Abstract

Gorse, Ulex europaeus, is an invasive weed that has serious agricultural, economic and ecological impacts. Although various biological control agents have been released in New Zealand, these have showed no noticeable impact on gorse populations. One such agent, Sericothrips staphylinus, was introduced to New Zealand in 1990 and although laboratory impact studies indicated it was a highly promising gorse biological control agent, it has not been as effective as was hoped. We hypothesized this was due to predation by natural enemies. This study investigated the predation and oviposition rates of three phytoseiid mites (Amblydromalus limonicus, Amblyseius herbicolus, and Neoseiulus cucumeris) that have been found on gorse plants in New Zealand on three S. staphylinus stages (1st instar larvae, 2nd instar larvae, and prepupa) in both choice and non-choice conditions. In non-choice conditions, A. limonicus had the highest predation and oviposition rate across all three immature stages, and N. cucumeris had the lowest. Amblydromalus  limonicus, A. herbicolus, and N. cucumeris all had their highest predation rate when consuming 1st instar larvae, and their lowest predation rate when consuming prepupa. In the choice experiment, all three predatory mite species consumed their highest proportion of 1st instar larvae, and their lowest proportion of prepupae.  The oviposition rate of all three mite species in the choice experiment was similar to the oviposition rate when presented with 1st instar larvae only. The results from this study confirm that A. limonicus, A. herbicolus, and N. cucumeris can predate and reproduce on S. staphylinus 1st instar larvae, 2nd instar larvae, and prepupa. This indicates that predation may be the reason why S. staphylinus is an ineffective biocontrol agent in New Zealand.