Diapause and population fluctuations in Megastigmus specularis Walley and Megastigmus spermotrophus Wachtl. (Hymenoptera, Torymidae).
The length of diapause and fluctuations in populations of Megastigmus specularis Walley and M. spermotrophus Wachtl were studied in relation to the annual seed crops of their food-plants Siberian fir (Abies sibirica) and Douglas fir (Pseudotsuga menziesii) in Finland, where the trees are not native and are cultivated mainly for ornamental purposes. Adults of M. specularis emerged mainly after the 1st winter, whereas larvae of M. spermotrophus remained in diapause for several years. The number of emerging adults was positively correlated with the flowering intensity of the food-plants. The different duration of diapause in the 2 species appeared to be an adaptation to the different cone crop fluctuations of their food-plants. The most important factor influencing the population dynamics of M. specularis seemed to be the annual amount of sound seeds.