Development of the willow sawfly, Nematus oligospilus, at different temperatures, and an estimation of voltinism throughout New Zealand

Cohorts of willow sawfly (Nematus oligospilus) were reared in the laboratory on crack willow (Salix fragilis) at seven constant temperatures between 11.3 and 28.8 degrees C. Larvae developed through 5-7 instars. Body size (length) of adult females was influenced by temperature and was strongly correlated with fecundity (R-2 = 0.8973). The developmental threshold (t(o)), determined through linear regression, was calculated to be 8.1 degrees C, and the thermal accumulation above t(o) required to complete one generation was 321 degree-days. The number of degree-days available for seasonal development of the sawfly at 10 sites throughout New Zealand was calculated. Allowing for the period of diapause, it was estimated that N. oligospilus might potentially develop through up to seven generations per year in the North Island and three generations per year in the south of the South Island.