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Diamondback Moth larvae can damage several cole crops in the Willamette Valley by chewing out holes in the foliage and contaminating the heads. This page provides some basic information about the pest and discusses management and controls methods. Detailed scouting and monitoring techniques are provided as well as a risk assessment for spray decisions.
Diamondback Moth (Plutella xylostella)
Diamondback Moth pupa Diamondback Moth adult (Photos by Ken Gray)
Diamondback Moth larvae and “window panes” associated with their feeding (Photo by Dan McGrath)
How to ID Pest
Diamondback larvae are smaller than most caterpillars that infest broccoli and cauliflower, about 8 mm when fully grown. Diamondback moth larvae are ready to move up into the broccoli and cauliflower to form their pupa when they are about one half inches long. The larva has two legs (prolegs) on the last segment forming a V-shape at the rear end. A very distinctive feature of this pest is when disturbed, the larvae wiggle frantically backwards and drop off the leave attached to a silken line. This behavior cue is useful for identifying the small larvae in the field. Adult moths are small, slender, and grayish brown. Male moths display three ivory diamond-shaped markings on their back. Although you will see pictures of diamondback eggs in the literature, they are very difficult to detect in the field. The eggs are laid closely oppressed to the under surface of the leaves. When the tiny larvae hatch, they immediately begin to mine the leaf mesophyll below the egg. This produces tiny depressions in the leaf surface called “window panes”. The larvae leave the upper surface of the leaf (epidermis) intact. This feeding behavior is unique to the diamondback larvae. When scouting a field, the percentage of leaves with fresh of window panes is an indication of egg laying pressure by the diamondback moth.
During a normal winter, the adult diamondback moth overwinters in trash and debris in and around crop fields. Adults emerge in early summer and lay eggs singly or in twos or threes mainly on the upper sides of leaves. The eggs hatch in 4 to 8 days. Larvae feed mostly on the undersides of outer or older leaves of older plants, chewing out small holes, and mature in 10 to 30 days. They then spin loose white cocoons, which they attach to leaves, stems, and the heads of broccoli and cauliflower, where they pupate. Adults emerge in 10 to 14 days. There may be four to six overlapping generations each year.
The 45th parallel latitude occurs in Salem, Oregon. Above the 45th parallel, the diamondback moth typically does not survive the winter. But this depends on the winter. In some winters, the moth population dies out and must reinvade the Willamette Valley from the south. In many cases, when the diamondback populations die out, it reestablishes itself in the valley by riding in on brassica seedlings imported from California or it simply flies north one planting at a time. In either case, when the diamondback population has to reinvade the Willamette Valley, the population gets a late start. Fewer generations occur during the growing season and pest pressure is diminished.
During a mild winter when the diamondback population survives the winter here in the Valley, the moths get an early start. With increasing acreages of overwintering radish and cabbage seed crops, we see more incidences of multiple generations of diamondback moth per season. This can increase pressure from this insect pest and raises concerns about the management of insecticide resistance.
Crops Affected & Damage
Cabbage and Broccoli are the preferred host of this pest, but it also attacks other Cole family crops. Larvae feed mostly on outer or older leaves of older plants, chewing out small holes, or at the growing points of young plants. They also feed on floral stalks and flower buds. In Oregon, the damage from this pest is not so much from feeding as from contamination by pupae. Late stage instars crawl into stems infesting the harvested portion of broccoli and cauliflower crops.
"Window pane" damage from Diamondback Moth larvae (Photo from the Ministry of Agriculture Food & Rural Affairs)
Scouting & Monitoring
Scouting for Diamondback Moth larvae and pupa can be done by pulling 10 leaves from 10 different spots in the field, totalling 100 leaves. Inspect for signs of “window panes”, larvae and pupa on each leaf.
Wing traps with a pheromone lure and a sticky bottom can be used to monitor adult Diamondback Moth populations. Place the traps just above the crop canopy.
Wing trap with pheromore lure (Photo from North Dakota State University)
Sex pheromone traps are useful tools for detecting the flights of the adult diamondback moth. Researchers recommend that wing trap or delta trap styles with sticky inserts be used to capture adult moths. Pheromone traps should be suspended about 3 to 5 feet (1 to 1.5 meters) high at the field’s edge (Figure 9). Traps indicate when moths have arrived in an area and give an indication of their relative numbers. High numbers of adults (>100 moths per week per trap) captured in the traps during bloom to early pod development provide an early warning that significant larval infestation may follow. Fields should be monitored for larvae then to assess the numbers of larvae present.
Risk assessment for Diamondback larva contaminating broccoli and cauliflower should be made using a combination of pheromone trap counts (an indication of egg laying pressure) and field scouting. During field scouting, the scout moves across the field pulling a leaf from a plant every time she steps across a row. Scouts generally pull ten leaves, examine them, then pull an additional ten leaves. Chose leaves that are not too small and not too big. leaves should be mature and unfolded, but not old and discolored. The larva count, the larva instar or developmental stage, and the development stage of the crop should all be considered. The greatest risk of contamination by diamondback moth occurs when the larva are numerous, the larva are mature (about one half inch long) and the broccoli or cauliflower buds are expanding and elongating. The risk is that the fourth instar diamondback larva will crawl up into the elongating heads and form their pupa there.
Natural Control: A number of natural factors can affect populations of diamondback moths negatively. For example, heavy rainfalls and irrigation can drown many small larvae (first or second instar). Humid conditions associated with rainfall or irrigation can favor the development of lethal fungal diseases, such as Entomophthorales. In addition, cool, windy weather reduces adult activity and females often die before they lay all of their eggs.
Biological Control: Various parasitoid wasps -- Diadegma insulare (Cresson), Microplitis plutellae (Muesebeck), and Diadromus subtilicornis (Gravenhorst)] and the egg parasite Trichogramma pretiosum can be important factors in controlling diamondback moth populations. In addition, fungal diseases play an important role in controlling moth populations. Disease outbreaks typically occur later in the growing season when diamondback moth populations are larger and weather conditions are more favorable for the fungi to develop. Generalist insect predators (predaceous arthropods), such as ground beetles, true bugs, syrphid fly larvae, lacewing larvae and spiders play a small but important role in reducing diamondback numbers.
Cultural Control: Young larvae are particularly susceptible to heavy rainfall and sprinkler irrigation.
Chemical Control: Bt (derived from the bacterial Bacillus thuringiensis) is a microbial protein that is poisonous to moths and butterflies. It is biologically rational in the sense that it is highly targeted. It is a stomach poison ingested only by insects attaching the crop plant. Bt is not highly toxic to other species of insects. It is not highly toxic to mammals. It has a short residual life in the environment. Bt, however, is a gentle insecticide that may not control major outbreaks of this pest.
The effectiveness of Bt is enhanced when it is applied according to pheromone moth trap counts, in other words, early in the development of the diamondback larvae. Apply Bt when the larvae are small. When infestation levels are low, Bt can be used to help conserve beneficial insects. When infestation levels are low, Bt can be used in combination or in rotation with other insecticides, thus providing a different mode of insecticidal action than other chemicals. This can help slow the development of insecticide resistance in diamondback populations.
Please consult the PNW Insect Management Handbook for alternative pesticide recommendations.
References and Citations
"Diamondback Moth Factsheet". Oregon State University. Accessed December 9, 2011. (http://insects.ippc.orst.edu/pdf/reb39.pdf).
Fleischer, Shelby. 2009. "Diamondback Moth." Pennsylvania State University College of Agricultural Sciences, Etomology. (http://ento.psu.edu/extension/factsheets/diamondback-moth).
Hollingsworth, Craig S. (Ed.). 2011. Pacific Northwest Insect Management Handbook. Corvallis: Oregon State University.
Natwick, E.T. 2010. "UC IPM Pest Management Guidlines: Cole Crops, Diamondback Moth." University of California Agriculture & Natural Resources. (http://www.ipm.ucdavis.edu/PMG/r108301311.html).
Fresh Market Vegetable Production, Insect management, Processed Vegetable Production, Willamette Valley, Broccoli, Cabbage, Diamondback Moth, Cauliflower, VegNet