Cylas formicarius Fabricius Coleoptera Curculionidae

Natural History

Distribution. Sweetpotato weevil now is found in tropical regions of Africa, Asia, and North and South America. Its origin is believed to be Africa or India despite the fact that the sweetpotato plant likely evolved in northwestern South America. Thus, the association between this crop and its principal pest is relatively recent. Sweetpotato weevil was first noted in the United States in Louisiana in 1875, and then in Florida in 1878 and Texas in 1890, probably entering by way of Cuba. It is now found throughout the coastal plain of the southeast from North Carolina to Texas. It also is found in Hawaii and Puerto Rico.

Host Plants. This weevil feeds only on plants in the plant family Convolvulaceae. Although it has been found associated with several genera, its primary hosts are in the genus Ipomoea. Among vegetable crops only sweet potato, I. batatas, is a suitable host. In the United States, some cultivars of sweet potato are called yams, and these are susceptible to sweetpotato weevil. True yam, Dioscorea spp., belongs to the plant family Dioscoreaceae. True yams are not a host of sweet-potato weevil and are rarely cultivated except in tropical areas. Native plants can be important hosts of sweetpotato weevil. Railroad vine, Ipomoea pes-caprae, and morningglory, I. panduratea, are among the suitable wild hosts. Cockerham et al. (1954) gave information on host relations.

Natural Enemies. Several natural enemies are known. Wasps such as Bracon mellitor Say, B. punctatus (Muesebeck), Metapelma spectabile Westwood (all Hymenoptera: Braconidae), and Euderus purpureas Yoshimoto (Hymenoptera: Eulophidae) have been reared from sweetpotato weevil larvae in the southeastern United States. There have been no studies of parasitoid effectiveness, but these species seem to be infrequent. Among predators, ants (Hymenoptera: Formicidae) seem to be most important. Diseases, especially the fungus Beauveria bassiana, have been observed to inflict high levels of mortality under conditions of high humidity and high insect density, but ield conditions are rarely conducive for disease epizootics. Jansson (1991) provided a complete review of natural enemies.

Life Cycle and Description. A complete life cycle requires 1-2 months, with about 35-40 days common during the summer months. The generations are indistinct, and the number occurring annually is estimated to be 5 in Texas, and about 8 in Louisiana. The adults do not undergo a period of diapause in the winter, but seek shelter and remain inactive until the weather is favorable. All stages can be found throughout the year if suitable host material is available.

  1. The 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, and seals the egg within the oviposition cavity with a plug of fecal material, making it difficult to observe. 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 ovi-position, in preference to depositing eggs in stem tissue. The egg is oval and creamy white. Its size is reported to average 0.65-0.79 mm long and 0.410.50 mm wide. There is little distinct sculpturing on the surface of the egg. Duration of the egg stage varies from about 5-6 days during the summer to about 1112 days during colder weather. Females apparently produce 2-4 eggs per day, or 75-90 eggs during their life span of about 30 days. Under laboratory conditions, however, Jansson and Hunsberger (1991) reported mean fecundity of 122 eggs, and others reported about 50-250 eggs per female.
  2. 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 0.29-0.32 mm, 0.43-0.49 mm, and 0.750.78 mm for instars 1-3, respectively. Duration of each instar is 8-16, 12-21, and 35-56 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. (See color figure 36.)
  3. The mature larva creates a small pupal chamber in the tuber or stem. The pupa is similar to the adult in appearance, though the head and elytra are bent ventrally. The pupa measures about 6.5 mm long. Initially the pupa is white, but with time this stage becomes grayish, with darker eyes and legs. Duration of the pupal stage averages 7-10 days, but in cool weather it may be extended up to 28 days.
  4. 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
Sweetpotato weevil larva.
Beauveria Bassiana Cylas Formicarius
Sweetpotato weevil pupa.

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 5.5-8.0 mm long. 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 (Mullen, 1981). 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 pre-oviposition period is seven days. A sex pheromone produced by females has been identified and synthesized (Heath et al., 1986). (See color figure 127.)

Good summaries of sweetpotato weevil biology were found in Reinhard (1923), Sherman and Tama-shiro (1954), and Cockerham et al. (1954).

Damage

Sweetpotato weevil is often considered to be the most serious pest of sweet potato, with reports of losses ranging from 5-97% in areas where the weevil occurs. Some of the insect density and yield component relationships were documented by Sutherland (1986). Basically, he found that there was a strong positive relationship between vine damage or weevil

Sweet Potato Weevil
Adult sweetpotato weevil.

density, and tuber damage. However, the plants exhibited some compensatory ability, with the relationship between vine damage and yield nonlinear. Talekar (1982), in contrast, found few significant relationships between weevil numbers and damage.

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 ter-pene 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.

Management

  1. Over 90% of larvae are found in the upper 15 cm of the tubers and basal 10 cm of the vine. Early in the season larvae are found about equally in the vine and tuber, but later in the season most occur in the tubers (Jansson et al., 1990a). Distribution of sweetpotato weevil in fields is aggregated (McSorley and Jansson, 1991). Pheromone traps show great promise for monitoring of adult population density (Jansson et al., 1990c). Weevils respond to low concentrations of pheromone, and apparently will move up to 280 m to a pheromone source (Mason et al., 1990). The sex pheromone also shows great potential for mating disruption and mass trapping (Jansson et al., 1991a).
  2. Planting-time applications of insecticides are sometimes made to the soil to prevent injury to slips or cuttings. Both granular or liquid formulations have been used. Systemic insecticides are preferred. Post-plant 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. Because of 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.

Cultural Practices. Cultural practices are sometimes recommended to alleviate weevil problems. 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. Miyatake et al. (1995), for example, documented dispersal rates of 150 m per day, with dispersal more rapid in the absence of suitable hosts.

Sanitation is particularly important for weevil population management. Discarded and unharvested tubers can support large populations, 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.

Soil conditions affect weevil damage potential. Because adults crawl into cracks in the soil to gain access to tubers, efforts are made to reduce soil cracking. Irrigation, mulching, and hilling are common approaches, and planting in sandy soil is preferable to soil with high clay content because cracking is less likely.

Host-Plant Resistance. Considerable research has been conducted on host-plant resistance in sweet potato, and several commercially available varieties exhibit low levels of resistance (Waddill and Conover, 1978; Barlow and Rolston, 1981; Mullen et al., 1980, 1985). Unfortunately, resistance is not adequate when plants are exposed to high weevil population densities, and severe damage results. Talekar (1987) suggested that adequate resistance might not be isolated by conventional plant-breeding techniques. Son et al. (1991) identified chemical oviposition stimulants that may be the basis for resistance in some cultivars.

Biological Control. Entomopathogenic nemato-des seem to be the organisms with the greatest potential for practical biological suppression of sweetpotato weevil (Jansson, 1991). Several strains of Steinernema carpocapsae (Nematoda: Steinernematidae) and Hetero-rhabditis bacteriophora (Nematoda: Heterorhabditidae) penetrate the soil and tubers, killing weevil larvae. At least in the soils of southern Florida, infective nematodes are persistent, remaining active for about four months. In some cases nematodes are more effective than insecticides at reducing damage (Jansson et al., 1990b, Jansson et al., 1993).

Other Methods. Other methods of suppression are sometimes used, especially for post-harvest treatment of tubers. Post-harvest treatment not only prevents damage in storage, but allows shipment of tubers to areas where sweetpotato weevil is not found but might survive. Traditionally, post-harvest treatment has been accomplished with chemical fumigants, but they have fallen out of favor. Irradiation is potentially effective, though older stages of insects are less susceptible to destruction (Dawes et al., 1987; Sharp, 1995). Storage in controlled atmospheres, principally low oxygen and high carbon dioxide, is very effective for destruction of weevils, but requires good storage conditions (Delate et al., 1990).

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