The diamondback moth is a pest of cole crops, especially cauliflower. This pest was introduced from Europe before the middle of the nineteenth century and is now found everywhere its host crops are grown. The moth has trouble overwintering in cooler regions although trade winds disperse the moth throughout the United State during the growing season.

The adult diamondback moth is small, grayish-brown with fringed hind wings. Probably the best characteristic for identifying the adult is the pattern of white diamonds that is present in a line down the insect’s back when at rest. Both coloration and the manner in which the wings are held create this pattern. The eggs of the diamondback moth are round, white and smaller than a pencil point. Eggs are found in groups of one, two or in rare cases 3 and will most often be located on the underside of leaves adjacent to a large vein. The larvae are pale green with a black head capsule and sporadic black hairs. In contrast to some of the other cabbage worms the larvae of the diamondback moth will wiggle furiously when picked up. The larvae are capable of producing silk and may be observed hanging by a silken thread. The pupa is approximately one quarter of an inch long, covered with a loose silken cocoon and will frequently be found stuck to the plant.

The diamondback moth overwinters as an adult in the debris of the previous year’s crop. In the spring when temperatures increase the moth will mate and begin a host search. Oviposition occurs on the under side of leaves, usually immediately adjacent to a vein. Eggs are usually oviposted in groups of singles, doubles or triples although more than one group may be oviposited per leaf. Larvae will feed for approximately ten days before beginning pupation. The pupal case of the diamondback moth is made of silk and is used to adhere the pupa to the plant. An adult moth emerges from the pupal case in 5 to 10 days depending upon the temperature. In Pennsylvania the diamondback moth will have between 4 and 7 generations in a growing season.

The diamondback moth may be able to overwinter in the southern areas of Pennsylvania during mild winters. When populations successfully overwinter, it greatly increases both the number and intensity of moth generations. Following harsh winters it is unlikely that moths have successfully overwintered. During these seasons immigrating diamondback moths are the result of prevailing winds from the south. Due to the variability in Pennsylvania winters, it is impossible to predict which winters may enable overwintering.

Adult diamond back moths will feed in small amounts although the damage is so slight that it will often go unnoticed. It is the larvae of the diamondback moth that can cause significant economic damage.

Larvae have two distinct feeding styles. Shortly after emerging from the egg, larvae are extremely small and will feed by mining (feeding on the leaf tissue between the upper and lower epidermis) into the leaf. The larvae themselves are not visible during this time (because they are inside the leaf). The only evidence is a long tortuous white trail which is the result of underlying dead tissue. As the larvae grow they become too large to feed as a leaf miner. It is at this point that larvae begin to burrow through the leaf tissue resulting in many small holes in the leaf. This type of damage is most commonly called "shotgun damage" due to the appearance of many small "shotgun pellet" holes.

Economic damage is most severe after heading begins. Larvae will burrow into the developing head destroying its marketability. The recommended treatment threshold for diamondback moth depends upon the plant growth stage. From seedling to cupping the recommended threshold is when 20 percent of the plants are infested. After heading begins, the recommended threshold decreases to 5 percent infestation.

The corner stone of good pest management is monitoring. Optimal monitoring for the diamondback moth is the use of pheromone traps to monitor adults. When the adult population rises, begin scouting the plants for larvae and eggs.

When spraying, be sure to use a high-pressure air blast sprayer (>200 psi) with 3 hollow cone nozzles per row. One nozzle should point straight down into the row, while the other two spray from either side. Additionally, use a spreader-sticker to increase coverage. This procedure sprays the plants from 3 sides and reduces the pesticide shadow that occurs as a result of the change in topography between the leaf and the vein. Keep in mind that it is this region, adjacent to the veins, where the diamond back moth oviposits its eggs. For proper control it is critical to ensure that optimal spray coverage has been achieved.

In some cases, the intensity of summer infestations of the diamondback moth can be reduced by eliminating the overwintering habitat. Because the diamondback moth overwinters in the debris of the previous year's field, all plant material should be either plowed under or raked to a central location and burned. Additionally, fields should be rotated as far as possible between years. This makes it more difficult for adults who have managed to overwinter to find the current year's crop.

For additional treatment options see the current Pennsylvania State University Vegetable Production Guide. (http://www.ento.psu.edu/vegetable/default.htm)

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