Use of natural enemies to achieve biological control of the severe and persistent glasshouse pest, the potato-tomato psyllid : A thesis submitted in partial fulfilment of the requirements for the Degree of Doctor of Philosophy at Lincoln University

The tomato-potato psyllid (TPP), Bactericera cockerelli, is a serious pest of solanaceous crops. Several populations of TPP have become pesticide-resistant, increasing the need for alternative control options, such as biological control. However, biological control of TPP has remained challenging. The parasitoid Tamarixia triozae, has shown some potential in greenhouses but has yet to demonstrate any commercial value. As an alternative option, the potential of the mirid bug, Engytatus nicotianae, was tested, either alone or combined with T.triozae, as a biocontrol agent of TPP on greenhouse tomato plants. The experiments were as follows: a) Laboratory no-choice and choice-feeding assays to investigate the consumption and preferences of E. nicotianae on different TPP life-stages; b) Two greenhouse cage-experiments with whole tomato plants to assess the potential of E. nicotianae as a TPP preventive agent; c) A greenhouse cage-experiment to investigate the efficacy of a combination of E. nicotianae and T. triozae on TPP infested plants. The no choice experiment showed the ability of Engytatus to feed upon eggs and the first four TPP nymphal stages, while the choice experiment highlighted a clear preference by E. nicotianae for the first two TPP instars, followed by the third instar and then the eggs. There was consumption of fourth instars only when all other life-stages had been consumed. The first cage experiment (b1) measured the productivity of whole plants in the presence or absence of TPP and in the presence or absence of introduced E. nicotianae. The E. nicotianae-only treatment produced the same numbers of fruits and flowers as the control (no insects), even though the mirid feeds on tomato leaves and stems, whereas the TPP-only treatments produced fewer fruits and flowers. E. nicotianae did not play a role in significantly preventing TPP-infested plant production loss. However, E. nicotianae markedly reduced TPP numbers; further, it prevented the species from establishing in four of the seven replicates of this experiment, and delayed TPP outbreaks in the remaining three replicates. The second cage experiment (b2) focused solely on the efficacy of E. nicotianae in preventing TPP from establishing and returned remarkable results, preventing TPP infestation in 11 of 14 replicates and markedly delaying TPP outbreak in the remaining 3 replicates. In the third cage experiment (c) the combination of E. nicotianae and T. triozae markedly reduced the number of TPP throughout the trial, showing a greater efficacy than E. nicotianae alone, which played only a marginal role in the control of TPP. These findings suggest that E. nicotianae could be considered to be firstly a useful preventative biocontrol tool and secondly an augmentative biocontrol tool in TPP pest management in tomato glass house productions and points to the need to further investigate how to optimize E. nicotianae mass rearing and release densities for maximum efficacy.