Long-term efficacy of Encarsia dispersa Polaszek (Hymenoptera: Aphelinidae) for the biological control of Aleurodicus dispersus Russell (Hemiptera: Aleyrodidae) in tropical monsoon Australia

Introductions of Encarsia dispersa Polaszek (Hymenoptera: Aphelinidae) for the biological control of spiralling whitefly, Aleurodicus dispersus Russell (Hemiptera: Aleyrodidae), were made directly onto Thursday Island, Torres Strait Queensland, in May 1994. Its efficacy was monitored intermittently for 7 years on 10 Acalypha wilkesiana Müll. Arg. (Euphorbiaceae) and two Terminalia catappa L. (Combretaceae) plants. Marked declines in whitefly densities were recorded on all hosts approximately 10 months after parasitoid releases. Declines in whitefly densities over the first 2 years were mostly matched with increases in per cent parasitism; these declines generally coinciding with wet seasons, with small resurgences occurring over the first dry season following the first decline. Whitefly densities and per cent leaves infested on T. catappa, and on A. wilkesiana with flat leaves were low by the latter half of the study. In contrast, A. wilkesiana with convoluted leaves seemed to inhibit the parasitoid by providing physical refuges for whitefly nymphs, resulting in higher whitefly numbers with low parasitism, particularly in dry conditions. Ten to 12 years after these initial declines, whitefly densities have remained low. Adults and larvae of Cryptolaemus affinis Crotch (Coleoptera: Coccinellidae) and larvae of Acletoxenus quadristriatus Duda (Diptera: Drosophilidae) were observed preying on whitefly nymphs during the study. Overall, sampling data suggested that A. dispersus was under biological control. It may be prudent to maintain A. wilkesiana with convoluted leaves (acting as host refuges) on the Torres Strait islands to retain parasitoid persistence, thereby maintaining controlled and stable populations of A. dispersus. In addition, because of the pest's wide host range and the ability for hosts with closed leaf structures, such as A. wilkesiana with convoluted leaves, to harbour higher pest numbers, movements of these plants within or out of tropical areas could pose a significant quarantine risk.