The role of wolf spiders (Araneae: Lycosidae) on the biological control of the bollworm, Helicoverpa(Lepidoptera: Noctuidae), in cotton crops

Abstract

Thesis by publication.%%%%%%%%%%%%1. Influence of crop management and environmental factors on wolf spider assemblages (Araneae: Lycosidae) in an Australian cotton cropping system – 2. Ecological and molecular approaches for assessing common prey and Helicoverpa larva consumption by wolf spiders in a Bt-cotton field – 3. Consumptive and non-consumptive effects of wolf spiders on cotton bollworms – 4. Intraguild interactions between two wolf spider species lead to non-additive effects on the biological control of the cotton bollworm – 5. Prey encounter, prey vulnerability and nutritional content in a ground arthropod trophic web in cotton crops – 6. A killer killed? Wolf spider mortality after feeding on ground crickets – General discussion – Appendices.%%%%Cotton bollworm larvae (Lepidoptera: Noctuidae, Helicoverpa spp.) that survive on genetically modified ‘Bt cotton’ contribute to the risk of widespread resistance to Bt toxins. A resistance management technique in cotton fields involves deep tilling of the soil to kill overwintering pupae ('pupae busting'), which is incompatible with the agronomic practice of minimum tillage. As a biological control alternative in minimum-tillage cotton fields, ground predators can kill Helicoverpa spp. larvae as they descend from the plant to pupate in the soil, or moths emerging from underground. In this thesis, I examine the impact of biological control from wolf spiders (Araneae: Lycosidae on ground-dwelling stages of Helicoverpa spp., as a strategy for Bt resistance management in minimum-tillage fields. Wolf spider diversity was higher in complex minimum-tillage cotton plots compared to simple tilled cotton plots. Predation events of general prey were rare to obseve in the field, and gut-content tests that a low proportion of wolf spiders (2.1%) potentially killed IgG-marked Helicoverpa spp. larvae, bu this is likely due to the low rate of spider recapture in cotton plots due to migration. In encosed containers, the three largest and abundant species of wolf spiders Tasmanicosa leuckartii, Hogna crispipes, and Hogna kuyani all kill high proportions of the 5th instar Helicoverpa spp. larvae on the soil. Tasmanicosa killed Helicoverpa before and after pupation; in glasshouse enclosures, a single Tasmanicosa can reduce by 38% the number of larvae surviving to pupation, and by 66% the number of larvae surviving to moth emergence. The increase in abundance (one or two Tasmaniacosa or Hogna individuals) in glasshouse enclosures did not increase Helicoverpa mortality. Increasing spider abundance and diversity (Tasmanicosa + Hogna) in glasshouse enclosures reduced Helicoverpa survival compared to one Tasmanicosa only, but this effect was not additive, suggesting that antagonistic interguild interactions between wolf spiders can limit biological control on Helicoverpa. In the presence of the ground cricket Teleogryllus commodus (a prey commonly observed in the field), Tasmanicosa still killed high proportions of Helicoverpa larvae in laboratory containers. However, transience…