The role of wolf spiders (Araneae: Lycosidae) on the biological control of the bollworm Helicoverpa spp. (Lepidoptera: Noctuidae) in cotton crops
thesisposted on 28.03.2022, 15:08 by Dalila Rendon
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 acquired toxicity by Teleogryllus which led to spider mortality can disrupt biological control of Helicoverpa. In addition to consumptive effects of direct predation, wolf spiders also exerted non-consumptive effects on Helicoverpa in laboratory containers, Helicoverpa larvae spent less time on a cotton boll and more time on the soil in the presence of a spider. Additionally, increased loss of cotton boll mass likely reflects changes in Helicoverpa foraging behaviour induced by the presence of spiders. Considering the setting (laboratory, glashouse, field), and the interactions with intraguild predators and alternative prey, wolf spiders showed various strengths and limitations in their capacity to control Helicoverpa. Given the high diversity and abundance of wolf spiders in cotton fields throughout the cropping season, and the high proportion of Helicoverpa spp. larvae and moths that spiders kill even in the presence of alternative prey, wolf spiders should be considered important biological control agents when implementing pest and Bt resistance management strategies.