Euscepes postfasciatus Fairmaire Coleoptera Curculionidae

Natural History

Distribution. West Indian sweetpotato weevil is found in Central and South America, the Caribbean

Islands, and many islands in the Pacific, including Fiji, Guam, Japan, New Zealand, and Samoa. It seems a native of the western hemisphere, and probably the Caribbean region. In the United States, it is known only from Hawaii, Puerto Rico, and the Virgin Islands. However, the proximity and tropical climate of southern Florida suggest that this state could be invaded by dispersants from Cuba, Dominican Republic, Jamaica, or elsewhere. Because West Indian sweetpotato weevil survives on Bermuda, which has a mild but not tropical climate, it is possible that the potential geographic range of this insect in United States could include much of the Gulf Coast area or southeast.

Host Plants. This weevil feeds on plants in the genus Ipomoea of the plant family Convolvulaceae. Among vegetable crops only sweet potato, I. batatas, is a suitable host. Native plants can be important hosts of sweetpotato weevil. Railroad vine, Ipomoea pes-caprae, and many other Ipomoea spp. are suitable hosts.

Natural Enemies. Little is known concerning the natural enemies of this insect. The fungus Beauveria bassiana has been consistently associated with this insect, and damage by this insect is typically greater during the dry season when fungi are less effective. This relationship is merely a correlation, however, and further research is needed to determine if there is a cause-and-effect relationship. Some parasitoids have been noted. In Hawaii, Eupelmus cushmani (Crawford) (Hymenoptera: Eupelmidae) attacks larvae infesting the above-ground portions of the host plant. In Peru, Eurydinoteloides sp. and Cerophala sp. (both Hymenoptera: Pteromalidae) also attack larvae.

Life Cycle and Description. The life cycle can be completed in 4-6 weeks. Thus, several generations per year can develop in the tropical climates where these insects are found.

  1. The female excavates a shallow cavity with her mouthparts in the sweet potato tuber or vine as a receptacle for her egg. Eggs are laid singly. The dimensions of the oval, yellowish egg given by Sherman and Tamashiro (1954) were 0.38 mm long and 0.34 mm wide. However, Raman and Alleyne (1991) reported that the egg was only 0.12 mm long and 0.10 mm wide. Both reported that the female seals the eggs within the oviposition cavity with a drop of fecal material, and that the oviposited egg appears as a raised-dark spot on the plant. The same authors suggested that this behavior does not apply to eggs laid on foliage, and that these eggs fail to hatch. The incubation period of the eggs is 7-9 days at 24°C and 11-15 days at 18°C. Females are reported to produce about 95-362 eggs during their life time, with mean fecundity estimated at 180 eggs.
  2. Upon hatching, larvae burrow deeper into the vine or tuber to feed. Larvae are legless, with a well-developed head. They are whitish and attain a length of about 5 mm. They greatly resemble larvae of sweetpotato weevil, Cylas formicarius; differences in setal patterns are pictured by Sherman and Tama-shiro (1954). Mean head capsule widths for the five instars are 0.18,0.25,0.37,0.56, and 0.78 mm for instars 1-5, respectively. Duration of the feeding period of the larval stage requires 25 (range 18-31) days at 24°C and 45 (range 40-53) days at 18°C (Raman and Alleyne, 1991). This does not include the nonfeeding or prepu-pal portion of the ifth instar, however, which is about five days. Sherman and Tamashiro (1954) indicated the number of days after oviposition in which the various instars were found as: first 7-14; second 10-15; third 12-16; fourth 13-21; fifth 17-30; and prepupal 21-32. Further, they indicate that the minimum time spent in each was 3, 2, 1, 4, 4, and 4 days for instars 1-5 and the prepupal stage, respectively. Thus, the duration of the larval stage at about 27°C, including the prepupal period, is likely about 25 days.
  3. The larva prepares a small pupation cell at the inal larval feeding site. The pupa is white initially, but becomes tan with age. The pupa measures about 4.0-4.9 mm long. In form, it greatly resembles the adult, except that the snout is pressed against the ventral surface and the elytra are incompletely developed and twisted ventrally. Duration of the pupal stage is 710 days.

West Indian sweetpotato weevil larva.

West Indian sweetpotato weevil larva.

Coleoptera
West Indian sweetpotato weevil pupa.

Adult. The adult of West Indian sweetpotato weevil is small and inconspicuous. It measures only 3.24.0 mm long and is reddish brown to grayish black. The tip of the elytra bear an indistinct whitish or yellowish transverse bar. The entire body, including the legs, is covered with a dense covering of short, stiff, erect bristles and scales. The adult often feeds before emerging from the pupation site. The adults feed on either the vine or the root, and have a tendency to be gregarious in the adult stage. Thus, several individuals may feed together, forming a large cavity. This species is much less apparent than the co-occurring sweet-potato weevil, Cylas formicarius (Fabricius). Often West Indian sweetpotato weevil feigns death when disturbed, drops from the plant, where it blends in with the soil particles, and avoids detection. Adults are long lived, often surviving more than 200 days if provided with adequate food. An interesting aspect of adult biology is that they apparently are flightless. Therefore, dispersal is dependent on walking by adults, or transport of infested tubers or other plant material containing larvae or other unapparent stage; the transport of plant material is undoubtedly the principal means of long-distance dispersal.

The biology of this species was reviewed by Sherman and Tamashiro (1954), and Raman and Alleyne (1991). Pierce (1918) provided a detailed description, and pictures the different stages.

Damage

This is the most severe weevil pest of sweet potato in the South Pacific and Caribbean areas. Successful cultivation of sweet potato on some Caribbean islands hinges largely on the successful management of this insect, which is known locally as "scarabee." It damages tubers both in the field and storage. Insect damage induces terpenoid production in sweet potatoes, making them bitter and unpalatable. Larvae infesting the tuber often burrow deep into the tuber,

Insects Sweet Potatoes
Adult West Indian sweetpotato weevil.

leaving little evidence of their presence on the surface. However, the center of the tuber may be thoroughly tunneled and packed with fecal material. Eventually, the tuber becomes blackened, and often infected with plant pathogens. Vines that are infested become wrinkled, cracked, or collapsed.

Management

Insecticides. Insecticides are often used to protect plants from injury. Planting stock sometimes is dipped in insecticide before treatment; this is particularly important if the planting stock is already infested. Insecticide is also incorporated into the planting bed and applied to foliage. Systemic insecticides are preferred because so much of the life cycle occurs inside the plant.

Cultural Practices. Several cultural practices materially affect West Indian sweetpotato weevil abundance (Sherman and Tamashiro, 1954). Foremost is sanitation. Propagation of sweet potato is often by apical cuttings or "slips," which can be infested. Plant debris is often infested and may serve as a breeding site for weevils between crops; such material should be destroyed. Ipomoea weeds should be removed from areas near crop fields as they serve both a source of initial infestation and provide harborage between crops. Crop rotation is particularly effective, because the adults are flightless. Storage of tubers should be distant from fields used to culture sweet potato.

Host-Plant Resistance. Considerable work has been done to assess naturally occurring host plant resistance. Varieties with thin stems are less injured by weevils. Unfortunately, they produce less satisfactory yields and their tubers tend to grow close to the surface, where they are more easily accessed by weevils. However, though numerous varieties or isolates contain some elements of resistance, considerable work remains to be done before commercially acceptable varieties with reliable resistance are available. It is likely that host-plant resistance will prove to be only a component of the management program, and cannot be relied upon to provide complete protection from attack (Raman and Alleyne, 1991).

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