Mortality factors affecting the leaf-mining stages of Phyllonorycter (Lepidoptera: Gracillariidae) on oak and birch: 1. Analysis of the mortality factors

Abstract

Mortality of life stages of Phyllonoycter species in mined leaves of oak and birch was investigated in a Cheshire locality during 1974. Phyllonotycter species are mostly bivoltine, but P. cavella is univoltine. Parasitism and host-feeding (predation) by Hymenoptera caused most mortality. Parasitism was heaviest in the first generations, with ectoparasitism predominating, although endoparasitism caused inore mortality in the second generations. The incidence of host-feeding increased to leaf-fall when more mines contained host-fed remains than either parasites or healthy Phyllonoycter. Host-feeding occurred particularly in the first three (sap-feeding) larval instars; ectoparasitism affected mainly the two subsequent (tissue-feeding) larval stages and pupae; and endoparasitism occurred in all stages although affected hosts are killed only from the fourth larval instar onwards. Highest mortality was suffered by tissue feeders in the first generation but by sap feeders in the second. Total mortality was greatest in second generation mines and, in a given generation, survival curves for Phyllonotycter were similar on both tree species. The observed density of second generation mines relative to first in the sample was higher than predicted from first generation mortality levels, and this is discussed.