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Diamondback Moth

Diamondback moth (Plutella xylostella) 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 was introduced into North America from Europe about 150 years ago. It now occurs throughout North America, wherever its host plants are grown. The most serious recent infestations occurred in 1995 and 2001. 

Life Cycle

Diamondback moth has four life states: 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 and generations usually overlap.  Therefore after the first generation all life stages may be present in the field at the same time. This can cause problems with optimizing control that should be targeted at the destructive larval stage.

Adults

Figure 1 - Adult diamondback moth at rest

The adult moth is approximately 8 to 9 mm (1/3 in.) long with a wing span of 12-15 mm (1/2 in.) (figure 1).

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 following emergence from the pupa although egg-laying can continue for about 10 days.

In the field, moths will flutter up out of the canopy as the crop is disturbed.

Eggs - 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.

Larvae

Figure 2. Diamondback moth larvae

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 (figure 2). The posterior end of the caterpillar is forked.

Larvae moult three times during the larval stage which lasts about 10 to 21 days, depending upon temperature and the availability of food. At maturity the larvae are spindle-shaped (broader at the middle and tapering at both ends) and about 12 mm (1/2 in.) long.

The diamondback moth larva is easily identified by its peculiar reaction to being disturbed. It will wriggle backwards 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.

Pupae

Figure 3. Diamondback moth pupa

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 (figure 3). The pupal stage lasts from five to 15 days, depending on environmental conditions.

Damage

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 depend on two factors – over-wintering populations to the south and strong south winds that transport the moths north into Manitoba, Saskatchewan and 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 can go through multiple generations and infestations can cause millions of dollars of damage.  In 2001 the moths arrived early, in high numbers and resulted in severe infestations crops across the Prairie Provinces.  The early arrival and favourable environmental conditions resulted in four generations in some areas.

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.  For example, the first generation larvae of the 2001 infestations were found feeding on early growth stages of canola in late May and resulted in stunted plant growth.

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 considerable. 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.

Pest Management

Monitoring

  • Adult - The presence and relative abundance of the diamondback moth in an area can be determined by using pheromone traps. These traps cannot predict the potential for crop damage, but trap counts can provide an early warning of a possible infestation. Environmental conditions will determine how many eggs are laid and whether the larvae emerge and survive.

  • Larvae - Monitor (late May through early September) diamondback moth larvae by:

    • Observing the plants in early stages for larvae feeding on the leaf tissue or

    • Beating the plants in an area measuring 50 cm x 50 cm  and counting the number of larvae dislodged from the plants.

  • To obtain an accurate count, repeat the procedure in at least five locations in the field. Crops should be monitored at least twice a week during the growing season.

Natural Controls

  • Weather - Natural factors can have a profound impact on diamondback moth populations. Cold, 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.

Biological factors

  • Diamondback moth 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.

Chemical Control

  • 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.

Action threshold

  • Canola, mustard - threshold numbers based on crop stands averaging 150 to 200 plants / m2. In areas where stands are thinner the action threshold should be adjusted accordingly.  A nominal threshold of 25 – 33% defoliation with larvae still present can be applied for canola in the seedling and rosette stage.
    • immature and flowering fields – 100 to 150 larvae per m2
    • flowering and podded fields – 200 to 300 larvae / m2
  • An insecticide application is recommended when larvae counts reach or exceed these levels.
  • A single, well-timed application of an insecticide registered for diamondback moth with either aerial or ground equipment is usually effective in controlling larval populations. Insecticide applications should be made when larval populations are high, because the effectiveness is reduced against adults or pupae.  Although the insecticide will kill adults, since the majority of the eggs are laid within the first 24 hours this strategy is unlikely to have a great effect on reducing the population.  Insecticides have little effect on eggs or pupae.
  • 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 a synthetic pyrethroid class of insecticide and apply it during the evening.
  • Insecticide selection will depend on cost, environmental conditions, days to harvest (pre-harvest interval), availability of product, the presence of other pests, and the presence of pollinating insects.

For information on insecticides registered for diamondback moth refer to the Guide to Crop Protection.

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