The enemy of my enemy is not always my friend: Susceptibility of honey bee to entomopathogenic nematodes.

Entomopathogenic nematodes (EPNs) are effective biocontrol agents against soil-borne insect pests, yet the risks they pose to beneficial insects are understudied, particularly when applied outside soil environments. With growing interest in above-ground applications of EPNs such as on crop foliage or against pests of the western honey bee (Apis mellifera) in hives, it is essential to evaluate their effects on honey bees. We conducted a large-scale assessment of honey bee brood and adult worker susceptibility to Australian EPN isolates ex situ and in situ. At 25 °C, filter paper bioassays showed that larvae (final instar) were highly susceptible to all EPN isolates (LC50 range: 0.66 to 6.8 IJs/cm2), with Heterorhabditis indica (particularly isolates Hi.HRN and Hi.HRN2) being the most virulent. Increasing the assay temperature to 33 °C slightly reduced virulence across isolates, yet all remained pathogenic (LC50 range: 0.7 to 9 IJs/cm2). Similarly, adult workers were highly susceptible (LC50 range: 1.8 to 10 IJs/cm2), again with H. indica Hi.HRN2 showing the greatest virulence. Conversely, Steinernema feltiae (notably isolate Sf.CPBR2) was the least virulent species, although it was still capable of causing high mortality in both honey bee larvae and workers. Interestingly, parasitisation of honey bee larvae by Varroa destructor increased susceptibility to S. feltiae Sf.CPBR2 but not to H. indica Hi.HRN2. Under hive-like conditions, direct treatment of uncapped and capped brood cells with H. indica Hi.HRN2 resulted in <50 % larval and <33 % pupal mortality. Finally, H. indica Hi.HRN2 treatment of subcolonies caused 19 % larval, 5.5 % pupal and 18 % adult worker mortality. Overall, our results indicate that EPN above-ground applications require precautions to safeguard honey bees, particularly during honey bee foraging or for in-hive use.