Fig. 1. Prickly pear cactus

Pricklypear cacti (Opuntia spp.) are being threatened by the accidental introduction of the cactus moth (Cactoblastis cactorum) into Florida and its expansion to Alabama on the Gulf Coast and South Carolina on the Atlantic Coast. This moth, native to Argentina, is capable of spreading into Mississippi and other states westward.

The caterpillars of this moth are capable of complete destruction of entire plants and stands of cacti (Fig 1). This exotic pest is expected to have a catastrophic eff ect on the landscape of the western states and Mexico if its range continues to expand.

Fig. 2. Eggstick

The female moth lays eggs on top of each other like pancakes to form an "egg stick" that resembles a cactus spine (Fig. 2). Each female can lay 60-100 eggs in a single egg stick and can lay 200-300 eggs within a few days. Egg sticks are about an inch long and are usually on the undersides or other protected parts of the plant pads. The egg stage lasts 3-4 weeks.

The caterpillars burrow into a pad after hatching and feed together as a group. The infestation by the lightly colored young larvae may be difficult to detect without splitting the pad open.

As the larvae mature, frass and sap may be pushed out of openings in the pad and onto the ground. Eventually the pad will become transparent and hollow. Larvae may move to additional pads to complete development, especially if the initially infested pad is small. Larvae mature in 4-5 weeks.

Mature larva are bright orange to red with black spots or bands around their bodies (Fig. 3 and 4). Mature larvae leave the plant and pupate under dead leaves or between the pads where they spin white cocoons. The pupal stage lasts 15-20 days.

Fig. 4. Cactus moth larva

Fig. 3. Group of larvae inside the pad

Native cactus moths in the genus Melitaria, also occur in southeastern and western United States, but they do not devastate cacti like the exotic cactus moth. Larvae of native cactus moths feed singly inside cacti pads, and are easily distinguished by their solid blue to purple color.

Fig. 5. Adult cactus moth

The cactus moth is gray with a zigzag line across the outer fourth of the forewing (Fig. 5). The span of both wings varies from slightly less than an inch to an inch and third, with the female being larger than the male. Th is moth has three generations in Florida with the first flight period during late March, second at the end of July, and third during mid-September-October. Larvae from the last generation spend the winter inside pads of the cactus.

More information on the cactus moth and its host plants, please visit GRI's cactus moth Web site at www.gri.msstate.edu/cactus_moth.

Photographs courtesy of J.S. Peterson, USDA-NRCS (Fig. 1) and
Lyle Buss, University of Florida (Figs. 2, 3, 4, 5).