Biotic resistance: Facilitation between invasive Homoptera and invasive ants limits the establishment of an introduced weed biocontrol agent in New Zealand

Abstract

The boneseed leafroller moth Tortrix s.l. sp. ‘chyrsanthemoides’ (BSLR), originating from Western Cape Province, South African was introduced into New Zealand for the biological control (biocontrol) of a South African shrub boneseed Chrysanthemoides monilifera ssp. monilifera but has established only patchily. We investigated factors hypothesized to influence its establishment success. Field surveys and manipulative experiments ruled out climate as a factor and indicated that establishment failure was associated with predation, mainly by invasive ants of South American (Linepithema humile), and Australian (Doleromyrma darwiniana; Nylanderia sp.) origin that were attracted to invasive honeydew-secreting scale insects (Parasaissetia nigra and Saissetia oleae) found on boneseed. An exclusion experiment showed that unless invertebrate predators (mainly invasive ants and Vespula and Polistes wasps) were excluded, BSLR larvae did not survive to maturity on boneseed plants infested with scale insects. This study supports the notion that insect agents that feed externally on the host-plant are susceptible to predation in the presence of ant-tended Homoptera and that if ant-tended Homoptera are present, candidate agents should be prioritized accordingly.