To be a good killer: Characterizing the virulence of entomopathogenic nematodes against wireworms.

Wireworms (Coleoptera: Elateridae) are generalist insect pests that can inflict significant economic losses on various crops. The restricted use of certain insecticides applied until now to control wireworms and the lack of alternatives have led to fears that the populations of these insects will increase over the coming years, causing ever-greater yield losses. Entomopathogenic nematodes (EPNs) have emerged as potential tool for wireworm control but unfortunately the virulence varies among populations. Therefore, the main objective of this work was to study the factors that may influence the virulence of different EPN populations against wireworms. Two hypotheses were proposed: first, the specific morphology of EPNs could impact their capacity to infect wireworms, with smaller EPNs being more efficient in physically entering wireworms. Second, we explored whether symbiont bacteria associated with EPNs might influence their virulence by identify whether specific characteristics, such as the metabolic activity or biochemical properties of these bacteria, correlate with the efficacy of infection.
We performed an efficacy screening and selected EPN populations that readily infect and kill wireworms (Agriotes obscurus and A. lineatus). Thirteen EPN populations (Steinernema carpocapsae, S. affine, S. feltiae, S. poinari and Heterorhabditis bacteriophora) were considered, three of which were of commercial origin, three from external laboratories and seven from isolations made in the Wallonia region (Belgium) on sites with high wireworms’ activity (areas under organic farming, under pesticide treatment, in grasslands, and in forests). Larval mortality was checked 24 h, 48 h, 72 h up to 8 weeks after infection.
Then, nematode morphometry was evaluated by measuring the length and diameter of newly emerged IJs from G. mellonella (n=30). Symbiotic bacteria were isolated from each population, identified through molecular analysis techniques and the metabolic characterisation of bacteria was carried out via the Analytical Profile Index (API) tests.

We identified three populations with potential infectivity against wireworms, all belonging to the genus Steinernema.The identified symbiotic bacteria belong to the species Xenorhabdus nematophila, X. bovienii, Photorhabdus laumondii and P. thracensis. The assimilation of arginine dihydrolase, urease, gelatinase and mannitol differed between the bacteria. Finally, the multiple comparisons shows that characteristics of symbiotic bacteria (species, sugar assimilation, gram characteristics) do not appear to be involved in the virulence of EPN populations. Yet, nematode morphometry emerged as a potential determinant of EPNs infection rate. These data provide valuable insights for optimizing biological control strategies in agricultural applications, pointing out the need to consider the morphology of EPNs in designing future applications to control wireworms.