The geographical range structure of the Holly Leaf-miner. III. Cold hardiness physiology
1. The thermal tolerances of a phytophagous insect, the Holly Leaf-miner Phytomyza ilicis Curtis, a specialist on holly Ilex aquifolium L., were investigated to determine whether these contribute to the high levels of miscellaneous larval mortalities that play an important role in determining the local abundances of this species across its geographical range. 2. Temperatures of crystallization, lower and upper lethal temperatures and rapid cold hardening experiments indicated that the Holly Leaf-miner has a pronounced cold hardiness, exceeding that of its host plant. Acute exposure to cold does not therefore contribute directly to miscellaneous mortalities and neither do summer temperatures exceed the upper lethal limits of the larvae. 3. Holly Leaf-miner densities are north-skewed; in the south densities gradually taper off and disappear well north of the distribution limits of the host plant. The parasitoid Chrysocharis gemma causes high larval mortalities in the centre of the distribution of the Holly Leaf-miner and is absent in the cold north, with the level of parasitism being positively correlated with miscellaneous mortalities. Only C. gemma adults overwinter and the attacks on Holly Leaf-miner, starting late winter, are likely opportunistic. This, and C. gemma's absence in the cold north, suggest significant cold susceptibility in C. gemma larvae. Thus, the greater proportion of miscellaneous larval mortalities of the Holly Leaf-miner are possibly the result of pre-premature deaths when C. gemma larvae are killed by cold snaps in late winter.