Cylas formicarius (Fabricius, 1798) & Euscepes postfasciatus (Fairmaire, 1849)

 Sweet potato weevils

General

• Insects belonging to Beetles that attack plants of the genus Ipomées. C. formicarius belongs to the family Brentidae, while E. postfasciatus belongs to the family Curculionidae
• Widely distributed in countries of the intertropical zone and often causing major damage in tubers.
• The larvae of these 2 weevils are quite similar. They are small, whitish worms about 5-7 mm in length. Adults are quite easily identifiable.
  • The adult C. formicarius is elongated and roughly ant-like with an orange patch between the head and abdomen. It is 6 to 8 mm long.
  • The adult of E . postfasciatus , much smaller, measures about 4 mm. Brown in color, it is recognizable by the presence of its 2 white spots on the elytra.
• Both species are hosts of plants of the Convolvulaceae family. Among them, only the sweet potato is cultivated. C. formicarius is quite specific for the sweet potato even if this weevil can be found on wild Ipomeas or sometimes on cultivated Araceae (dachine).

• Sensitive botanical family (s)

• Affected production areas :

• Organs attacked

Symptoms, damage

• Symptoms :
  • Rare bites and perforations on leaves without impact on the crop.
  • Presence of galleries in the tubers of the sweet potatoes caused by the larvae, which strongly depreciates the commercial quality of the production. Damaged tubers are thus much more sensitive to secondary invading fungi and bacteria causing rots before and after harvest. Up to 97% of production can be lost.

• Signs : Presence of adult insects and larvae on plants, in tubers, and in culture (figures).

• Possible confusion :

Biology

• Development cycles : The 2 species have a similar biological cycle made up of 3 development phases (larval, pupation and adult stage). The duration of the complete cycle (from egg to egg) lasts 1 to 2 months. It varies according to the temperature for the 2 species.
  • The adult lays its eggs on the stems.
  • Upon hatching, the larvae enter the base of stems or tubers and tunnel as they feed. The entire larval phase as well as pupation takes place in the plant.
  • The adult finally leaves the galleries and evolves under the vegetation of the crop.
• Dispersion : Adults of C. formicarius are able to fly short distances. Concerning E. postfasciatus , its ability to fly seems very limited and the spread of this insect is mainly through the eggs present on the cuttings and the presence of wild Ipomeas which can play the role of reservoir plants.
• In the subtropical region, there is a decrease in the activity of pest populations and the intensity of damage during cool periods.

Protection

• Make rotations (5 years recommended) or put the plots in fallow to break the cycles.
• Give preference to short-cycle varieties when the pressure from these pests increases.
• Using leafless head cuttings reduces the risk of introducing plant material containing eggs and larvae.
• Delay the arrival of C. formicarius by removing  new crops from previous ones by 500 meters.
• Use plastic mulch or rice husk to act as a barrier to adults and to conserve soil moisture.
• Set up a system trapping with a sex attractant throughout the culture; it can be maintained for up to 6 months after harvest to continue to regulate the population. In the case of new plots, the device will be placed at the periphery of the plot in order to delay the introduction of weevils as much as possible. The trapping technique is available for C. formicarius only.
• Use biopesticides (1). The entomopathogenic nematodes Steinernema carpocapsae can be used; this specialty ( biological flood control ) is authorized in Guyana for this use.
• Reducing the harvest time of a plot helps minimize the impact and development of the pest.
• Remove crop residues after harvest to avoid maintaining weevil populations. It should be noted that the activity of the pigs in the plot is particularly effective because they consume the residual tubers.