A study of the holly leaf miner and its parasites

Abstract

This short term research was concerned with an Agromyzid parasite –Phytomyza ilicis, Curt, which attacks young unthickened leaves of Jlex aquifolium, causing the formation of brown yellow blotches or mines on the leaf surface. Few papers related to P. ilicis have been published, and no recorded work has been given for the North East area of England, despite the fact that the holly leaf miner /holly tree association provides a useful system for the study of plant-insect relationships. Aims of the investigation were to determine whether differences in levels of infestation existed between trees of varying age and sex within the same area. Variation in infestation with height and aspect was investigated, together with egg mortality, later mortality factors by bird-attack and different species of parasites was also examined. The life-cycle of P. ilicis and its parasites was recorded from the mature larval - adult stages. Adult Sphegigaster flavicornis were recorded from suction-traps only. The study was carried out within Hollingside Wood, Durham city G.R. NZ.276 405. Sixteen holly bushes were examined on the basis of their isolation or proximaty to other holly bushes, position with respect to other vegetation, height and diameter, degree of healthiness based on the number of leaves per twig, sex and position with respect to gradient within the main wood. Six suction-traps were set up within a group of three adjacent trees at varying height Intervals within the University field-station. The size of bushes examined varied between 1.7-9.6 metres. Problems with sampling are discussed, and it was decided to sample between 325-4300 leaves depending on the size of, the tree, The ideal sample size was 800-1000 leases, leaves v/ere examined for the presence of eggs, the number of eggs per leaf, the position along the midrib, the number of mines per leaf and their position on the leaf. Each mine was examined for bird attack, and for the presence of both larval and pupal parasites. Leaf-sections were obtained from each tree for leaves of the 1977 season, to determine whether cuticle thickness, influenced the level of infestation between trees. Cuticle thickness was found to be unimportant in determining whether a leaf was mined or not, however cuticle thickness ultimately is important since adult P. ilicis can only attack young leaves where the cuticle is undeveloped. A variation in the time of emergence for adult P. ilicis was observed when compared with accounts given by Miall and Taylor (1907), Downes (1931), although the general pattern of the life-cycle was similar. No significant difference in the level of infestation was observed between trees, although total mine and egg number varied significantly. Spatial distribution did not appear to influence the density of mines, and no significant difference in population density with regard to position within Hollingside Wood was found. Aspect and height influenced both total mine and egg number. Egg mortality was observed to vary between trees but differences were not significant. Egg density was observed to influence the viability of eggs within the leaf, but; the relationship was density-independent by the larval parasite, C. gemma, and by an undetermined mortality factor influencing survival in the early stages of development. The pupal parasites observed were C, syma and S. flavicornis