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Diamondback moth (Plutella xylostella) was introduced into North America from Europe about 150 years ago. It now occurs throughout North America, wherever its host plants are grown. Diamondback moth larvae feed on all plants in the mustard family (canola, mustard), cole crops (broccoli, cabbage) and on several greenhouse plants. In Western Canada, canola and mustard are its primary targets.
Although the diamondback moth occurs each year throughout the Canadian prairies and north central United States, the severity of the infestation varies considerably from year to year.
Diamondback moth has four life stages: egg, larva, pupa and adult. Crop damage is caused by the larval stage.
Normally, the diamondback moth takes about 32 days to develop from egg to adult. However, the time to complete a generation may vary from 21 to 51 days depending on weather and food conditions. There may be several generations per growing season. Generations usually overlap and all four life stages may be present in the field at the same time.
The adult moth is approximately 8 to 9 mm (1/3 inch) long with a wing span of 12 to 15 mm (½ inch).
At rest, the moth folds its wings over the abdomen in a tent-like manner. The folded wings flare upwards and outward at the tips. The wing tips are fringed with long hairs.
In the male, the forewing margins have a series of yellow wavy markings. When the wings are folded while the moth is at rest, these markings come together to form three yellow diamonds, hence the name diamondback.
Adult females lay an average of 160 eggs during their life span of about 16 days. Egg-laying occurs at night. The greatest number of eggs are laid the first night after emergence and egg-laying continues for about ten days.
In the field, moths will flutter up out of the canopy as the canopy is disturbed.
Figure 1, Adult of diamondback moth
Eggs are oval, yellowish white and tiny. They are glued to the upper and lower leaf surfaces singly or in groups of two or three, usually along the veins or where the leaf surface is uneven. The eggs hatch in about five or six days.
Immediately after hatching from the egg, larvae burrow into the leaf and begin mining the leaf tissue internally. After feeding within the leaf for about a week, the larvae exit from the underside of the leaf and begin feeding externally.
The larvae are pale yellowish green to green caterpillars covered with fine, scattered, erect hairs. The posterior end of the caterpillar is forked.
Larvae moult three times during the larval stage which lasts about ten to 21 days, depending upon temperature and the availability of food. At maturity the larvae are cigar-shaped and about 12 mm (½ inch) long.
The diamondback moth larva is easily identified by its peculiar reaction to being disturbed. It will wriggle backward violently and may drop from the plant, suspended by a silken thread. After several seconds, the larva will climb back onto the leaf and continue feeding.
Figure 2. Pupa (left) and larva (right) of diamondback moth
Larvae pupate in delicate, white, open-mesh cocoons attached to the leaves, stems or seed pods of the host plant. Initially, the pupae are light green but as they mature, they become brown as the adult moth becomes visible through the cocoon. The pupal stage lasts from five to 15 days, depending on environmental conditions.
An infestation of diamondback moths cannot be predicted based on the previous year's population because very few, if any, pupae survive the long, cold Canadian winters. Instead, the severity of the infestation in any given year depends on two factors: overwintering populations to the south and strong south winds to transport the moths north into Manitoba, central Saskatchewan and eastern Alberta in the spring.
In years when conditions are right for the moths - that is, when moths arrive on the wind in large numbers in early May and summer temperatures are hot - diamondback moth infestations can cause millions of dollars of damage.
Diamondback moth larvae feed on leaves, buds, flowers, seed pods, the green outer layer of the stems and occasionally the developing seeds within the older seed pods of canola and mustard. The amount of damage varies greatly, depending on plant growth stage, larval densities and size.
When larvae are small, damage is evident as small irregular holes or "shot holes" in the leaves. If larvae are numerous, they may eat the entire leaf, leaving only the veins.
When plants begin to flower, larger larvae often feed on the flower buds, flowers and young seed pods. Feeding damage during the early flowering stage can be extensive. Extensive feeding on the flowers will delay plant maturity, cause the crop to develop unevenly and significantly reduce seed yields.
When plants are fully podded and leaves begin to wilt or die in late July or early August, larvae will remove the surface tissue from the stems and seed pods. The seeds within a damaged pod will not fill completely and pods may shatter. Larvae may also chew into seed pods and eat the developing seeds.
Crop damage is usually first evident on plants growing on ridges and knolls in the field. Damage can only be prevented by early field monitoring and the application of insecticides, if larval numbers exceed the action threshold.
The presence and relative abundance of the diamondback moth can be determined by using pheromone-baited traps. These traps cannot predict the potential for crop damage, but trap counts can provide an early warning if high levels of the adult moths blow, and indicate regions that may be at higher risk. Environmental conditions will determine how many eggs are laid, and weather and natural enemies will determine how many larvae emerge and survive.
Monitor diamondback moth larvae by removing the plants in an area measuring 0.1 square metre area (about 1 foot square), beating them on a clean surface, and counting the number of larvae dislodged from the plants. To obtain an accurate count, repeat this procedure in at least five locations in the field. Crops should be monitored at least once a week during the growing season, and more frequently if populations approach economic levels.
Seedling stage: A nominal threshold of 25-33% defoliation, with larvae still present on plants, can be applied for canola at the seedling stage.
Immature to flowering plants: Control may be required in canola if larvae exceed 10-15 per ft2 of plants (100-150/m2) in immature to flowering plants.
Plants with flowers and pods: Control may be required in canola if larvae exceed 20-30 per ft2 (200-300/m2) in plants with flowers and pods.
Thresholds at all crop stages may be lower for Polish type canolas than for Argentine type canolas and higher for mustard.
Cool, windy weather reduces adult activity and females often die before they lay all their eggs. Heavy rainfall can drown small larvae and reduce numbers by more than half. Humid conditions within the crop following a rainfall can promote the spread of fatal fungal diseases throughout the diamondback moth population.
Diamondback moths are affected by diseases, parasites and predators.
Entomophthorales fungi cause natural disease outbreaks in diamondback populations. These outbreaks usually occur late in the growing season when populations are high. The rate of infection of diamondback moth larvae can be high enough to limit the development of additional generations late in the season.
In Western Canada, three species of parasitic wasps attack the diamondback moth. Diadegma insulare (Cresson) and Microplitis plutellae (Muesebeck) attack the larval stages while the third species, Diadromus subtilicornis (Gravenhorst), attacks the prepupal and pupal stages.
Flies, wasps, lacewings, plant bugs, pirate bugs, beetles, spiders and birds also prey on the diamondback moth larvae.
Despite the abundance of potential biological control agents, the only effective way of controlling a diamondback moth outbreak once the population exceeds the action threshold is to apply an insecticide.
Insecticide selection will depend on cost, environmental conditions, days to harvest, availability of product, the presence of other pests, and the presence of pollinating insects. The recommended insecticides are listed in the table below.
Insecticides should always be applied with enough water to ensure adequate coverage. Use high water volumes and label rates when the crop canopy is dense.
Injury to honeybees and other pollinating insects can be minimized by not spraying flowering crops. When it is necessary to apply an insecticide to a flowering crop, use Coragen, XenTari, Decis, Matador or Silencer; and apply it during the evening.
Table 1. Insecticides Registered for the Control of Diamondback Moth on Canola or Mustard in Canada.
|Product||Rate of product per acre||Preharvest Intervals (days)|
|XenTari||202 - 405 g||0||-|
|Decis* 5.0 EC||40-60 ml||7||7|
|Lorsban/Pyrinex/Nufos/ Citadel||405-607 ml||21||--|
|Malathion 500||220-340 ml||7||--|
|Malathion 85E**||105-168 ml||7||7|
*Do not apply at temperatures higher than 25ºC ** Do not apply at temperatures lower than 20ºC
For more information contact: John Gavloski, Entomologist, Manitoba Agriculture, Food and Rural Development, Crops Knowledge Centre, Carman, Manitoba, Canada, 204-745-5668.