Two-year cycle spruce budworm

Latin name: Choristoneura occidentalis biennis (Freeman)
French name: Tordeuse bisannuelle de l'épinette
Order: Lepidoptera
Family: Tortricidae
Synonym(s): Choristoneura biennis Freeman

Description

Distribution

Alberta, British Columbia, Yukon

This species occurs in higher elevation forests in the Rocky Mountains northward and westward through the central interior of British Columbia.

Micro-habitat(s)

Needle, Bud, Cone

Damage, symptoms and biology

Repeated defoliation by the two-year-cycle budworm causes crown dieback, and some tree mortality. Unlike one-year cycle species (eg. eastern spruce budworms) trees are not defoliated every year and overall damage is less severe.

Because of the two-year life cycle, in the southern part of the budworm’s range, heavier defoliation used to occur in odd years, whereas in the more northern part of the range (central interior) more severe damage occurred in even years. This pattern has changed. In recent decades, areas to the north of Prince George mature in odd-numbered years and those to the south in even-numbered years. These are the years of most intense damage. In British Columbia, it is estimated that over the last 300 years, an epidemic of this pest has occurred every 30 years on average, and lasted an average of 10 years.

The final instar larva is robust, measuring up to 25 mm. The head is brown, with black vertices. The rest of the body is dark brown with the dorsum of each abdominal segment marked with two pairs of ivory white spots. Earlier instars are more even-colored (brown).

Moths emerge from mid-July to early August, mate, and lay approximately 150 eggs in several masses on needles of the host tree. After hatching occurs 2 weeks later, the newly emerged larvae seek shelter to spin hibernacula in preparation for their first diapause before winter. This budworm overwinters as a second-instar larva protected in a hibernaculum (silken shelter) on a branch or on the trunk. Following overwintering, larvae start feeding from late May to early June, mine needles and buds for 3-4 weeks, then spin hibernacula and enter a second diapause to overwinter as fourth-instar larvae. Larval development is completed during the spring of the second year, which is when the greatest amount of tree damage occurs. A short pupation period precedes the emergence of adults in July.