The potential of Cleobora mellyi (southern ladybird) as a biological control agent of the invasive tomato-potato psyllid (Bactericera cockerelli) on potatoes

Abstract

Invasive species cause large annual economic losses worldwide, including the New Zealand agricultural and horticultural sectors, and threaten our native and endemic species. Insects are some of the most damaging invasive species in New Zealand horticultural production, causing crop losses either directly by plant damage, or indirectly by vectoring disease. The effective management of these insects is critical to maintaining viable industries, but must be done in such a way as to limit, where possible, environmental harm. In this thesis I focussed on the biological control of the invasive insect tomato-potato psyllid (Bactericera cockerelli), an exotic pest of solanaceous crops from Central America. The biological control agent I investigated was the southern ladybird (Cleobora mellyi). The tomato-potato psyllid was first detected in New Zealand in 2006, rapidly dispersing around the country. This pest also causes a physiological condition known as psyllid yellows, and vectors a bacterium (Candidatus Liberibacter solanacearum syn. psyllaurous) that causes zebra chip disease in potato plants. The psyllid, psyllid yellows, and zebra chip disease combined are estimated to have cost the potato industry NZ $120 million from 2006-2011. The southern ladybird was first introduced to New Zealand in the 1970s as a biological control agent for the eucalypt tortoise beetle (Paropsis charybdis), with further recent releases. Laboratory based consumption bioassays and behaviour analysis indicated that the southern ladybird had potential as a biological control agent, work which is developed furthered in this thesis. Under glasshouse conditions, I investigated two main objectives for my first experimental chapter: 1) if the southern ladybird could reduce tomato-potato psyllid densities on potato plants; 2) if there was a trophic cascade caused by top-down control resulting in differences in tuber number, weight, and dry matter. The results showed that over a 10 week sampling period, the southern ladybird significantly reduced TPP densities. Where there was only the psyllid, and no ladybird, no tubers were formed, while where there was the ladybird and psyllid, significantly smaller and fewer tubers were produced than the control and ladybird only. To test for prey choice, I conducted a laboratory bioassay using small experimental arenas. Both adult and larval ladybirds were trialled using the same methodology. A single ladybird in each arena was presented with a choice between 10 psyllid nymphs and either 10 green peach aphids, potato aphids, or greenhouse whitefly nymphs. Neither the adult nor larvae of the southern ladybird showed a preference for the psyllid over either aphid species, but did show an aversion to the whitefly as prey. I investigated the longevity of unmated adult southern ladybirds over a 12-week period using four treatments: 1) water only; 2) a floral resource (flowering buckwheat); 3) 10 psyllids/day; 4) flowering buckwheat + 10 psyllids/day. The unfed ladybirds showed 100% mortality within a two week timeframe. The ladybirds provided with only 10 psyllids/day, and those provided with only buckwheat survived significantly longer than those unfed. Ladybirds fed the combination of psyllids and buckwheat survived significantly longer than those fed only psyllids and the control. The southern ladybird showed some potential as a biological control agent of the tomato-potato psyllid. Further research, such as testing for intraguild predation, and replicating the results in this thesis in semi-field experiments is required to determine if full-scale field releases of this predator as a biological control agent should progress.