Shovon Chandra Sarkar

Agricultural producers in developed countries typically rely on insecticides to control insect pests. The rise in insecticide resistance in various pest species, the risk of chemical residues, and the potential impact of insecticides on non-target species have led to the exploration of alternative methods of management. Biological control, which involves the use of natural enemies to help manage pest populations, has been the focus of considerable research efforts. However, in the case of a newly invasive pest in an ecosystem, there is the specific question of whether any of the resident natural enemy species in the region are capable of effectively utilising the new pest as a food source. In this context, generalist predators are likely to be promising options because their ability to adapt to because of a wide range of prey species.

In early 2017 an invasive pest, tomato potato psyllid, Bactericera cockerelli Šulc (Hemiptera: Triozidae) was first detected on mainland Australia near Perth. It can cause significant losses to solanaceous crops directly via feeding and by transmitting the bacterium Candidatus Liberibacter solanacearum that causes zebra chip disease in potatoes, although this pathogen has not yet been detected in mainland Australia. Considering B. cockerelli’s demonstrated ability to develop resistance to insecticides, utilizing resident predatory natural enemies as biological control agents could make a valuable contribution to managing the pest. In this study, the species of B. cockerelli predators occurring in fields of Solanaceae in Western Australia were identified and the biocontrol potential of the two most abundant ladybirds, both generalist predators, were evaluated in laboratory and glasshouse experiments.

Sampling was conducted in fields of different solanaceous crops to identify the species of generalist predators that occurred in the environment of south-western WA. Predators were sampled using sweep netting. The predators found represented a broad taxonomic range and their populations in the field were higher when the B. cockerelli population were higher. The most abundant generalist predator was green lacewing (Mallada signatus Schneider (Neuroptera: Chrysopidae)) followed by ladybirds (Coleoptera). To screen the field collected predators for those which had been feeding on the B. cockerelli, PCR primer pairs were developed. Individuals from 7 insect taxa were analysed for the presence of DNA of B. cockerelli. Coleopteran predators were most frequently detected with B. cockerelli DNA. Hippodamia variegata Goeze (Coleoptera: Coccinellidae), an introduced ladybird species, was positive in 70% of cases and an Australian native ladybird, Coccinella transversalis Fabricius (Coleoptera: Coccinellidae), was positive in 65% of cases.

The biocontrol potential of H. variegata and C. transversalis were investigated and compared. An age-stage, two sex life table was formulated to assess each predator’s development and reproduction when utilizing B. cockerelli as a food source in contrast to a common aphid prey. The predatory potential of ladybirds on B. cockerelli nymphs was assessed in a closed arena and the effects of releasing ladybirds for the control of B. cockerelli on caged tomato plants were then evaluated. Both ladybirds could successfully develop and reproduce when feeding on B. cockerelli. A type II functional response was observed on their predation. In the greenhouse, the release of ladybird larvae significantly suppressed the development of the B. cockerelli population and positively influenced the plant chlorophyll content and biomass. The impact of C. transversalis was less than that of the introduced ladybird, H. variegata, reflecting their difference in voracity.

The capacity of the ladybirds to learn from their previous dietary experience and how this might impact prey preference was tested. Early life experience with B. cockerelli as a prey species significantly increased the preference of both ladybird species for the psyllid although the effect in C. transversalis was less than in H. variegata. This resulted in greater suppression of pest populations by the experienced predators on glasshouse grown tomato plants.

This study shows that predators that are already in WA fields can contribute to managing populations of the newly invasive pest B. cockerelli. Hippodamia variegata and C. transversalis show potential as biocontrol agents. Both species successfully develop and reproduce when utilizing the new prey species and were able to suppress the development of pest populations in the glasshouse. Prior exposure to B. cockerelli can influence the predators’ prey preference and alter the impact on the pest. Although a lower proportion of green lacewing tested positive for the presence of B. cockerelli DNA, the much higher numbers present in the field suggest it is another species that should be assessed.

The results have the potential to contribute the implementation of sustainable pest management solutions for the invasive B. cockerelli. They also demonstrate the value of examining generalist predators as possible biocontrol agents for newly invasive species and point to the possibility of improving the impact of augmentative release strategies through early life exposure of generalist predatory insects to target pests. This may be particularly useful in cases of invasive pest species.