Improving persistence of entomopathogenic nematodes in aboveground applications for pest control in apple orchards

Abstract: Entomopathogenic nematodes (EPNs) are effective biological control agents of a range of soil-borne insect pests. However, to date the use of entomopathogenic nematodes against aboveground pests remains negligible. The main reason for this is the poor persistence of EPNs in environmental extreme conditions such as direct sunlight, rapid temperature fluctuations and desiccation. The main objective of this study was to determine how persistence of EPNs can be improved when applied as aboveground sprayings in an apple orchard. A field trial was conducted in a randomized block design in which 5 different substrates (straw pellets, paper mulch, hemp, flax and wood chips) were applied around the stems of apple trees. Infective juveniles of Steinernema carpocapsae were applied as aboveground sprayings using a motorized knapsack sprayer. By putting wax moth (Galleria mellonella) larvae as bait in the substrates not only the EPN survival but also their potential to infect a host, after being a few days aboveground, could be tested. Presence and quantities of EPNs able to infect insects during several days after aboveground application were checked with real time quantitative PCR. Significant differences in survival rates and infection potential of S. carpocapsae were obtained depending on the presence and type of substrate at the base of the apple trees. In particular the presence of straw pellets could keep EPN for several days in a good condition. Our results indicate that the persistence of aboveground applied EPNs can substantially be improved by adding specific substrates around the stem at the base of apple trees. This opens perspectives towards a more efficient biological control by aboveground applied EPNs of pests which migrate (e.g. woolly apple aphid, Eriosoma lanigerum) or hibernate (e.g. codling moth, Cydia pomonella) on the tree stem in apple orchards.