Resistance of the woolly apple aphid (Eriosoma lanigerum) to control by entomopathogenic nematodes

The restricted use of effective insecticides against the woolly apple aphid (WAA), Eriosoma lanigerum, has led to this aphid becoming a major pest in apple orchards in Western Europe. We investigated the efficacy of six commercially available species of entomopathogenic nematodes (EPN) to control WAA. Only S. carpocapsae caused significant mortality in screening experiments in multiwell plates (between 20% and 40% mortality). However, the aphids were often parasitized but not killed by S. carpocapsae. Applying a plant-parasitic nematode (Pratylenchus thornei) killed as many aphids in multiwell plates as did S. carpocapsae. This showed that mortality of WAA due to S. carpocapsae was probably caused by a factor nonspecific to EPN application, e.g. excess stress. Normally, insects are killed by EPN through the endotoxins produced by their symbiotic bacteria, which are released into the insect hemolymph by the EPN vector. We discovered that the symbiotic bacteria of S. carpocapsae, Xenorhabdus nematophila, and H. bacteriophora, Photorhabdus luminescens, were unable to grow in the hemolymph of the WAA. In conclusion, our study suggests WWA are able to bypass control by the EPN-symbiotic bacteria mechanism by preventing growth of the symbiotic bacteria.