HOST-PLANT SELECTION IN THE HOLLY LEAF-MINER - DOES MOTHER KNOW BEST

(1) The holly leaf-miner (Phytomyza ilicis) is a monophagous leaf-mining agromyzid that attacks holly (Ilex aquifolium). (2) Twenty-five individual holly bushes maintained six- to eightfold differences in levels of infestation over 3 years (three generations of mines), despite being in close proximity. Differences in levels of infestation were due to differential oviposition by adult flies. (3) Infestation levels were negatively correlated (nominal P < 0.05) with nitrogen levels in young foliage at the time of oviposition in one generation of flies (1988); however, this result is difficult to interpret because several significance tests were carried out. No other characteristics of the bushes were correlated with levels of infestation. (4) Contrary to expectation, larval performance and host-plant selection by adult female were not correlated. There were no differences in mine size, larval growth rate, or size of adult females at emergence on heavily infested or lightly infested bushes. Percentage successful emergence was low, and did not differ significantly with mine density. (5) The heaviest source of miner mortality (miscellaneous larval deaths) was density-independent. Mortality of larvae surviving miscellaneous larval deaths was significantly spatially density dependent (revealed by k-factor analysis), due to parasitism by Chrysocharis gemma. Spatially density-dependent larval parasitism, and miscellaneous larval deaths were inversely correlated. (6) We discuss these results in the context of the ideal free distribution (Fretwell 1972), speculating that high densities of animals reduce the quality of intrinsically superior habitat patches via spatially density-dependent larval parasitism, leading to equal miner performance on all bushes.