Time-dependent behavioural plasticity influences oviposition latency in no-choice host-specificity trials for a candidate weed biocontrol agent

Laboratory no-choice trials are standard bioassays used to evaluate the host range of arthropods under inves-tigation as potential biological control agents against weeds. In these trials false positives can occur when oviposition and development occur on test plants that are not utilised as a host in the field. Deprivation from preferred hosts can lead to insects accepting a wider range of plant species for oviposition. Observations of what appears to be undiscriminating behaviour in host-specificity trials can result in a decision to not release what would otherwise be a safe, specialised agent. In conducting host specificity testing of the weevil, Listronotus sordidus (Gyllenhal) against the aquatic macrophyte, Sagittaria platyphylla (Engelm.) J.G.Sm. (Alismataceae) we observed oviposition on non-target species in no-choice quarantine laboratory trials ranging from only a few eggs being laid on test species to an abundance of eggs on others. While data showing high and very low (or no) oviposition provides a clear indi-cation of the weevil's likely acceptance of these plant species in the field, interpretation of the results becomes more difficult when intermediate levels of oviposition occur. In this study we sought to gain further insights into oviposition behaviour on 'intermediate' hosts. Oviposition by L. sordidus was observed over a range of time periods in no-choice host trials to determine if lower oviposition counts represented consistently low oviposition over time, or whether this was due to oviposition latency. Test species that were accepted for oviposition at intermediate levels at the conclusion of the trial were less likely to be oviposited on than the host during earlier time periods. Oviposition latency may indicate these species are not utilised as hosts in the field as we predict that the agent would initially reject these species and continue searching for a more suitable host. We recommend measures of time-dependent changes in oviposition be recorded in host-specificity trials, and that post-release host range studies could be used to validate the utility of oviposition latency as a predictor of host use in the field. In time, this information could be incorporated into risk assessment processes to reduce the risk of rejecting safe agents.