Biological control of the woolly apple aphid (Eriosoma lanigerum) using entomopathogenic nematodes

The woolly apple aphid (WAA), Eriosoma lanigerum, has become a major pest in apple orchards in Western Europe after restriction of effective insecticides. We evaluated the efficacy of commercially available species of entomopathogenic nematodes (EPN) (Heterorhabditis bacteriophora, H. megidis, Steinernema carpocapsae, S. feltiae, S. glaseri and S. kraussei) to control WAA. Only S. carpocapsae caused significant mortality in screening experiments in multiwell plates (between 20% and 40% mortality). Adding surfactants to the nematode suspension, to reduce the water-repellent properties of the cuticle and woolly structures of the aphids, did not improve control by EPN. In laboratory and semi-field experiments, parasitization rates of WAA by EPN were frequently higher than mortality rates, as insects were often parasitized but still alive. Parasitization rates of the aphids even increased 3 days after treatment, while mortality remained the same. Presence of EPN had no effect on aphid reproduction as numbers of embryos in parasitized and non-parasitized females were similar. Treating the aphids with the plant-parasitic nematode Pratylenchus thornei in the multiwell plates led to a similar mortality as applying S. carpocapsae. These observations indicate that mortality caused by S. carpocapsae to WAA throughout the study was probably caused by a factor non specific to EPN application, e.g. excess stress. This finding warrants careful interpretation of mortality observed in artificial conditions. Hypotheses for the lack of immediate death following successful parasitization of WAA by S. carpocapsae will be discussed.