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Phenology of the Sagebrush Defoliating Moth Aroga websteri (Lepidoptera: Gelechiidae), With Application to Population Irruptions

Abstract Larval populations of the sagebrush defoliating moth Aroga websteri Clarke (Lepidoptera: Gelechiidae) were sampled in a montane stand of big sagebrush (Artemisia tridentata Nutall) in northern Utah during 2008–2012 to develop degree-day (DD) estimates for moth phenology (single-sine method, base temperature 5°C). Degree-day estimates were determined also for populations studied previously (in 1972–1975) in a lowland valley stand of big sagebrush on the Utah–Idaho border. Peak larval abundances (of mostly first and second instars, and ranging from 0–2.8 larvae per 15 cm of terminal stem at individual locations in northern Utah in 2008–2012) occurred on average at 239 DD (accumulated beginning 1 January). Differences in DD accumulation and precipitation among years with endemic (low) and population irruption levels of the Aroga moth were examined using data from these sites and from historical accounts from Alturas, CA, during 1962–1964. The DD accumulation during mid-May through mid-June (when early instars of the moth are developing) was intermediate in years of population irruptions versus in endemic years with cooler or warmer late springs–early summers. Years with population irruptions were also characterized by high precipitation in June and July during late stage larval development. Thus, it appears that periodic population irruptions of the defoliator are associated with distinctive weather conditions. Additional research is needed to test whether these conditions can promote increased larval survivorship as associated with high nutritional value and palatability of sagebrush foliage for the moth.