Potential hosts for infective juveniles of entomopathogenic nematodes can vary considerably in quality based on the characteristics of the host species/stage, physiological status (e.g. stress, feeding on toxins), and infection status (heterospecific or conspecific infection). In this study, we investigated responses of the entomopathogenic nematode Steinernema riobrave to hosts (Galleria mellonella or Tenebrio molitor) that were previously parasitized with conspecifics or injected with the nematode-symbiotic bacterium, Xenorhabdus sp., to determine if there is a preference for previously parasitized/injected hosts and when this preference might occur. In no-choice bioassays, the number of juveniles infecting both host species decreased with increasing time post-infection. However, infective juveniles continued to infect previously parasitized hosts up to 72 h. Significant preference was exhibited by S. riobrave for 24 h post-infection G. mellonella larvae over uninfected, and by 24 h post-injection G. mellonella larvae over 48 h post-injection larvae. No significant preference was exhibited by S. riobrave for T. molitor hosts previously parasitized with conspecifics or those injected with bacteria in any treatment combination. Such preference for, or continued infection of parasitized insects, has the potential to impact nematode efficacy.

Entomopathogenic nematode infective juveniles are likely to encounter both uninfected and infected insects and host quality depends on the stage of the infection. We hypothesized that nematode response to infected hosts will change over the course of an infection. Here, we tested this hypothesis by focusing on the influence of host infection status on long-range attraction to host volatile cues. The attraction response of 3 nematode species (Steinernema carpocapsae, S. glaseri and S. riobrave) with different foraging strategies to infected and uninfected insects (Galleria mellonella and Tenebrio molitor) was tested at 24 h intervals from start of infection to emergence of infective juveniles from depleted host. As expected, based on their foraging strategies, S. carpocapsae was not very responsive to hosts, S. glaseri was highly responsive and S. riobrave was intermediate. Generally, the level of attraction did not change with time after infection and was similar between infected and uninfected hosts. An exception was S. glaseri infected T. molitor, which tended to be less attractive to S. glaseri than uninfected hosts. These results suggest that any influence of host infection status on infection behaviour is occurring at subsequent steps in the host-infection process than host attraction, or involves non-volatile cues.