HOST FOLIAGE IN THE SUSCEPTIBILITY OF FOREST SITES IN CENTRAL CALIFORNIA TO OUTBREAKS OF THE DOUGLAS-FIR TUSSOCK MOTH, ORGYIA PSEUDOTSUGATA (LEPIDOPTERA: LYMANTRIIDAE)

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Abstract
Host foliage was evaluated for nonpreference and antibiosis by field rearing Douglas-fir tussock moth larvae, Orgyia pseudotsugata (McDunnough), (instars 2–6) at typical outbreak and non-outbreak forest sites in central California. Nonpreference was evaluated by comparing frass production and antibiosis by comparing survival, pupation, and fecundity on the different sites. Typical outbreak sites had a lower site index and higher plant moisture stress than typical non-outbreak sites. Production of frass for all crown levels collectively was not significantly different between sites, although more frass was produced in the tops of trees on outbreak sites. Survival and pupation were also not significantly different between sites, but egg production was 28% higher on outbreak sites than non-outbreak sites. If differences in foliage quality did exist between sites, they did not affect the tussock moth enough to explain observed differences in population numbers.