Combining ecological and molecular methods to investigate predation of a lepidopteron pest complex of Brassica crops

Authors: FURLONG, M.J. - Queensland University - Australia; Rowley, Daniel; Greenstone, Matthew

Submitted to: Entomologia Experimentalis et Applicata
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/24/2014
Publication Date: 9/25/2014
Citation: Furlong, M., Rowley, D.L., Greenstone, M.H. 2014. Combining ecological and molecular methods to investigate predation of a lepidopteron pest complex of Brassica crops. Entomologia Experimentalis et Applicata. DOI: 10.1111/eea.12231.

Interpretive Summary: Conservation biocontrol is the modification of crop fields or farming practices to attract and retain natural enemies to kill insect pests. Crop fields contain many different natural enemy species whose impact on particular pests may not be known, making it difficult to choose the best candidates for conservation. Parasitic wasps and predatory insects and spiders are known to be important natural enemies of cabbage- pests, but their small size and cryptic feeding habits make them difficult to study. Also, predators sometimes consume parasitized pests, which reduces the parasites’ effectiveness as biocontrol agents. We used a combination of polymerase chain reaction (PCR) assays to detect pest DNA in the guts of predators, and field experiments in which these natural enemies were either allowed to attack the pests or prevented from doing so, to determine their importance in controlling the pests. The most important predators were spiders, which consumed an average of 57% and 88% of two pests, the cabbage cluster caterpillar and diamondback moth, respectively. Most important, the spiders attacked the youngest caterpillars, whereas the most effective parasitic wasp attacks older caterpillars, so that its effectiveness is not reduced by the spiders’ feeding. This information will be used by growers and consultants to devise pest management programs that enhance the survival and effectiveness of spiders and parasitic wasps in cabbage, for example by avoiding pesticides toxic to them, or by planting companion crops that provide alternate food for them throughout the growing season.

Technical Abstract: In southeast Queensland Brassica crops are attacked by Crocidolomia pavonana F. (Lepidoptera: Crambidae) in late summer and autumn, and Plutella xylsotella (L) (Lepidoptera: Plutellidae) from late autumn through spring. The impact of endemic predatory and parasitic arthropods on each pest was studied by physical exclusion from experimental cohorts of C. pavonana and P. xylostella during the 2006 growing season. Randomly selected cabbage plants were destructively sampled and predators on the soil surface were collected for molecular gut-content analysis for DNA of each pest. In May, mean predator density was 1.8/ plant, and 0.57 (95%CI=0.40-0.74) of the C. pavonana cohort was lost to predation. Spiders accounted for 92% of all predators (Lycosidae (68%), Theridiidae (5%), Salticidae (5%), Clubionidae (3%), unidentified (11%). Crocidolomia pavonana and P. xylostella were detected in 23% and 5% of the collected Lycosidae (n=26), respectively. In August the mean predator density was 2.1/ plant, and 0.88 (95%CI=0.83-0.93) of the P. xylostella cohort was lost to predation. Spiders accounted for 99% of all predators (Lycosidae (51%), Theridiidae (7%), Salticidae (2%), Clubionidae (5%), unidentified (32%). Remains of P. xylsotella were detected in Lycosidae (11%; n=95), Theridiidae (42%; n=12), and Clubionidae (40%; n=10). Plutella xylsotella larvae are small and pupate on the host plant while C. pavonana are larger and pupate in the soil; these inter-specific differences probably explain the relative importance of foliar (Theridiidae and Clubionidae) and epigeal spiders (Lycosidae) as predators of each species. Most predation was on first instars, which means there is little impact on the key P. xylostella parasitoid Diadegma semiclausum, which attacks later instars.