Simultaneous outbreaks of three species of larch web-spinning sawflies: influences of weather and stand structure

The first outbreaks of three species of larch web-spinning sawflies (Cephalcia lariciphila japonica Shinohara, Cephalcia koebelei (Rohwer) and Acantholyda nipponica Yano and Sato (Hymenoptera: Pamphiliidae) that occurred in 1993 in non-native larch (Larix leptolepis Gordon) plantations in Hokkaido, northern Japan, were examined in two distinct areas 500 km apart. We also examined annual changes in prepupal densities in the soil in seven larch stands chosen across a defoliated region in central Hokkaido. The influence of stand structure on sawfly density were examined in 30 larch stands. Defoliation by C. lariciphila, which took place in eastern Hokkaido, were restricted to a small area for 4 years, then increased rapidly from 1997 and reached 15,000 ha in 2000. Defoliation in central Hokkaido, which was caused by C. koebelei and A. nipponica, increased to 2600 ha in the next year after initiation of the outbreak, continued at this level for 4 years and diminished in 1999. In central and eastern Hokkaido, outbreaks were preceded by three consecutive years of high yearly average temperature, which occurred 2–4 years before the outbreaks. However, yearly precipitation did not change noticeably prior to the outbreaks. Annual changes in prepupal densities in central Hokkaido revealed that the density peaked earlier near the stands where the outbreak was first observed and peaked later in peripheral areas. This suggests that the outbreaks began in local epicenters and expanded to surrounding areas. Neither larch density (trees/area), tree size (height and diameter at breast height (DBH)) nor proportion of larch stems correlate with the prepupal densities although these factors varied considerably. Thus, after canopy closure, sawfly density does not seem to be influenced by the stand factors.