Diamondback Moth

The life stages of diamondback moth (family Plutellidae) are egg, larva, pupa, and adult. First instars are colorless or pale, tiny larvae (caterpillars) that mine leaves. Older larvae feed externally on buds, leaves, and terminals. Older larvae are grayish or pale green overall with numerous, pale tubercles (bumps), each with a tiny, black bristle. The larval body tapers at both ends and grows up to 2/5 inch long. Larvae have five pairs of prolegs (fleshy stubs on the abdomen) and a rear pair that protrude so when the caterpillar is viewed from above they form a distinct V-shape. Larvae when disturbed wriggle and commonly drop from plants on a silk thread.

Adults (moths) are 1/4 inch long with a wingspread of 3/5 inch. Adults have long, forward-projecting antennae and are slender and mostly brownish with blackish and pale markings. At rest where wings meet over the back most adults have a broad, pale, lengthwise band. In some individuals portions of the cream-colored, pale brown, or whitish band constrict into one or more diamond shapes, hence the species common name.

Females lay minute eggs, each about 1/50 inch (0.5 mm) long. Eggs are laid singly or in groups of several on the underside of foliage and in existing depressions in leaves. Each female lays an average of about 75 eggs during her life. Pupae are green to yellowish, about 1/3 inch long, and occur in a loose, silken cocoon commonly on the underside of leaves.

Diamondback moth can feed and reproduce throughout the year in locations with mild winters. In California there can be up to 10 generations per year. Diamondback moths generally are most abundant in spring through early summer, then again during fall. Development from egg to adult (1 generation) occurs in about 29, 16, or 12 days when temperatures average 68°, 77°, or 87°F, respectively.

Diamondback moth larvae feed only on Brassicaceae (crucifers) including cole crops, mustard weeds, and cruciferous ornamentals. Ornamental hosts include candytuft, stock, sweet alyssum, and wallflower. Larvae chew small circular holes in the underside of leaves, giving foliage a shothole appearance. When infested as seedlings, plant growth rate is slowed, plant parts may grow out distorted, and plants can be killed. Abundant larvae can extensively defoliate older plants, leaving mostly leaf veins. First instars chew inside or on buds, including flower buds.

Prior to the 1950s diamondback moth was generally a minor pest; this was mainly attributed to high rates of parasitism. After World War II the extensive use of newer insecticides that are toxic to natural enemies coincided with a widespread increase in diamondback moth abundance and damage. Diamondback moth was the first crop pest reported resistant to DDT. In the 1980s its resistance to pyrethroid insecticides was first documented. Diamondback moth is now notorious for developing insecticide-resistant populations and has become a common, serious pest of crucifers nationwide.

Early in the growing season rely upon cultural and preventive controls including exclusion and sanitation, conservation of natural enemies, and the application of selective and semi-selective insecticides. For guidelines on when to treat, see Establishing Action Thresholds.

Natural enemies commonly control diamondback moth when broad-spectrum, persistent insecticides such as pyrethroids are not applied. The ichneumonid wasp, Diadegma insulare (=D. insularis), is the most common larval parasite in southern California. The wasps Cotesia plutellae and Microplitis plutellae also extensively parasitize diamondback moth larvae. Trichogramma spp. parasitize and kill the moth eggs. Predators including predaceous ground beetles and various true bugs also prey on immature diamondback moths. Viruses commonly kill the late instars.

Most natural enemies of larvae do not kill diamondback moth until the last instar, and parasitized or initially infected larvae will still damage crops. However, conserved natural enemies can greatly reduce the abundance of subsequent generations of the pest and its damage. Biological control reduces the migration between crops and into growing areas from alternative hosts because individuals of the previous generation do not survive to adulthood. To encourage biological control rely on caterpillar-specific Bacillus thuringiensis (Bt), somewhat selective products such as certain insect growth regulators and spinosad. Excellent coverage under leaves can improve control with these insecticides. Especially early in the crop cycle employ nonchemical controls and practice other natural-enemy conservation methods as discussed in Biological Control and Protecting Natural Enemies and Pollinators.

Keep production areas free of cruciferous weeds (e.g., mustards) that are alternate hosts of diamondback moth and certain other pests. When greenhouses or nearby areas are lighted at night, exclude migrating adults by covering openings with screens of sufficient surface area to allow adequate ventilation. To reduce moth attraction, switch from always-on night lighting to motion-activated security lights. Screen individual seedling flats to exclude migrating adults and larvae. Use row covers and exclusion netting to keep moths from field crops; hold row covers above plant surfaces with hoops or other supports to eliminate oviposition through the mesh. Ensure covers and screens are properly maintained.

Biological and cultural controls are organically acceptable management methods. The botanicals azadirachtin, neem, and pyrethrins without piperonyl butoxide, or PBO (PyGanic), entomopathogenic nematodes (Heterorhabditis and Steinernema spp.), and the microbial insecticides Bacillus thuringiensis ssp. aizawai, Bt ssp. kurstaki, and certain spinosad formulations (Entrust Naturalyte, Entrust SC) are acceptable for organic production.

Visually inspect hosts at least weekly to detect larvae and their chewing damage. Especially if Bt sprays are planned, use species-specific, pheromone-baited, sticky traps to determine when adults are flying. Adult captures indicate females are laying eggs and larvae are present or soon will be. At these times Bt applications are warranted as they are most effective against young larvae. Reapply Bt every 7 to 10 days while adults are trapped because Bt is not persistent and not all eggs are laid or hatch at the same time; rotate to another effective insecticide if populations increase.

Focus on excluding adults from growing areas and particular groups of plants and directly controlling the larvae. Especially early in the crop cycle, rely on exclusion, excellent sanitation, conservation of natural enemies, and the application of selective or semi-selective insecticides where feasible.

Diamondback moth has developed resistance to many insecticides, but the extent of resistance to particular modes of action varies among pest populations. For example, some populations may be resistant to spinosad, but others may still be susceptible. Applying ineffective, broad-spectrum insecticides and especially repeating applications of insecticides with the same mode of action can promote resistance in this and other pests while killing or disrupting the activities of natural enemies. If a thoroughly applied product does not control diamondback moth do not reapply that mode of action; switch to a different insecticide mode of action.

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