Analyses of the Northern Distributional Limit of Aedes albopictus (Diptera: Culicidae) With a Simple Thermal Index

Worldwide invasion of Aedes albopictus (Skuse) has become a serious concern in vector control. In temperate regions, an important goal has been to predict northern range limits after new introductions or global climate change. We propose a simple thermal suitability index (TSI) that can be calculated from monthly mean temperatures and may predict the northern distributional limits of Ae. albopictus. Analyses of known distributions revealed that TSI yields a common criterion (TSI = 1) below which establishment is thermally prohibited throughout the northern hemisphere. However, temperature conditions around northern limits differ among regions, depending on thermal continentality (the degree of seasonal temperature variation). Thermal continentality is high on the Asian Continent, intermediate in North America and Japan, and low (i.e., oceanic) in Europe. On the Asian Continent, there may be maximal winter coldness above which establishment is prohibited irrespective of summer reproduction. In Europe, establishment is possible under summer warmth lower than in other regions but there may be minimal summer warmth below which establishment is difficult despite high winter survival during mild winters. Among the thermal indices that have been commonly used as establishment criteria, the annual mean temperature of 10 degrees C proved to be a common criterion irrespective of thermal continentality, whereas coldest month means and warmth during the reproductive season (often expressed as accumulated degree-days above 11 degrees C) are strongly influenced by thermal continentality. Although annual mean temperatures can be a practical criterion, TSI was more informative in evaluating thermal suitability for this species.