Woolly bear caterpillars

Pest status

In the early spring, some woolly bear species (Apantesis blakei and Apantesis williamsii Grote) can be occasional pests in pasture and field crops.

Image 1. Apantesis blakei larva foraging in April. 
Photo credit: Samantha Plett.

Image 2. An Apantesis blakei adult. 
Photo credit: S. J. Barkley

Normally these caterpillars feed on native grasses, but during dry springs or when pasture conditions are poor, they move into adjacent field crops and can cause sporadic outbreaks in southern Alberta. Population outbreaks tend to last 1 to 5 years, with long periods of low populations between outbreaks.

Hosts include wheat, sugar beets, alfalfa, canola, rye, barley, oats, corn, sorghum, sunflowers, beans, lambsquarters, dandelions, and geranium among others.

In extreme cases, populations as high as 100 larvae per m² have been recorded, although this is rare. The risk of outbreak depends on the population size, fall food availability, and spring grassland and pasture condition. Dry fall conditions followed by dry spring conditions may increase risk.

Life cycle

This species overwinters as a late-instar larva. Larvae become active in early to mid-April, depending on temperature, and finish feeding and developing.

The adult moths mate and lay eggs in May. Eggs hatch immediately and larvae develop until undergoing a mid-summer diapause. In the fall, these larvae become active again and will continue to develop. If food is widely available, larvae may complete feeding and developing prior to overwintering, leading to a very short spring feeding period.

Monitoring

There are no economic thresholds for this insect, but it is important to inspect overwintered and early-seeded crops for defoliation early in the season, especially fields adjacent to native grassland or pasture. If spring conditions are adequate, or if larvae finish development before spring crop emergence, then these insects cause no or little economic harm.

References

Byers, J.R. 1988. Woolly-bear caterpillars of Grammia blakei (Grt.) (lepidoptera: Arctiidae) – an occasional pest of native range pastures and adjacent crops in southern Alberta and southwestern Saskatchewan. The Canadian Entomologist. 120: 927-935.