Diamondback Moth

Host Plants

Diamondback moth larvae feed on all cole crops (plants belonging to the mustard family), including broccoli, cauliflower, cabbage, and rutabaga.

Regions impacted

Found throughout North America (particularly the Canadian prairies and north central United States), South America, and Europe.

Description

• The adult moth is small at roughly a 1/3 inch long with a wing span of ½ to one inch. The male is dark brown and sports three light-colored diamond patterns on his back. The female moth is lighter in color, and its diamond markings are more muted. When sedentary, the moth folds its wings in a tent-like manner; this is when the 3 diamond markings are most prominent. The moths are not particularly strong fliers but, when disturbed, will flutter a few feet off the ground for short distances.
• Diamondback eggs are oval, pale yellow/white, and measure less than 3/16th of an inch. The female deposits the eggs singly or in groups of two or three, usually in depressions or irregularities on the surface of leaves or other plant parts.
• The larvae or caterpillars are about ½ an inch long and tapered at each end. They are pale yellow to green with scattered fine hairs.
• Larvae pupate in a white silk cocoon, usually formed on the leaves of the host. Pupae start light green but turn brownish as the maturing adult moth becomes visible through the cocoon.

Life Cycle

• The diamondback moth overwinters as an adult in milder climates. In the early spring, females lay eggs on leaf surfaces which hatch in about five or six days.
• The larvae burrow into the leaves, feed for a week or two, and then emerge onto the host’s exterior.
• The larvae complete four larval stages lasting 10 to 20 days, after which they spin a white silk cocoon on the lower portion of the host plant.
• Total development time from the egg to pupal stage averages 25 to 30 days, depending on climate and feeding conditions.
• The number of generations averages 3 to 5, with climate as the primary factor.

Damage and Detection

• Plant damage is caused by moth larvae feeding on leaves, stems, buds, and flowers.
• They generally attack the area between the larger veins and the leaf’s midrib. The amount of damage varies based on the larvae infestation density and the plant growth stage.
• When larvae are tiny, the damage is visible as small holes in the leaves.
• If larvae are numerous, the entire leaf may be eaten, leaving only the veins.
• Most serious damage is caused by the caterpillars burrowing into the heads of plants such as cabbage and cauliflower, causing them to be stunted or/or malformed (or aborted altogether).

Prevention and Control

Cultural controls

• Moths can travel with transplants – inspect and ensure they are free of insects before planting.
• Inspect crops at least once a week during the warmer months.
• Clear and destroy weeds around the crop to reduce the survival of larvae.
• If possible, utilize overhead sprinkler irrigation, as rainfall appears to be a factor in suppressing young larvae populations.

Biological controls

• Helpful agents such as predators and wasp parasitoids can be part of the solution – encourage their presence.
• Diamondback moth is known to have numerous natural enemies that attack all life stages of this pest. Adult moths are often attacked by birds and spiders, while ants, lacewings, big-eyed bugs, and staphylinids beetles are reported to cause mortality in the larvae.
• Use of trap crops: A trap crop, or sacrificial crop, is a plant that attracts garden pests, usually insects, away from your nearby crop. Collards and tomatoes have been found effective at attracting Diamondback moth away from cabbage crops.

Host Plant Resistance

Host crops have varying degrees of natural resistance to attack by the diamondback moth, but a key factor is the presence or degree of leaf wax.

Insecticides

Adequate crop protection will likely require the regular application of insecticides. Resistance to insecticides, however, is strong, including with Bacillus thuringiensis (Bt) products. The use of Bt is strongly recommended, given it spares helpful parasitoids, but it, like other insecticides, will require frequent rotation to other products and chemical mixes.