Diamondback Moth

Diamondback moth larvae (caterpillars) grow to be up to 0.3 inch long. They are wider in the middle and taper at both ends. The two prolegs on the last segment form a distinctive V-shape at the rear end. When disturbed, the caterpillars wriggle rapidly or attach themselves to a leaf by a silken thread and drop off, similar to the behavior of leafroller caterpillars. Mature diamondback moth caterpillars are smaller than other caterpillars that are common in cole crops.

The caterpillars mature in 10 to 14 days during warm weather and spin a loose cocoon and pupate on leaves or stems. Female adults (moths) lay tiny, roundish eggs singly on the undersides of leaves. Eggs are difficult to find.

Diamondback moth may be active throughout the year, especially in coastal areas. The pest is most numerous in spring to early summer and in the fall.

Larvae feed mostly on outer or older leaves of plants. Young stages rasp the undersides of the leaves, creating damage with a characteristic "window paning" appearance, in which the upper surface of the leaf remains intact and becomes transparent. Older larvae chew small holes or feed at the growing points of young plants and chew floral stalks and flower buds.

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 the caterpillars bore into flower buds or broccoli and cauliflower heads, causing contamination and economic injury. Injury to leaves alone is not usually serious, except when the wrapper or cap leaves of cabbage are injured.

Keep records of diamondback moth while monitoring for other caterpillars. Natural enemies and insecticides applied to control other pests might keep the diamondback moth under control in most fields in California. Keep in mind that broad-spectrum pesticides applied for other pests can sometimes increase diamondback moth infestations by eliminating natural enemies.

Rotate out of cole crops so that they do not grow in the same field in consecutive years. Plant all cole crops in a given field at the same time, rather than staggering planting dates between adjacent blocks of the same field. Blocks with differently aged cole crops enable diamondback moth to move through the field and cause sustained, continuous damage.

Natural enemies often control diamondback moth in California. In Southern California, an ichneumonid wasp, Diadegma insularis, is the most common parasitic wasp that attacks diamondback moth. Trichogramma pretiosum, another type of parasitic wasp, may also attack the moth eggs. Predators such as ground beetles, spiders, syrphid fly larvae, and true bugs can also play an important role in biological control of diamondback moth. Naturally occurring microbial diseases are not known to effectively control this pest. Consider planting insectary plants to attract natural enemies of diamondback moth.

Use biological control in an organically certified crop, as well as sprays of Bacillus thuringiensis and the Entrust SC formulation of spinosad.

Twice per week during the following periods, sample fields for caterpillars and record the number of diamondback moth larvae separately from other species:

• during the seedling stage or right after transplanting,
• at thinning, and
• just before heading.

In cabbage, regularly monitor wrapper leaves for damage after heading. Adults frequently migrate in from fields being harvested or disced, so carefully check border rows if numbers were high in adjacent fields.

No economic thresholds have been developed for diamondback moth in California. Insecticide application may be necessary if there is significant feeding on growing points.

Diamondback moth has developed resistance to multiple insecticides with different modes of action. Therefore, it is especially important to rotate insecticide modes of action to manage this pest. Do not apply more than two insecticides with the same mode of action within a 10- to 14-day period.