(Lepidoptera: Tineidae or Plutellidae)

1. BIOLOGY AND ECOLOGY

The diamondback moth, Plutella xylostella (Plutella maculipennis of some earlier authors), is an occasional pest of canola across the prairies. Their name is derived from the series of diamond-shaped figures formed by these while marks when the wings are folded at rest. The 'diamonds' are present on both sexes but most obvious on the male moths.

The moths are small, 12 mm long, with an 18-20 mm wing-span. They are grey or brown, with tannish-white marks on the margin of the forewing. While at rest, the wings are folded, roof-like, over the body. They hold their antennae forward when at rest.

Minute, disc-shaped, pale-green or lemon-yellow eggs are laid singly, or in twos or threes, on either side of a leaf. A female may lay 100-300 scale-like, greenish-white to creamy-yellow eggs. The eggs are laid singly or in small groups.

The mature larva is a small, pale-green to bluish-green caterpillar, about 8 mm long. It has a narrow head and hindquarters (i.e., somewhat spindle-shaped). It has 5 pairs of logs on the rear half of the body.

Occurring throughout North America (Harcourt, 1956), diamondback moths occasionally migrate into southern Manitoba in May and June, laying their eggs on canola and other host plants.

The eggs, larvae, and pupae of these moths can be found o their host plants in the agricultural areas of southern Manitoba.

4. Hosts:

In canola growing areas, most of the crops will not have emerged by the time the moths arrive. Thus, many eggs are laid on cruciferous weeds and volunteer canola.

On the prairies, diamondback moths are most frequently observed in canola fields. Other favoured food plants include broccoli, Brussels sprouts, cabbage, cauliflower, horseradish, commercial mustard, radish, and turnip (Butts and McEwen, 1981; Burgess et al., 1983; Lasota and Kok, 1989; Stewart et al., 1990). They may also be seen occasionally on stinkweed, peppergrass, shepherd's purse, and wild mustards. They do not attack cereals.

1. Life Cycle.

The moths overwinter In the southern U.S.A. and Mexico in protected locations (e.g., beneath leaves lying on the ground). Diamondback moths are believed to migrate into southern Manitoba from the south in late May and June. In Europe and in the Far East, researchers have shown that these moths can travel, aided by winds, for up to about 1600 km.

Small, green larvae hatch within 3 to 6 days of the eggs being laid (Harcourt, 1957). They enter the leaves to feed on, or 'mine', the internal leaf tissue. After feeding within the leaf for about a week, the larvae move to the outside of the leaf. Larvae feed for 10-30 days, depending upon food supply and temperature, to reach a mature length of about 12 mm.

The smooth, pale yellowish-green larvae commonly feed on the undersides of leaves. When disturbed, the larvae will move backward rapidly, lashing the body violently. Some larvae may drop from the plant on a fine silken thread. They will remain hanging several centimeters below the leaf until the danger is past and use the thread to climb back onto the leaf.

Pupation takes place in delicate, whitish, loose net-like cocoons. These cocoons are attached to the host or nearby plants.

The adult moths emerge in 7-14 days. The female moths mate once and egg-laying begins almost immediately. Female moths have been reported to lay 100-300 eggs during their 2- to 3-week life-span. The moths do not fly far since they are weak fliers. They feed on nectar from wild flowers at dusk and lay eggs after dark.

There are at least 3 generations a year in Manitoba. All larval instars may be found on the plants at the same time. The first larval generation has only leaves to feed on and is not numerous. Generally, only the second generation larvae cause yield loss when flowering and pod formation is at a peak (often about the last week in July). The third generation is likely to affect only unusually late-maturing crops.

Although there seem to be some diamondback moth larvae around every year, they are only an occasional post.

These insects do not overwinter on the prairies. Moths are carried into Canada from the southern U.S. on northerly winds in early May or June. The number of spring migrants and their establishment is weather-dependent, and infestations vary greatly from year to year. The last major outbreak of diamondback moths in Manitoba was during the summer of 1988.

Rainfall is a natural control agent. Young larvae are easily dislodged from plants by rain and can drown on the soil surface or in water trapped on the plants. Warm, dry weather seems to promote the build-up of high population levels.

Other factors influencing diamondback moth abundance are weather conditions during the egg-laying period. Cool, cloudy weather reduces moth flight activity. Heavy rain can disrupt mating and egg-laying. If inclement weather persists, many female moths die before egg laying is completed. These observations have been confirmed in various laboratory studies (e.g., Moller, 1988).

1. Importance:

Larvae of the diamondback moth feed on the foliage of cruciferous plants from the seedling stage to harvest. They can greatly reduce the quality and yield of the crop.

Damage by young larvae is characterized by small mines and holes or ‘windows’ in the leaves and surface stripping on the undersides of leaves. The amount of leaf area lost probably causes little yield reduction in any but the most extreme cases.

Older larvae feed on flowers, young pods and the surface tissue of stems and mature pods, usually from mid-July to early August. Most damage is caused by feeding on the surface of filling and maturing pods. The seeds under damaged areas do not fill properly and the pods are more susceptible to early shattering.

In severe cases, feeding damage shows from a distance as abnormal whitening. After an infestation is controlled in a crop at the pod stage, there is not enough time for a new infestation to develop.

Various insect parasites, including braconid, ichneumonid, and trichogrammid wasps (Sivapragasam et al., 1986), attack the larvae of the diamondback moth. Predators, including spiders and birds, may also kill the larvae and adults. Little is known about the effectiveness of these natural enemies in reducing populations. Generally, most workers believe that they have little impact during outbreaks of the diamondback moth.

1. Population Monitoring Techniques:

When monitoring for diamondback moth larvae on canola, closely inspect the middle and lower parts of the plant, especially before pod formation. The 1st-instar larva mines in leaf tissue and, like the egg, is nearly impossible to find. Later instar larvae feed on the lower leaves are easier to find. They tend to wriggle backwards vigorously when disturbed.

Some workers recommend that you gently pull out a canola plant and count all the larvae present. Because the plant covers an area of about 1 square-foot, this method may be better than dislodging all the larvae (hopefully) and trying to count the larvae on the ground. The latter is next to impossible when you are standing out in a field of nearly-mature canola. Similarly, sweeping insects from mature canola with an insect not is a poor means of determining larval density.

When diamondback moth populations are extremely high, the pupal cocoons may be present in the tens or hundreds on the maturing canola plants. Sometimes, close examination will reveal the presence of a parasite's cocoon within the diamondback moth's cocoon.

On cabbages and other vegetables, one must make direct counts of the larvae on individual plants. Some workers recommend examining about 150 plants per field.

Sticky traps have been used by some workers for monitoring adult moth populations (Butts and McEwen, 1981; Sivapragasam and Saito, 1986). Yellow trap are sometimes used, especially in vegetable crops. Simple yellow traps can be made from Bristol board, painted yellow and coated with Stickem or Tanglefoot. These traps should be checked biweekly for the presence of the moths.

2. Threshold/Action Population Level:

It takes a severe infestation of small larvae to cause appreciable damage. The action threshold for diamondback moths in canola is about 300 larvae/M². There have been reports of as many as 45 larvae per plant during severe Infestations. If pod damage begins to show when the larvae are still small and numerous, control measures are warranted.

Economic thresholds for diamondback moth in cauliflower are about 5 larvae/plant (Stewart and Sears,1988). Heads must be kept free of larvae, especially during the 2 weeks before harvest. If drought conditions are occurring at the same time, the threshold may be lower.

2. Management Alternatives - Non-Chemical:

Several factors influence the potential abundance of this pest. They include the size of the spring influx from the south and the availability of suitable food for the first-generation larvae. If the moths arrive before the emergence of the preferred hosts, alternate host plants will be used. These include volunteer weeds on summer fallow. Tillage reduces the availability of alternate host plants. Thus, tillage may reduce the establishment of first-generation larvae.

Still at the research stage, new cultivars, resistant to feeding by the larvae, may offer the best hope for long-term control. Cauliflower, with glossy leaf waxes, is one example in the vegetable industry. For canola, work is being carried out with chemicals given off by the plants and affecting feeding and egg-laying behaviour. It may be many years before suitable canola cultivars are developed.

Generally, insecticides should be applied from the air to control the feeding larvae. High water volumes (e.g., 4 gal./acre) and uniform spray coverage are required, especially when the crops are dense. Droplets should be in the 150-250 micron size range. Spray failures occur when insufficient insecticide penetrates to the lower leaves of the plant.

Diamondback moth larvae on canola can be controlled by far insecticidal sprays: i.e.,

(g Al/ha) Interval

Azinphos-methyl

canola........ 300 30

Deltamethrin..... 5-7.5 14

Malathion........ 350 7

Methidathion

canola........ 250 30

Trichlorfon

canola........ 1120 21

Restrictions and Precautions –

Azinphos-methyl (Guthion): May be applied by air or ground equipment. Works as a contact poison. When mixing the Guthion SC, pour the required amount into full tank of water and then agitate. Do not apply to crops in bloom or allow spray drift toward beehives. Do not re-enter treated fields without protective equipment within 2 days of application.

Deltamethrin (Decis 5 EC): Do not make more than 3 applications per year. (Only 1 application per year by air). Do not apply at temperatures above 25C. Untreated borders (15 m using ground equipment; 100 m using aerial equipment) must be left around wetlands, sloughs, rivers, houses, and farm buildings.

Malathion: Do not apply at air temperatures below l8C. May be applied by air or ground equipment.

Methidathion (Monitor 480): Do not feed or allow animals to graze on treated crops. Apply in minimum of 110 L water/ha by ground; or 22 L water/ha by air.

Do not apply when bees are present.

Trichorfon (Dyfox): Do not apply to flax after flowering. Do not contaminate streams, lakes or ponds. May cause spotting of automobile paint, wash car immediately if accidental exposure occurs.

Methidathion (Monitor 480) and deltamethrin (Decis 5.0 EC) are also registered for the control of diamondback moth larvae on broccoli, brussels sprouts, cabbage and cauliflower, using ground equipment. Monitor is applied at a rate of 1.1 to 2.25 1/ha. Cauliflower and cabbage have a 7 day pre-harvest interval whereas broccoli and brussels sprouts have a 14 day pre-harvest interval. Decis is applied at a rate of 150-200 ml/ha. Do not use within 3 days of harvest.

Chlorpyrifos (Lorsban 4E) is also register for the control of diamondback moth in canola. There is a 21 -day pre-harvest interval. It may be applied by air or ground equipment. It works as a contact and stomach poison (also by inhalation). Apply when insects exceed economic threshold levels in sufficient water for good coverage. When mixing add approximately 1/4 the amount of water required to the spray tank and with agitation add the Lorsban 4E. Maintain sufficient agitation during mixing and application.

Insecticide resistance has been reported, mainly in southern areas of the U.S.A. (Shelton et al., 1993). Resistance became a serious issue in the late 1980's. Larvae can develop resistance to both conventional chemical insecticides and to microbial insecticides, including Bacillus thuringiensis (Tabashnik et al., 1992). Considering the fact that outbreaks in Manitoba , result from massive influxes of moths from the south, farmers should be aware that resistance may occur. Post-spray checks should be made to ensure that the Insecticide applied was effective.