The Population Dynamics of an Alien Insect: Andricus quercuscalicis (Hymenoptera: Cynipidae)

Abstract

SUMMARY (1) We describe the results of the first 10 years of a long-term study of the population biology of a cynipid gall wasp, Andricus quercuscalicis (Burgsdorf 1783), an alien herbivorous insect that appears to have reached the limits of its geographical distribution within England. (2) We document the annual fluctuations in abundance of the sexual generation on Quercus cerris (L.), the Turkey oak, and the agamic (parthenogenetic) generation on Q. robur (L.), the English oak, as measured by gall densities per shoot of the host tree. The sexual generation has fluctuated 6-fold and the agamic generation has fluctuated 10-fold over the 10-year period. (3) Resource use and rates of parasitism vary substantially between the two generations. The agamic generation on Q. robur uses 35-60% of the acorn crop in any given year, whereas the sexual generation on Q. cerris uses only 2-8% of the male flowers. There was virtually no parasitism of the agamic generation, while the sexual generation suffers 25-35% mortality from a group of five native parasitoids. (4) The two generations appear to be loosely coupled. It is impossible to predict the abundance of one generation from the numbers in the previous generation, either within or between years. Three features of the system contribute to this uncoupling: (i) the insect is food limited in the agamic generation; (ii) acorn production fluctuated 20-fold, independent of gall densities; (iii) there were two large variable migration mortalities between each generation. (5) There was extremely high tree-to-tree variation in the rate of galling on both species of oak, and these differences were consistent from year to year. Some trees have never been galled, while others support predictably high gall densities losing over 90% of their acorns every year. The possible causes and consequences of this heterogeneity are discussed. (6) The invasion of A. quercuscalicis appears to have had less impact on the native guild of oak-galling cynipids than might have been predicted. Competitive effects appear to be negligible, as the wasp attacks tissues on the two oak species that are not utilized by other cynipids in the U.K. The effect of adding an abundant new host on the dynamics of shared natural enemies was reduced because virtually all the parasitoids emerging from the sexual galls on Q. cerris were males, and the agamic galls were almost parasite-free.