The yellowmargined leaf beetle (Microtheca ochroloma) was first recorded in the U.S. in Mobile, AL, in March 1947. The beetle is now distributed along the Gulf Coast from Florida to Texas as well as in Georgia and North Carolina. The adult beetle is about 5 millimeters long and predominately dark brown, bronze, or black. The margins of the elytra or hardened forewings characteristic of beetles are marked with a margin of yellow or brown, a characteristic which gave this species its common name. Each elytra also has four rows of deep punctures.
The yellowmargined leaf beetle is a particular problem on Chinese cabbage and other leafy brassicas in the Glades, especially for organic growers. Most damage occurs in the spring when both the larvae and adults are found feeding on crucifers, where they feed on the foliage and leaf margins, making small holes. Adults and larvae often defoliate the host. Larvae, especially early instars, work in groups to strip individual stems. Other crops affected include broccoli, cabbage, cauliflower, collards, mustard, radish, turnip, and watercress.
Survival And Spread
Under favorable conditions, the beetle can complete its life cycle in less than one month. In Florida, adults remain active throughout the winter and more than one generation may occur per year during mild Gulf Coast winters. The egg is bright orange, elongate, and laid singly or in small groups on plant stems, under fallen leaves, or on the soil surface. The egg stage lasts about four to five days. The larva is grayish to yellow-brown, covered with a fine layer of hairs, and has a dark, sclerotized head capsule that is brown or black. The mature larva spins a peculiar blackish network around itself prior to pupation. Pupal cases are attached to the undersides of leaves, and their dark color stands out against the green foliage. The pupal stage lasts five to six days. New adults stay in the pupal case for about two days before emerging. Larvae are gregarious during their early instars, but become solitary later. Mature larvae spin loose, net-like pupal cases on foliage.
One reason the yellowmargined leaf beetle is such a problem to growers in Florida: The beetle’s host plants thrive in the cool months from October to April, and this period comprises the growing season for leafy brassicas in South Florida. During these months, hard frosts or freezes are rare and the adult beetles can continue to feed and reproduce throughout the winter on an ample food supply. The yellowmargined leaf beetle is an introduced pest and has no known predators or parasites in the U.S. The yellowmargined leaf beetle is not a major problem for conventional growers, as it usually is controlled by foliar insecticides used against other insect pests. It can be a bigger problem for organic growers who cannot use these chemical insecticides.
By Gene McAvoy, Regional Vegetable Specialist IV, University of Florida/IFAS