Predicting Sirex noctilio and S. nigricornis emergence using degree days

The study of temporal interactions between native insects and alien invaders can be facilitated by the ability to forecast adult emergence. We used field-collected adult emergence data of Sirex noctilio Fabricius (Hymenoptera: Siricidae), a woodwasp native of Eurasia that has recently invaded northeastern North America, and Sirex nigricornis Fabricius, a woodwasp native to North America, to develop and test cumulative degree-day (CDD) models. Five data sets were collected each in Ontario, Canada (S.noctilio) and Louisiana, USA (S.nigricornis) over 4years; three data sets were used to develop models and two were used to test them. Males and females of each species were modelled separately. After testing several potential temperatures, chosen thresholds for CDD were 0 degrees C lower threshold and 25 degrees C upper threshold for both Sirex spp. We used a three-parameter Gompertz growth function to model Sirex spp. emergence against CDD. Models predicted 10% emergence of S.noctilio in Ontario after 1239 and 1280 CDD, for males (start date=1 April; R-2=0.91) and females (start date=1 April; R-2=0.86), respectively. Models predicted 10% emergence of S.nigricornis in Louisiana after 3980 and 5016 CDD, for males (start date=1 May; R-2=0.83) and females (start date=1 March; R-2=0.73), respectively. Cumulative degree-day models predicted 10 and 90% emergence of woodwasp populations with less error (1-13%) than they did 50% emergence (5-27%). For both Sirex spp., male emergence began a few days before and concluded at about the same time as that of females. In southern Ontario, models predict that S.noctilio adults will be in flight between 1015 and 2430 CDD (1 April start date for CDD; from early-July until mid-September). In Louisiana, models predict that S.nigricornis adults will be in flight between 3854 and 4700 CDD (1 May start date for CDD; from early-October until late-November).