TRUE ARMYWORMS
(Lepidoptera: Noctuidae)
I. BIOLOGY AND ECOLOGY
Armyworms get their name from their behaviour of moving, army-like, across fields, as a result of population pressures and starvation, in search of food (Walkden, 1950; Davidson and Lyon, 1987;Tashiro, 1988).
The true (or common) armyworm, Pseudaletia unipuncta, resembles another pest of grasses, the bronzed cutworm. Unlike the bronzed cutworm, armyworm larvae do not have a metallic sheen. Armyworms also differ in their feeding behaviour.
The eggs are greenish-white and laid in clusters. Smaller than the head of a pin, they look like tiny white beads.
The young larvae are nearly naked, smooth, pale-green in color, and have a looping crawl (like inchworms) until about half-grown.
The mature larvae are large (3-4.5 cm in length and about 5mm wide), hairless, and variable in color, ranging from dark greenish-brown to black (Philip and Mengersen, 1989). Viewed from the side, they have long, pale white, pale orange and dark brown stripes along the length of the abdomen.
The larvae have a distinctive yellowish- to yellowish-brown colored, mottled head capsule. Viewed from the front, the head capsule has 2 inverted crescents. Another distinguishing feature for the larvae is that each proleg has a dark band on its outer side. The pupae are brown and resemble other cutworm pupae.
The adult armyworm moth is about 2.5 cm long, light-brown to reddish-brown in color, and has a small but prominent, white spot, centered on each of Its fore-wings (Bereza, 1974; Tashiro, 1988). It has a wingspan of about 4 cm (Davidson and Lyon, 1987).
2. Geographical Distribution:
Although native to North America (Arnett, 1985), true armyworms do not survive Canadian winters. New infestations occur most years from annual flights originating in the U.S.A. (Guppy, 1961; Fields and McNeil, 1984; Ayre, 1985, Ayre and Lamb, 1990). In Canada they are most common from southern Saskatchewan and eastward (Beirne, 1971).
In southern Manitoba, they have been reported from various regions. They often seem to show up from in the Central, Interlake and Eastern regions. There are rarely 2 successive outbreaks in the same area.
3. Habitat:
Armyworms can be found in grasslands, pastures, and lawns and in small grain and forage crops (Philip and Mengersen, 1989).
4. Hosts:
True armyworms have been reported on various native grasses and turfgrasses and, in agricultural areas, on small grains and forages (Breeland, 1958; Rings, 1977, Philip and Mengerson, 1989). During times of extremely high populations, they may move into gardens and consume anything that is green.
Armyworms feed primarily on members of the grass family, damaging crops such as oats, wheat, fall rye, corn, barley and forage grasses (e.g., brome and couch grass).
Broad-leafed plants such as alfalfa, cabbage and turnips may also be attacked but the damage is minor. They have also been reported feeding on some weeds (e.g., green foxtail).
Armyworms probably migrate into southern Manitoba as adults on wind currents from the southern U.S.A., early in the year. Few studies of armyworm migration have been made but there is some evidence that they can move up to 1600 km (Hendrix and Showers, 1992). In the spring, the larvae resume feeding until they are full grown and then enter the pupal stage in late April.
The moths are active at night, feeding on nectar, mating, and searching for oviposition sites. Lodged plants provide a favourable habitat for moth concealment and egg laying. During June, adult female armyworms lay their tiny, white eggs at night, in clusters of up to 100, at the bases of plants. The eggs hatch in 1-2 weeks.
Lodged plants also provide the high humidity required for establishment of the larvae. The larvae feed at night on the leaves and crowns of their hosts. During the day, they rest at the bases of grass plants, often under a deep cover of thatch. There are 6 larval instars. It takes 3-4 weeks for the larvae to reach maturity.
When mature, the larvae construct pupal chambers or cells in a few centimeters below the surface of the soil. The pupae are about 20 mm long and reddish-brown in color. Pupation may last about 2 weeks.
There are usually 2 generations per summer in Canada with the first generation doing the greatest damage in late June and early July (Ayre and Lamb, 1990).
Larval populations of true armyworms seem most abundant in July when the larvae are full-grown, their feeding is greatest, and damage is most apparent.
The effects of temperature and photoperiod on adult behaviour has been studied by various authors (Guppy, 1961; Turgeon et al, 1983; Fields and McNeil, 1984). Cool, moist, spring weather (associated with heavy growth of grasses and small grains) favours outbreaks of true armyworms (Davidson and Lyon, 1987).
True armyworms do most of their feeding damage during their last few days as nearly-mature larvae. In
Manitoba, the damage is usually most apparent during July.
Larvae feed on the leaves, stripping the leaf margins and move up the plants to feed on the panicles and flowers, stripping off the awns and kernels. Dense, lodged cereal crops are more susceptible to damage from armyworm larvae.
In cam, armyworms will strip the leaves and feed on the developing ears.
Armyworms are often controlled naturally by various fungal diseases.
Insect parasites occasionally attack the immature stages. For example, tachinid flies attack the larvae (Arthur and Powell, 1989). One of the commonest tachinid parasites is Winthemia quadripustilata (the red-tailed tachinid fly).
Tiny, parasitic braconid wasps attack the eggs.
Various predators (e.g., ants, ground beetles, wasps, toads, birds, domestic fowl, skunks) can also affect local armyworm populations.
II. MANAGEMENT
In Turf
Because armyworms do most of their serious damage to turf during their last few days as larvae and because they are most difficult to kill when fully-grown, it is important to carry out regular inspections of suspect turf. The best time to look for armyworm larvae is in the evening or early morning when they are feeding up on the plants.
Armyworms damage grass by skeletonizing the blades or, when fully-grown, severing entire plants at the crown. Look for armyworm damage in the wetter areas of the turf. The armyworm larvae can be sometimes be found by carefully examining the thatch layer on the edges of dead patches of turf. They will coil up when disturbed. The best time to find them is at night, with the aid of a strong flashlight.
Larval densities can be determined by drenching a 1 m square of suspect turf with a weak solution of pyrethrins. Count the larvae coming to the surface over a 10 minute period to obtain an estimate of their density.
In Crops
Influxes of moths from the south, during May and June, can be monitored using light traps (Ayre and Lamb, 1990). Pheromone traps can also be used in monitoring programs (Hill and Roelofs, 1980).
In cereal and grass crops, check at least 5 areas of a field in late-June. Pay special attention to patches of lodged plants and weedy areas. During the day, look for notched leaves on the plants. Late at night, with the aid of a flashlight, shake plants over a 1 -m
If sampling for inactive larvae during the day, look under plant debris, lumps of soil, in soil cracks, and deep in the leaves. Day-time checking for armyworms is difficult.
Controls are rarely necessary in Canada but severe local infestations may require action. Although there is little solid basis for setting armyworm action thresholds, densities of 50-65 larvae/m² probably warrant chemical control. In agricultural situations, some workers say that control is usually necessary when larval densities exceed 10 larvae/m².
Only infested areas of the field should be treated. Spraying should be done in the evening when armyworms are feeding on the plants.
There is little benefit in applying a chemical once the armyworm is nearly full-grown, pupae are present, parasitism is extensive, or the crop is nearing maturity. By that time most of the damage will have been done.
As with many turf pests, good turf maintenance practices will tend to help injured grass to recover form light infestations of the true armyworm. Because the larvae hide at the bases of grass and in the thatch during the day, thatch removal may remove some protection from natural enemies and assist in keeping populations low.
In Turf
Carbaryl and methoxychlor are registered for the control of armyworms in turf. For best results, mow the turf and remove the clippings and irrigate the turf before applying the insecticide in the late afternoon or evening. Do not mow or irrigate for several days after making the insecticide application.
In Crops
Selection of the insecticide to use will depend on the crop. The insecticides used on various field crops are outlined below: i.e.,
Rate Preharvest
(g Al/ha) Interval
Barley, Oats and
Wheat
Carbaryl...... 1125-225 14
Malathion..... 560-1400 7
Methomyl...... 340 20
Trichlorfon ... 560 21
Rye
Carbaryl..... 1125-2250 14
Malathion.... 560-140 7
Corn
Methomyl 280-560 3
Restrictions –
Carbaryl: Sevin is of moderate acute mammalian toxicity and is highly toxic to bees. Avoid contamination of food, feed, water supplies, streams and ponds. Do not apply tank-mix combinations unless your previous experience indicates the mixture is effective and will not result in application problems, excessive residues or plant injury. Do not store where temperatures may exceed 38C.
Malathion: Do not apply at air temperatures below, 18C.
Methomyl: Apply to corn as soon as young larvae appear and then at 3 to 5 day intervals.
Lorsban is another example of one of the products registered for armyworm control. Chlorpyrifos (Lorsban 4E [0.355-1.485 1/acre]) is registered for the control of true armyworm in barley, oats and wheat. There is a 60-day pre-harvest interval. It may be applied by air or ground equipment. It works as a contact and stomach poison and by inhalation. Apply when insects exceed economic threshold levels in sufficient water for good coverage. Use higher rates for heavy infestations.
When mixing Lorsban, 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.
Lorsban may be tank mixed with the following herbicides: i.e , Avenge 200-C, Banvel plus 2,4-D Amine; Buctril M; MCPA Ester and Amine; Tordon 202C; 2,4-D Amine - and Ester. When tank mixing always add the herbicide to the spray tank and then add Lorsban 4E. If Lorsban 4E is added first, settling out may occur, causing plugging of lines or nozzles.
Do not apply directly to water or where runoff could occur to adjacent aquatic sites. Avoid use when bees are actively foraging.
Trichlorfon (e.g., Dyfox [0.6 1/acre]) is also registered for the control of armyworm in flax. There is a 21-day pre-harvest interval. It may be applied by air or ground equipment.