BERTHA ARMYWORM
(Lepidoptera: Noctuidae)
The bertha armyworm, Mamestra configurata, is an important pest of canola on the Canadian prairies.
Moths are predominantly dark-colored, with a dark patch of pale green on the thorax and at the base of each wing. A silvery spot and silver fringe on the forewings are also characteristic. They have a wing-span of about 38 mm.
Pinhead-sized eggs are white when laid, and then gradually darken as the larvae develop inside the eggs.
Newly emerged larvae are small, 3 mm long, pale green with a pale yellowish stripe along each side. The larvae moult 6 times and pass through color phases of green and pale-brown before becoming large velvety black caterpillars. Mature larvae are 4-5 cm long, with a light brown head and a broad pale orange strips along each side. However, some remain green or pale brown throughout their larval life.
Pupae are reddish-brown and resemble other cutworm pupae.
The bertha armyworm is native to the Canadian prairies and parkland region of western Canada (Hegdekar, 1983).
In Manitoba, small, localized outbreaks of bertha armyworms have occurred in the areas of Carman, Cromer, Hamiota, Minitonas, and Neepawa. They seem to be more common in the area north of Brandon and west of Lake Manitoba, where an outbreak occurred in 1971-1974. However, the bertha armyworm may occur wherever canola is grown In Manitoba.
The eggs and young larvae are found on the lower leaves of the host plants. The pupae are present in the soil below these same plants.
4. Hosts:
Bertha armyworms are general feeders on broad-leafed plants. They have been especially harmful to canola and flax on the prairies. They do not feed on grasses or cereal crops. Sweet clover, alfalfa, field peas, and garden vegetables are occasionally attacked (Anon., 1976). They may also food on some weeds, including lamb's quarters.
There is only one generation per year. Like other moths and butterflies, the bertha armyworm passes through 4 stages: i.e., eggs, larvae, pupae, and adults.
Bertha armyworms usually begin to overwinter as pupae in the soil in late-August (Hogdekar, 1983). Moths emerge from overwintering pupae in mld-July. Soon after emergence, mated females lay their eggs on the undersides of canola leaves in clusters of 50-150. The eggs are laid in a honeycomb arrangement, one layer thick.
Larvae emerge in about 7 days later. The tiny green caterpillers disperse after hatching (Butcher and
Bracken, 1976). Their pale-green color makes them very difficult to see on the undersides of leaves.
When disturbed, the larvae drop from the leaves suspended by fine silk threads. The larvae are not highly mobile. They migrate from an infested field only when the food supply is short or when the crop is over-ripe. There are 6 larval instars and a pre-pupal stage.
After feeding, the mature larvae burrow 5-15 cm into the soil where they pupate and overwinter.
Larvae may be common, in an area where their hosts occur, from mid-June to late-August.
Photoperiod induces diapause in August. As daylength decreases, mature larvae leave their host plants
and enter the soil to overwinter. Overwintering in the soil may protect the pupae from both high and low temperatures.
Photoperiod and temperature may also affect adult moth behaviour and egg development (Howlander
and Gerber, 1986a,b; Gerber and Howlander, 1987). Cool temperatures may delay flights of adults and slow egg maturation.
Young larvae chew irregular holes in the lower leaves but normally cause little visible damage. Feeding damage to leaves and pods is first noticeable from early to mid-August. The last two larval instars (the 5th- and 6th-instars) are the most damaging. About 80% of all eaten plant tissue, including the maturing pods, is consumed by these instars. Damaged pods shelf more rapidly while drying in the swath.
Heavy infestations may reduce yields because of defoliation and seed pod consumption. Crop losses due to pod feeding will be most severe if there are few leaves.
Larvae can continue to feed on swathed canola for a few days until the crop does. However, by the time of swathing, most larvae have dropped to the ground to pupate. In some years, early-seeded canola can be swathed prior to damage.
Bertha armyworms have only reached widespread outbreak levels about 6 or 7 times since they were first noted in 1922. Periods of pest abundance usually last 1-3 years. There appears to be no regular sequence to the outbreaks. Large increases in rapeseed/canola acreage may contributed to dramatic increases in the populations of bertha armyworms.
Many diseases and parasites attack the larvae (Turnock, 1988). Viral diseases are common during severe outbreaks and may contribute to sudden declines of armyworm populations (Wylie and Bucher,1977; Bucher and Turnock, 1983). Larvae that are infected by a virus appear melted onto the plant. Fungus-attacked larvae look as if they have been preserved on the plant.
Insect parasites may kill up to 75% of the armyworms. Ichneumonid wasps and tachinid flies are fairly common and may parasitize many larvae (Wylie, 1977; Arthur and Powell, 1989, 1990). Unfortunately, damage to the crop has often been done before the larva are killed by parasites.
II. MANAGEMENT
1. Population Monitoring Techniques:
The young green-colored larvae, as noted above, are very difficult to see on the host plants (Anon.,
1976). Careful examination of the lower leaves is required. As they become mature, the larger larvae (about 3.5 cm long) are more conspicuous (usually brown or velvety black in color, with a yellowish band along each side).
Checking for larvae can be done by the agronomist or farmer starting in mid-July. Shake canola plants to dislodge the larvae and then count all the larvae on the ground over a 1 mē area.
From a distance, infested canola fields look pale-white because the larvae eat the outer green layer of the stems and pods exposing white tissue.
The moths fly at night and may be seen near outdoor lights or, at dusk, feeding on flowers. Pheromone monitoring of bertha armyworm moths indicates when field sampling is necessary (Steck et al., 1979). Wing traps, baited with synthetic sex attractants, can be used to locate populations of male moths.
Bertha armyworms can be controlled with a foliar spray applied when larvae are actively feeding and before the destructive 5th- and 6th-instar larvae appear. The detection of larvae before these stages requires field inspections in late July or early August, depending on when the peak moth flight occurred.
Control is recommended when larval population densities exceed the economic threshold of 20-30 larvae/Mē. A population of 20 larvae/Mē can consume the equivalent of 65 kg canola seed/ha (1.16 bushels/acre).
Some workers suggest that 10 larvae/Mē can damage pod and warrant treatment. Certainly, control should be initiated well before pod damage reaches 20% (Bracken, 1987).
Cultivation in the fall and spring will reduce populations of pupae by exposing them to freezing, diseases and predators (Anon., 1976).
Commercial formulations of the bacteria, Bacillus thuringiensis kurstaki, are not considered to be effective against this lepidopterous pest.
In Canola
An aerial application an insecticide, once per season in August, when the larvae are present in high
numbers and feeding heavily, will control bertha armyworm in canola.
Some of the insecticides commonly used against bertha armyworm in canola are given below: i.e.,
Rate Preharvest
(g Al/ha) Interval
Chlorpyrifos
Canola only...... 350-490 21
Deltamethrin
Canola & Mustard.. 5-15 14
Methomyi............... 250 8
Methamidophos
Canola only....... 280-560 10
Restrictions and Precautions -
Chlorpyrifos: May be applied by air or ground equipment. Works as contacted 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 of the amount of water required to the spray tank and with agitation add the Lorsban 4E. Maintain sufficient agitation during mixing and application. Very toxic to bees, fish, birds, aquatic organisms and other wildlife. Do not apply or drift on to workers or other persons. Do not apply directly to water or where runoff could occur to adjacent aquatic sites. Avoid use when bees are actively foraging.
Deltamethrin: Do not make more than 3 applications per year. (Only 1 application per year by air). Do not apply at temperatures above 25"C.
Methomyl: Sprays or drift must not contact workers, other persons or animals. The area being treated must be vacated by unprotected persons. May be fatal or cause blindness if swallowed. Poisonous if inhaled. Use higher rates for mature insects, dense canopy or when infestations are heavy. When mixing, fill spray tank one quarter to one half full of water an add Lannate L directly to spray tank. Mix thoroughly. Once dissolved, continued agitation is not required. Do not use air agitation.
Cypermethrin: This insecticide (e.g., Cymbush [O.O8-0.113 1/acre] and Ripcord [0.028 1/acre (ground) or 0.035 1/acre (air)]) is also registered for the control of armyworm in canola. There is a 30-daypre-harvest interval.
In Flax
Lannate [0.4-0.5 1/acre] can be used to control bertha armyworm in flax. There is an 8-day pre-harvest interval. Note the precautions and restrictions noted above for methomyl.