Geographical and annual variations in the proportion of extended diapausing individuals of Illiciomyia yukawai (Diptera: Cecidomyiidae) with reference to an adaptive significance of its bimodal emergence pattern

Illiciomyia yukawai (Diptera: Cecidomyiidae) induces leaf galls on Illicium anisatum (Schisandraceae) and exhibits 1-year (yr) and 2-yr type life history patterns. Geographical and annual variations in the proportion of 2-yr to 1-yr type individuals of I. yukawai were studied from 1977 to 1994 and in 2008, 2009 and 2014. An analysis of geographical variation at 13 different census fields indicated that the proportion of 2-yr type individuals significantly increased with the decrease of mean annual temperature reflected by altitudinal and latitudinal gradients. However, a multiple regression analysis indicated that the annual variation in the proportion was not correlated with temperature, gall density, abundance of host resources or parasitism by hymenopteran parasitoids. Thus, we eliminated the effects of these external factors that have been thought to determine the annual variation. We need to take alternative intrinsic factors, instead of external factors, into consideration to explain the annual variation. The fall of galled leaves occasionally caused a higher mortality of 2-yr type individuals than those of 1-yr type. Nevertheless, 2-yr type individuals exist. We considered that the existence of 2-yr type individuals of I. yukawai has an adaptive significance to diversify risks against catastrophic events such as the serious shortage of host buds and high percentage of parasitism that happen more frequently than the high mortality caused by the fall of galled leaves.