The impact of entomopathogenic nematodes on a non-target, service-providing longhorn beetle is limited by targeted application when controlling forestry pest Hylobius abietis

Entomopathogenic nematodes are being applied to tree stumps on coniferous clearfell sites in Europe for inundative biological control of the large pine weevil (Hylobius abietis; Coleoptera: Curculionidae), a major forestry pest. We investigated the risk that two nematode species, Steinernema carpocapsae and Heterorhabditis downesi (Nematoda: Rhabditidae), present to longhorn beetle Rhagium bifasciatum (Coleoptera: Cerambycidae), a service-providing, non-target saproxylic insect on clearfell sites. In a Petri dish assay, larvae of R. bifasciatum were less susceptible (LC50 24–42) than pupae or adults (LC50 ≤ 10) to infective juveniles (IJs) of S. carpocapsae and H. downesi. S. carpocapsae and H. downesi reproduced within R. bifasciatum larvae, pupae and adults, with up to 130,000 IJs emerging per insect. When we applied 1.8 million IJs (half the number recommended per tree stump containing pine weevil) to deadwood logs in laboratory and field experiments, both nematode species infected more than 50% of R. bifasciatum within the logs. Field application at a lower, more realistic rate (18,000 IJs per log) resulted in 0–11% infection. The two nematode species caused similar rates of infection within logs and infected R. bifasciatum larvae more than 4 cm deep within the wood. On six clearfell sites sampled one to twelve months after S. carpocapsae had been inundatively applied to tree stumps for suppression of pine weevil, <10% of deadwood logs contained infected R. bifasciatum and