Seasonal life history and impact of Nepytia janetae (Lepidoptera: Geometridae): an emerging pest in Southwestern montane forests.

Abstract

Nepytia janetae is a previously innocuous non-eruptive species that has recently incurred multiple devastating outbreaks in the American Southwest. We report information on the life cycle, biology, and impact of this species learned during the first 3 known outbreaks in spruce-fir and mixed-conifer forests of the Pinaleño Mountains and White Mountains of Arizona and the Sacramento Mountains of New Mexico. N. janetae is a univoltine, autumn- and winter-feeding wasteful defoliator with 3-yr eruptive outbreaks. Outbreaks terminate with parasitism, viral infection, starvation, and in one outbreak with heavy rainfall during egg deposition. Conifer mortality varied between outbreaks but approached 100% in some stands in all three study areas. Only severely defoliated trees died. Mortality was associated with defoliation severity, moisture availability in the last growing season of the outbreak and/or in the first post-outbreak growing season, and bark beetle activity. Other site and stand variables associated with defoliation and mortality varied between outbreaks, highlighting the need to evaluate multiple outbreaks before identifying factors related to susceptibility and vulnerability to a new pest species. The emergence of this insect as a serious pest is probably related to warming climate but the mechanisms are unclear; the only consistent pattern is an association with low or early loss of snowpack in either the year immediately preceding or the first year of the outbreaks. Differences in host specificity and larval coloration between the distant mountain ranges indicate that N. janetae may be a complex of multiple species or subspecies.