Whitney Cranshaw, Colorado State University, Bugwood.org
Plutella xylostella (Linnaeus)
The diamondback moth (DBM) tends to be the smallest of three lepidopteran larva that infest crucifer crops in Pennsylvania (the other two species are the imported cabbageworm, and the cabbage looper). On a per larva basis, the diamondback moth causes less damage than the other two species, however, populations can be high enough to cause severe damage.
DBM overwinters as an adult in warmer climates. It probably overwinters in most years in the warmest parts of Pennsylvania, but may not overwinter in the cooler parts. Adults are weak fliers, but populations are known to migrate long distances and annually reinvade areas well into Canada. Immigration also occurs as eggs or other life stages on transplants, including immigration of insecticide-resistant strains. Care should be taken to ensure clean transplants. Adults initiate activity at dusk, and continue well into the night. New adults, which emerge from pupa in the morning, mate as early as the evening of the first day of emergence. Adults feed only on water drops or dew and are short-lived (12 to 16 days). In side view, adult wing-tips turn upward; in top-view, there is a weak pattern of diamonds down the back. Egg-laying starts soon after mating, and lasts for abut 10 days. Eggs are laid before midnight. About 250-300 eggs are deposited per female. Eggs are flat ovals, laid singly or in small cluster, typically in concavities of leaf surfaces. Eggs hatch in 4-8 days. Larvae go through 4 instars: the range of days per instar is 3-7; 2-7; 2-8, and 2-10 for the 1st - 4th instar. Larvae wriggle, move backwards, and spin down silk threads when disturbed. Larva can also be recognized by a body that is tapered on both ends, and a pair of prolegs at the tail end that forms a distinctive 'V'-shape.
Crop damage is caused by larval feeding. First instars mine leaf tissue, thereafter feeding occurs on the undersurface of the leaf, resulting in a windowpane from the remnants of the top leaf surface. The windowpane will dry, crack and fall out over time. Pupation lasts 5-15 days in a loose silk cocoon. Total development from egg to pupal stage ranges from 17-51 days, and averages 25-30 days.
DBM is a specialist on both cultivated and wild plants in the crucifer family, but does have preferences within this plant family. Cultivated areas hold the largest populations, but weedy mustards, yellow rocket, etc., can help maintain populations. The plant chemicals that are common in this plant family are used by DBM as movement arrestants, feeding stimulants, and egg-laying stimulants. Mustards and collards are preferred egg-laying sites and are sometimes used to concentrate populations, or to help bring populations into a test plot. It may be feasible to use these in a trap crop practice. Intercropping may influence host-seeking or egg-laying behavior. Some literature reviews suggest that intercropping tests have not been consistent; others suggest that crops like white mustard (Brassica hirta) or rape (B. juncea) can be quite trap crops - an example planting would be a band of trap crop every 15-20 rows of cabbage. DBM oviposited in the trap crop, and parasitism rates can be high in those crops. Host plant resistance occurs on glossy-leaf cabbages (cabbages without the waxy bloom, so leaves are green).
DBM is notable among our pests in just how much populations can suffer large amounts of mortality from natural enemies when those natural enemies are in the farmscape. Parasitoids have repeatedly been extremely important for keeping populations of this insect in check. In fact, DBM was not considered a pest until widespread use of broad spectrum insecticides were in use. This probably killed off the parasitoids, while the DBM developed pesticide resistance - and DBM is one of our best at developing insecticide resistance. Parasitoid populations may be influenced by intercropping or other methods of providing nectar resources in the farmscape. Adult parasitoids live longer when provisioned with nectar. The larval parasitoids appear to be the most important, particularly Diadegma insulare, although there are many larval parasitoid species. Egg parasitoids have been used in a repeated mass-release method.
Rainfall is a useful mortality factor. Rainfall, and sprinkler irrigation, kills young larvae. Drip or furrow-irrigated crops tend to have higher populations.
Row covers, when placed on the crop prior to immigration of any life stage, are an effective control.
The sex pheromone used by females to call in males is known and available commercially for monitoring. It is a good management practice to put several DBM pheromone traps in each field, to help you know when moths are flying. At a higher concentration and distribution, scientists are experimenting with the sex pheromone to disrupt mating, with some notable success in Japan and Florida. However, pheromone disruption tests are difficult to conduct, and may be difficult economical if you still have to spray for other pest species. Results from Florida show dramatic reduction in sprays for DBM (from ~13-15 sprays down to 3 sprays) when plots are greater than 16 acres in size. It is unclear what the results would be in areas that do not have the type of pressure to warrant large numbers of sprays.
In addition to monitoring with a pheromone-baited trap, monitoring for larva or feeding damage, and limiting sprays to when thresholds are exceeded, consistently reduces the amount of insecticide applications. Thresholds tend to be very low for seedling establishment, rise considerably while there is vigorous vegetative growth, and then are reduced again when the marketable part of the crop (e.g., broccoli florets, cupping stage of cabbage) is growing. Counting from 40-50 plants is needed to give a good estimate of larval density. In Texas, 0.3 larvae per plant (1 larva per 3 plants) are tolerated; in Florida the threshold is 1 feeding hole per plant. The Commercial Vegetable Production Guide suggests a threshold of 20% infest cabbage plants prior to heading, then drops the threshold to 5% when heads begin to form.
Refer to the Commercial Vegetable Production Guidefor control recommendations.
Pesticides are poisonous. Read and follow directions and safety precautions on labels. Handle carefully and store in original labeled containers out of the reach of children, pets, and livestock. Dispose of empty containers right away, in a safe manner and place. Do not contaminate forage, streams, or ponds.
Authored by: Shelby Fleischer, Professor