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The army cutworm (Euxoa auxiliaries) is the larval form of a species of noctuid moth that is a pest especially on early spring crop fields in the great plains of North America. Euxoa auxiliaries is found from Kansas to California and from Texas up through Central Canada. The adult moth, known as the grey miller moth, is a grey-brown moth with a wingspan of 4-5 cm. In the late summer, a moth may lay a thousand or more eggs on bare soil, especially in cultivated or overgrazed fields of low elevation prairie lands. The larvae hatch and feed for a short time, and then overwinter underground as partially grown larvae. In early spring the larvae once again become active, feeding at night on a wide range of host plants and hiding in the soil or under rocks and dirt during the day. They eat many field crops including wheat, alfalfa, barley, potato, sugarbeets, other vegetables and grasses and broad-leaved weeds. When a food source is depleted, the army cut worm can move en mass to find more food. Once mature, they are up to 4-5 cm long. Mature larvae then burrow back down into the soil to pupate, emerging as moths in late spring/early summer.

Army cutworms are seasonal migrants. The spring batch of newly hatched adult moths migrate hundreds of kilometers to aggregate in specific locations at high elevation scree slopes in the Rocky Mountains, especially those adjacent to flower-filled alpine fields. Here they over-summer, feeding on nectar at night and aestivating in large numbers in crevices of the rocky scree slides during the day. The adult moths build up large reserves of body fat during this time, becoming up to 60% fat by weight. Studies in the 1980s found that these moths provide an important food source for grizzly bears between the months of June-August. Grizzlies return to these “insect aggregation sites” year after year, where a grizzly can eat up to 47% of its annual energy budget in one month (about 1 million army cutworm moths). Studies have investigated the effect of pesticide exposure to grizzy bears since army cutworms provide a significant portion of their diet. These studies find the moths contain negligible amounts, possibly because although the larvae are regularly treated with pesticides in fields, much of the fat sequestered in army cut worm moths is accumulated from alpine nectar sources which are not sprayed.

(Glogoza 2000; Robison et al. 2006; Robison 2003; Anweiler 2003; Bjornlie and Haroldson 2003)

Adult moths have been described as spraying large volumes of non-noxious liquid when disturbed, for unknown reason; perhaps as defense? (Eisner and Eisner 1992) If the larva is disturbed, they roll up into a tight curl.

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