Diamondback moth, Plutella xylostella

Photo courtesy of Alton Sparks, University of Georgia

The diamondback moth’s caterpillar larvae chew small circular holes in leaves from the undersides, giving the leaves a shot-hole appearance. Very high populations can defoliate plants. It is a particularly avid attacker of cruciferous vegetable crops, like cabbage, kale and watercress. Affected flowers include perennials like candytuft and annuals like sweet alyssum.

Appearance

The adult is a small, slender, grayish-brown moth with pronounced antennae. It is about 6 mm long and marked with a broad cream or light brown band along the back. The band is sometimes constricted to form one or more light-colored diamonds on the back, which is the basis for the common name of this insect. The larvae are colorless in the first instar, but thereafter are green. The yellowish pupa is 7 to 9 mm in length.

Life cycle

The diamondback moth has four instars. Initially, the feeding habit of first instar larvae is leaf mining, although they are so small that the mines are difficult to notice. The larvae emerge from their mines at the conclusion of the first instar, molt beneath the leaf, and thereafter feed on the lower surface of the leaf. Pupation occurs in a loose silk cocoon, usually formed on the lower or outer leaves. The duration of the cocoon averages about 8 ½ days (range 5-15 days). Adult males and females live about 12 and 16 days, respectively, and females deposit eggs for about 10 days. Total development time from the egg to pupal stage averages 25 to 30 days, depending on weather, with a range of about 17 to 51 days. The number of generations varies from four in cold climates such as southern Canada to perhaps 8 to 12 in the south.

Cultural control

Because these pests feed on a large variety of plant species, keep production areas free of weeds (e.g., mustards) that serve as hosts to diamondback moths. For greenhouse operations, exclusion of winged adults can be accomplished by covering openings to greenhouses with screens. Screens are especially important when lights are used at night in greenhouses to control flowering because lights attract adult moths. Individual seedling flats may also be covered with screens to exclude adults and larvae. Row covers can be a practical measure to exclude moths in field production as long as the mesh prevents entry of adults, and the row cover is held above the plant surface to eliminate oviposition through the fabric. Also, intermittent overhead irrigation can disrupt oviposition by diamondback moth.

Scouting and treatment

If Bt sprays are planned, use pheromone traps to determine adult flight activity and mating. Once adults are caught in traps, it is very likely that larvae are present and Bt should be applied as soon as possible because it is most effective against young larvae. Use regular visual inspections of plants to detect larvae and their damage. Diamondback moth may be resistant to many insecticides.

Biological control

A number of parasites, both tachinid flies and parasitic wasps, attack Lepidoptera larvae and reduce their population growth rate. Viruses do not usually kill the larvae until later instars. The parasitic stingless wasps Cotesia plutellae, Diadegma insulare and Microplitis plutellae are commercially available for control of diamondback moth. Applying insecticides other than Bacillus thuringiensis (Bt) products are likely to exclude parasites because the residues are lethal to these beneficial insects.

Source: University of California Agriculture & Natural Resources IPM department, University of Florida Entomology Department