Army cutworm

Life cycle

There is one generation per year.

The army cutworm (Euxoa auxiliaris) larvae eat the foliage of wheat, oats, barley, mustard, flax, alfalfa, sweetclover, peas, cabbage, sugar beet, various weeds (notably stinkweed) and grasses.

The female moths each lay about 1,000 eggs in soft soil in late August through October. The eggs hatch in a few days to two weeks. The larvae feed above ground on plant foliage at night and remain below ground during the day. Development stops when the ground freezes; larvae are usually about half-grown by this time. They remain inactive throughout the winter just beneath the surface in loose soil.

Larvae begin to feed in April and continue to feed until pupation in May to early June. Moths appear in June for a brief flight period, aestivate (summer hibernation) in buildings and under trash and clods during June and July, and then become active again for the egg-laying period.

Damage assessment

Damage description

Damage can be of any severity up to complete defoliation. In severe infestations, the area defoliated has ranged from an individual field to thousands of acres with larval densities of up to 200 per square metre. The first signs of damage are holes in leaves and semi-circular notches eaten from the edges of leaves.

Sampling and monitoring methods

Army cutworm moths have been monitored in southern Alberta with pheromone traps since 1978. Sample larvae as for other subterranean cutworm species. Mark an area of soil 50 cm by 50 cm. During the day, larvae should be within the top 5 to 7 cm of soil surface. Count the larvae within each 50 cm of row in the sample area. Multiply the number of larvae by four to give the number of larvae per square metre. Repeat the process in different areas of the field.

Economic thresholds

Economic thresholds for this insect have not been rigorously tested. However, the following guidelines may be helpful.

The economic threshold is dependent, among other factors, on the crop infested. Mustard is more susceptible to damage from this cutworm than are cereals and alfalfa. A density of five cutworms per square metre is sufficient to destroy a mustard field, whereas cereals and alfalfa can withstand cutworm populations of up to 50 per square metre. The latter crops can resume growth after attack whereas mustard cannot.

Other factors affecting the economic threshold are the plants' growth condition and the number of weed hosts in the field. A weedy field will suffer less damage than a clean one. Forage crops and pastures must be watched closely in April and early May for the presence of these larvae. Plants that have adequate moisture and are vigorously growing with 12 to 15 cm of top growth can withstand four larvae per 30 cm of row without loss of yield. If plants are under 10 cm in height and two or more larvae per 30 cm of row are present, chemical treatment is required.

Control

Effects of weather

Each outbreak year is usually preceded by a year with an abnormally dry July and wet autumn. A July with less than 3.8 cm of rainfall and a mean temperature of 17 degrees Celsius or higher is favorable to a population increase. However, an outbreak may not occur in the following year unless a dry July is followed by a total of over 11.4 cm of precipitation in August to October, with most of it in September.

Army cutworms are reduced in a wet July when moths are drowned or covered with mud during aestivation. A dry fall delays egg-hatching by up to 2 months; eggs and first instar larvae are very susceptible to desiccation and are killed in the soil. Because of these mortality factors, an abundance of moths in early fall does not necessarily mean a cutworm outbreak the following year.

Cultural practices

Spring crops can escape damage if they emerge after the cutworms have pupated. This is the usual situation and occurs when cutworm development is advanced by favorable fall or spring weather or crop seeding is delayed by wet weather. Crops can also be seeded later in the season to avoid attack by this pest.

Mechanical

If cutworms are marching, plow a steep-sided trench across the path of advance. Line the trench with a plastic sheet to ensure that larvae will not climb out of the trap. Use of insecticides on larvae in this limited space is economical.

Natural enemies

Five or possibly 6 species of wasp parasites have been recorded for army cutworm in Canada, mostly from Alberta.

Copidosoma is a tiny parasitic wasp (about 2 mm in length) which lays a single egg in cutworm larvae. The egg produces multiple embryos from which over a thousand offspring may be produced. 60% or more of army cutworm larvae were parasitized by this organism during an outbreak in Alberta in 1990. Copidosoma prolonged the larval stage of army cutworm into June; the cutworms caused damage to spring crops, an unusual circumstance since they normally have pupated by that time.