How to Manage Pests

UC Pest Management Guidelines

DESCRIPTION OF THE PEST

Diamondback larvae are small (about 0.33 inch when full grown) compared to other caterpillars in cole crops. The larval body is wider in the middle and tapering at both ends with two prolegs on the last segment forming a distinctive V-shape at the rear end. When disturbed the larvae wiggle frantically or rapidly attach a silken line to a leaf and drop over the edge. They feed mostly on outer or older leaves of older plants chewing out small holes or at the growing points of young plants. They will also feed on floral stalks and flower buds. Larvae mature in 10 to 14 days and spin a loose cocoon on leaves or stems for pupation. Adult moths lay their tiny, roundish eggs singly on the undersides of leaves; eggs are difficult to find. Although they may occur all year round, especially in coastal areas, diamondback moths are often abundant in spring and early summer, and populations may rise again in fall.

DAMAGE

Diamondback moth infestations are most serious when they damage the crowns or growing points of young plants or Brussels sprouts. This injury can severely stunt growth. Sometimes diamondback moth caterpillars may also bore into heads of broccoli or cauliflower, or in the flower buds of stalks, causing economic injury and contamination. Injury to leaves is not usually serious, except when the wrapper or cap leaves of cabbage are injured.

MANAGEMENT

Natural enemies and insecticides applied to control other pests keep the diamondback moth under satisfactory control in most fields in California, but keep records of diamondback moth as you monitor for other caterpillars.

Biological Control
Natural enemies often effectively control diamondback moth in California. In southern California, the ichneumonid wasp, Diadegma insularis, has been identified as the most common parasite. Trichogramma pretiosum may also attack diamondback eggs. Various predators such as ground beetles, true bugs, syrphid fly larvae, and spiders can be important factors in controlling populations. Microbial diseases are not known to be a significant mortality factor.

Organically Acceptable Methods
Biological control and sprays of Bacillus thuringiensis and the Entrust formulation of spinosad are organically acceptable management tools.

Monitoring and Treatment Decisions
Check fields during the seedling stage, at thinning, and just before heading. Also, record diamondback larvae numbers when you make your twice-weekly samples for other caterpillar pests. In cabbage fields, regularly monitor wrapper leaves for damage after heading. Adult moths frequently migrate from fields being harvested or disced under, so carefully check border rows if populations were high in adjacent fields. No treatment levels have been developed for diamondback moth in California; however, treatment may be required if significant injury to growing points is occurring.

PUBLICATION

UC IPM Pest Management Guidelines: Cole Crops
UC ANR Publication 3442
Insects and Mites
E. T. Natwick, UC Cooperative Extension, Imperial County
Acknowledgments for contributions to Insects and Mites:
W. J. Bentley, UC IPM Program, Kearney Agricultural Center, Parlier
W. E. Chaney, UC Cooperative Extension, Monterey County
N. C. Toscano, Entomology, UC Riverside

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