synthesized.
A symptom of infestation by sweetpotato weevil is yellowing of the vines, but a heavy infestation is usually necessary before this is apparent. Thus, incipient problems are easily overlooked, and damage not apparent until tubers are harvested. The principal form of damage to sweet potato is mining of the tubers by larvae. The infested tuber is often riddled with cavities, spongy in appearance, and dark in color. In addition to damage caused directly by tunneling, larvae cause damage indirectly by facilitating entry of soil-borne pathogens. Even low levels of feeding induce a chemical reaction that imparts a bitter taste and terpene odor to the tubers. Larvae also mine the vine of the plant, causing it to darken, crack, or collapse. The adult may feed on the tubers, creating numerous small holes that measure about the length of its head. The adult generally has limited access to the tubers, however, so damage by this stage is less severe than by larvae. Adult feeding on the foliage seldom is of consequence.
Insecticides. Planting time applications of insecticides are commonly made to the soil to prevent injury to the slips or cuttings. Either granular or liquid formulations are used, and systemic insecticides are preferred. Postplant applications are sometimes made to the foliage for adult control, especially if fields are likely to be invaded from adjacent areas, but if systemic insecticide is applied some suppression of larvae developing in the vine may also occur. Due to the long duration of the plant growth period, it is not uncommon for preplant or planting time applications to be followed by one or more insecticide applications to the plant or soil at mid season. Insecticides are also applied to tubers being placed into storage to prevent reinfestation and inoculation of nearby fields.
Insect Management Guide for sweetpotatoes
Cultural practices. Cultural practices are sometimes recommended to alleviate weevil problem. Isolation is frequently recommended, and it is advisable to locate new fields away from previous crops and distant from sweet potato storage facilities, because both can be a source of new infestations. However, despite the infrequency of flight by adults, dispersal can occur over considerable distances. Dispersal rates of 150 m per day have been observed, with dispersal more rapid in the absence of suitable hosts.
Sanitation is particularly important for weevil population management. Discarded tubers and unharvested tubers can support large population, and every effort should be made to remove such host material. Related to this, of course, is the destruction of alternate hosts; control of Ipomoea weeds is recommended.
Biological control. Entomopathogenic nematodes seem to be the organisms with the greatest potential for practical biological suppression of sweetpotato weevil. Several strains of Steinernema carpocapsae (Nematoda: Steinernematidae) and Heterorhabditis bacteriophora (Nematoda: Heterorhabditidae) penetrate the soil and tubers, killing weevil larvae. At least in the soils of southern Florida, the infective nematodes are persistent, remaining active for up to four months. In some cases nematodes are more effective than insecticides at reducing damage.
Other methods. Other methods of suppression are sometimes used, especially for postharvest treatment of tubers. Postharvest treatment not only prevents damage in storage, but allows shipment of tubers to areas where sweetpotato weevil is not found but might survive. Traditionally, postharvest treatment has been accomplished with chemical fumigants, but they have fallen from favor. Irradiation is potentially effective, although older stages of insects are less susceptible to destruction. Storage in controlled atmospheres, principally low oxygen and high carbon dioxide, is very effective for destruction of weevils, but requires good storage conditions.
Author: John L. Capinera, University of Florida
Photographs: J.L. Castner, University of Florida
Project Coordinator: Thomas R. Fasulo, University of Florida
Publication Number: EENY-27
Publication Date: February 1998. Latest revision: June 2006.
Copyright 1998-2006 University of Florida
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Department of Entomology and Nematology
Division of Plant Industry
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larvae
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damage to sweet potato tuber