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Cylas formicarius, Sweet potato weevil (Coleoptera: Brentidae)

Christopher Carlton

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Description

The sweet potato weevil, Cylas formicarius, is a major threat to sweet potato production worldwide. These insects are easy to distinguish morphologically. Adults are about 6 mm long with dark blue metallic heads and abdomens and orange legs and thoraces. The forewings are hardened into protective elytra, similar to most other beetles. Hindwings are folded and hidden under the elytra unless deployed for flight. Although sweet potato weevils and other beetles in the family Brentidae possess a rostrum or snout on the front of the head, they are not true weevils. Instead, the family Brentidae (straight-snouted weevils) is a related group within the weevil group of families. The antennae are not elbowed (geniculate), which distinguishes them from typical weevils in the family Curculionidae. Males and females exhibit differences in the shapes of the antennae (sexual dimorphism). Females have lobes on the anterior end of the antennae and males do not.

Eggs are minute, oval and cream colored, and are found in small depressions produced by the female on the outside of sweet potato tubers or roots. Larvae are C-shaped, soft, legless and grublike, and live entirely within the tubers. Pupae are 5 mm in length and pale in color, darkening before adult emergence.

For many years, sweet potato weevils in the Western Hemisphere were referred to as Cylas formicarius elegantulus, but the subspecies epithet has been recently abandoned, and no subspecies are currently recognized.

Life Cycle

Female sweet potato weevils release a sex pheromone to attract males. Egg development is highest after the release of the sex pheromone. After mating, females lay eggs for 62 to 104 days. Chemicals produced by the sweet potato plant (pentacyclic triterpene in storage roots and boehmeryl acetate in the periderm layer) play a role in stimulating oviposition (egg laying). The eggs are deposited into a feeding hole on a stem or root and covered with a fecal plug, which maintains moisture and protects the eggs. After four to nine days, the larvae hatch. Soon after hatching, larvae feed on the eggshell. Later stages (instars) move and feed on the host plant horizontally, tunneling through in the storage root. Larvae transform into pupae in four to 15 days, depending on temperature. Development is optimal when temperatures are between 27 C and 30 C. In lower temperatures, development may slow, but the insects do not diapause. Development is complete when adults emerge and exit the storage root to begin the life cycle anew.

Ecological Significance and Pest Status

Sweet potato weevils have a limited host range on plants in the family Convolvulaceae. Together, adults and larvae readily consume most parts of the sweet potato plant in the field and in storage. Adults preferentially feed on the leaf veins, which are more nutritious than the thin areas (lamellae) between veins. Feeding injury resembles an area of pitting and furrows. Most damage is done by larvae, which reduces yields as a result of tunneling. Both adult and larval feeding induces terpenoid production, which causes bitter-tasting tubers that are unmarketable for human consumption. Without control tactics, sweet potato weevil larval tunneling can cause crop losses ranging from 5% to 100%, depending on the level of infestation. Because of this, Louisiana growers rely on science-based control methods to prevent crop loss.

Control and Management

LSU AgCenter scientists have been conducting research on sweet potato weevil biology and control for many years. In fact, the earliest records of the species in the U.S. are from Louisiana during 1875. Current management relies on quarantine protocols, monitoring, isolating movement of infested material, chemical control and host-plant resistance. Because sweet potato is the primary host for sweet potato weevils, protection through quarantine programs is mandated in Louisiana. Growing, packing and storage locations are designated with green or pink certificate permit tags that designate sweet potato weevil-free and sweet potato weevil-infested areas, respectively. Movement is restricted for pink tag areas to prevent pest transport. The Louisiana Department of Agriculture and Forestry also enforces a required pheromone-assisted monitoring system for tag and quarantine assignment. If sweet potato weevils are found on or in fields or pheromone traps, the operation receives a pink tag and a three-year quarantine. Synthetic sex pheromones are utilized to monitor populations in each area, detecting even low populations.

Pesticides in rotation, including fumigants, pre- and postplanting applications, and foliar sprays, are commonly used as chemical control tactics. In accordance with Louisiana regulations, sweet potato farmers maintain rigorous chemical spray schedules. Products currently listed for sweet potato weevils in Louisiana are Imidan 70-W (formulated phosmet) and Brigade 2EC (formulated bifenthrin) (25%). Other chemicals are approved but not as effective against sweet potato weevils.

Lastly, resistant varieties of sweet potato can provide protection by negatively impacting weevil development. Planting resistant varieties requires little to no extra input investment from farmers. Characteristics contributing to resistance or susceptibility include root density, dry matter and starch value, overall plant chemical makeup, vine thickness and root depth. Categories of resistance include tolerance (plant can recover or limit loss/injury), antixenosis (plant affects insect behavior) and antibiosis (plant affects insect fitness). Chemical characteristics may influence interactions between plants and insects, and mediate insect behavior, insect host finding, insect feeding and oviposition. These can act as deterrents, toxins, stimulants and attractants. For detailed recommendations, consult the latest LSU AgCenter Pest Management Guide.

References

  • Chalfant, R.B., R.K. Jansson, D.R. Seal, and J.M. Schalk. 1990. Ecology and management of sweet potato insects. Annual Review of Entomology 35: 157–180.
  • Cockerham, K.L., O.T. Deen, M.B. Christian, and L.D. Newsom. 1954. The biology of the sweet potato weevil. Agricultural Experiment Station Technical Bulletin 483. 30 pp.
  • Hammond, A., R.N. Story, M.J. Murray, C.R. McCown, and D. Ring. 1997. Evaluation of selected soil and foliar insecticides on sweet potato weevil, white grubs and banded cucumber beetles. Arthropod Management Tests 22: 167–169.
  • Louisiana Department of Agriculture and Forestry. 2022. Sweet potato dealer permit information- pink tag areas. (accessed 28 November 2024).
  • LSU AgCenter. 2023. Louisiana Insect Pest Management Guide (pp. 13-17). (accessed 3 September 2024).
  • Nottingham, S.F., K.-C. Son, D.D. Wilson, R.F. Severson, and S.J. Kays. 1989. Feeding and oviposition preferences of sweet potato weevil, Cylas formicarius elegantulus (Summers), on storage roots of sweet potato cultivars with differing surface chemistries. Journal of Chemical Ecology 15: 895– 903.
  • Painter, R.H. 1951. Insect resistance in crop plants. Soil Science 72: 481.
  • Starr C.K., D.D. Wilson, R.F. Severson, S.J. Kays. 1997. Sexual dimorphism in the sweet potato weevil, Cylas formicarius (F.) (Coleoptera: Brentidae). Canadian Entomologist 129: 61–70.
  • Sutherland, J.A. 1986. A review of the biology and control of the sweet potato weevil Cylas formicarius (Fab). Tropical Pest Management 32: 304–315.
  • Wang, Y., and S.J. Kays. 2002. Sweet potato volatile chemistry in relation to sweet potato weevil (Cylas formicarius) behavior. Journal of the American Horticultural Society 127: 656–662.
  • Wilson, D.D., K.C. Son, S.F. Nottingham, R. F. Severson, and S.J. Kays. 1989. Characterization of an oviposition stimulant from the surface of sweet potato Ipomoea batatas storage roots for the sweet potato weevil, Cylas formicarius elegantulus. Entomologia Experimentalis et Applicata 51: 71–75.


Authors

Schyler Thibodeaux
Christopher E. Carlton





A decayed sweet potato cut in half, showing a weevil invested interior

Sweet potato weevil infestation (Schyler Lee Thibodeaux).

A close-up side view of a small weevil beetle with a long snout and dark, shiny, textured wing covers; a 5 mm scale bar is shown below.

Adult sweet potato weevil, side view (Nathan C. Arey).

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6/13/2025

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6/17/2025 7:34:54 PM
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