Silverleaf whitefly (SLW) and green vegetable bug (GVB) are important pests in the Australian cotton industry. However, the particular natural enemy species involved in suppressing population development of these pests is poorly understood. Such information would be valuable in developing strategies for the conservation of these natural enemies, especially in terms of insecticide selection, management decisions and understanding sources of the beneficials (e.g. other crops, native vegetation).

We used primers specific to either SLW or GVB DNA to identify the presence of SLW or GVB DNA in the gut of predators. Positive results would indicate that predator had consumed either SLW or GVB. We used a pre-existing primer for GVB and developed a new primer for SLW, however, both primers required refining to improve sensitivity. We tested these primers against a wide range of insects and spiders found in cotton crops and confirmed that the primers reacted with the appropriate DNA and did not cross react with that from other species. We also confirmed that the risks of contamination due to the sampling process was low, eg SLW DNA contaminating a sweep net or beatsheet leading to contamination of insects collected from the sheet.

Over two cotton seasons we collected a wide array of potential predator species and tested them for the presence of either SLW or GVB DNA. Twenty-four predator species tested positive for the presence of SLW DNA. These included a range of spiders (Night stalker, Lynx, Orb weavers, Tangle web and Jumping spiders), predatory bugs (Brown smudge bugs, Damsel bugs, Big-eyed bugs, Minute pirate bugs), “facultative” predatory bugs (Green mirids and Apple dimpling bugs), predatory beetles (Red and Blue beetles, 4 lady beetle species), Green lacewing larvae and “phytophagous” thrips adults and larvae (probably mostly Frankliniella spp.). GVB abundance was low, limiting the value of results. Nevertheless, a number of predators tested positive for GVB DNA including several spider species (Tangleweb, Lynx, Nightstalker and House spiders), Damsel bugs, Red and Blue beetles and Striped ladybeetle adults.

Detailed observations were made of predatory behaviour and the appearance of SLW nymphs after being attacked by different predators and these observations and images will be useful for identifying nymphs that have suffered predation in the field.

Attempts to correlate the abundance of predator groups with the abundance of SLW showed no significant relationships except for Apple dimpling bug where abundance increased as SLW abundance increase and total thrips (adults and larvae) which declined as SLW abundance increased. There were no significant relationships between SLW abundance and the proportion of any predator group testing positive for SLW DNA. This lack of correlation possible reflects the generalist nature of the predatory species, so they are not necessarily going to respond numerically in abundance to a single prey species.

A ‘predation’ index was calculated by multiplying the abundance of the species at the site by the proportion positive for SLW DNA. Across the two years of study Red and blue beetles, thrips adults and larvae, Brown smudge bugs, Apple dimpling bugs, Big-eyed bugs, Minute pirate bugs and Lynx and Night stalker spiders potentially have the biggest effect on SLW abundance. This study has provided the first step in using molecular techniques to identify beneficial species important for control of SLW for conservation in Australian cotton systems.