Predicting the realised host-range of Leptinotarsa texana with open-field experiments

The leaf beetle Leptinotarsa texana is a potential biological control agent for silverleaf nightshade Solanum elaeagnifolium in Australia. However, previous research has shown that L. texana can develop on the non-target crop eggplant Solanum melongena in quarantine cage experiments. We used open-field experiments in the agent's native range of Texas, USA, to clarify the realised host-range of L. texana in relation to S. melongena. We conducted five replicated field experiments to investigate: (1) the extent that L. texana would show a preference for oviposition when presented with both S. elaeagnifolium and S. melongena in an open-field context, and (2) the extent that larvae would move from defoliated S. elaeagnifolium onto either nearby S. melongena plants, or bypass S. melongena in search of S. elaeagnifolium. We found, in the open-field experiments, that adult females prefer S. elaeagnifolium over S. melongena plants for settling and oviposition and, in fact, observed no oviposition on S. melongena in any experiment. However, the low levels of oviposition observed in the S. elaeagnifolium experimental plants (the control) was insufficient to conclusively demonstrate that S. melongena is not within the realised host-range of L. texana adults. We also found evidence that L. texana larvae can cause spill-over damage to S. melongena in situations where co-occurring S. elaeagnifolium is defoliated. Finally, we demonstrate the use of an argument map to visually represent the contributions of multiple, sometimes conflicting, field and laboratory experiments to biological control agent risk analysis.