The Intergovernmental Panel on Climate Change (IPCC) estimated that the global mean surface air temperature will increase about 2 degrees C above the 1990 level by 2100. Such an increase in temperature may result in an increase in the number of generations per year of most insects. Several simulation models have been constructed to predict the increase in the number of generations, but these require a great deal of calculation to obtain estimates for many insect species at various locations under various scenarios of global warming. This paper proposes an analytical method to enable a quick estimation. Two assumptions are used: (1) The developmental rate of insects is approximately given by a linear function of temperature (T ), with a developmental zero (T₀) and a thermal constant (K ). (2) The increase in temperature is approximately even throughout the year. Let m be the current annual mean temperature at a given location, and Delta T the potential increase in temperature under global warming at the location. If the temperature before the DT-rise is higher than T₀ in summer and lower than (T₀-DT ) in winter, the increase in the number of generations per year, which is denoted by DN, is approximately given by

DN = DT[c + d (m - T₀ )]/K,

where c and d are constants. The validity of the approximation is checked using the "smoothed daily normals" in Japan obtained as a running mean of 15 successive daily normals where a daily normal is defined as the averaged daily temperatures for 30 years from 1960 to 1990. The estimates are c = 204.4 and d = 12.46. Using this formula, linear isoclines of DN are plotted in a graph of T₀ versus K. By plotting the T₀ and K of various insects in this graph, we can estimate the increase in the number of generations under global warming in temperate zones. (Copyright by the Japanese Society of Applied Entomology and Zoology)


Figure 1.   The effect of 2 degrees warming on the increase in the number of generations at a location having a current annual mean temperature m = 15 degrees centigrade. Lines with numbers 1, 2, 3, 4, and 5 refer to the isoclines showing the increase in the respective number of generations. Circles show the mean value of T₀ and K for different arthropod groups.