Quantitative analysis of the causes of outbreaks in leaf-mining insects: application to the common leaf-miner Leucoptera malifoliella (Lepidoptera: Lyonetiidae) on apple tree

Leaf-mining by insects usually have limited detrimental consequences on host-trees. Yet, Leucoptera malifoliella, a common leaf-mining moth on Rosaceae, is among those leaf-mining species that can exhibit occasional, sometimes even regular eruptive impacts on their hosts, in particular on apple trees. Here, we address the different causes involved in a particular case of nascent outbreak of this moth observed in the wild that is independently of the artificial conditions that prevail in densely planted apple orchards. Relevantly analyzing the reasons of eruptive outbreaks remains difficult however, as multiple causes are involved and their respective influences on mining-impact level are largely intermingled. Thus, in order to disentangle the respective contributions of each cause in quantitative terms, a specifically dedicated approach, the “Melba” procedure, was implemented and applied to the data issued from an exhaustive sampling of the foliage of the studied wild apple-tree. Out of the three main causal components of mining impact level, neither  the average clutch-size nor the severity of mated females in selecting those leaves acceptable for oviposition substantially differ from the ranges of values encountered in other leaf-mining species having high but non eruptive mining impacts on their respective hosts. In fact, the recorded nascent outbreak is mainly attributable to the unusually high density of mated females attracted to the investigated host tree. Accordingly, metapopulation effects obviously play a major role in triggering the recorded eruptive impact.