Abstract
The blacklegged tick Ixodes scapularis is the primary vector for the bacterium causing Lyme disease in eastern North America and for other medically important pathogens. This species is vulnerable to attack by fungal pathogens and arthropod predators, but the impacts of interactions between biocontrol agents have not been examined. The biocontrol agent Met52®, containing the entomopathogenic fungus Metarhizium brunneum (=M. anisopliae), controls blacklegged ticks with efficacy comparable to chemical acaricides. The brush‐legged wolf spider Schizocosa ocreata is a predator of I. scapularis that reduces their survival under field conditions. We conducted a field microcosm experiment to assess the compatibility of Met52 and S. ocreata as tick biocontrol agents. We compared the fits of alternative models in predicting survival of unfed (flat) and blood‐fed (engorged) nymphs. We found the strongest support for a model that included negative effects of Met52 and S. ocreata on flat nymph survival. We found evidence for interference between biocontrol agents, with Met52 reducing spider survival, but we did not find a significant interaction effect between the two agents on nymph survival. For engorged nymphs, low recovery rates resulted in low statistical power to detect possible effects of biocontrol agents. We found that nymph questing activity was lower when the spider was active above the leaf litter than when the spider was unobserved. This provides the first evidence that predation cues might affect behavior important for tick fitness and pathogen transmission. This study presents field microcosm evidence that the biopesticide Met52 and spider Schizocosa ocreata each reduced survival of blacklegged ticks Ixodes scapularis. Met52 reduced spider survival. Potential interference between Met52 and the spider should be examined at larger scales, where overlap patterns may differ. Ticks were more likely to quest when the spider was inactive, suggesting the ticks changed their behavior to reduce danger.
Highlights
In a Before-After-Control-Impact study, we found that use of Metarhizium brunneum strain F52 (Met52) for tick control in residential yards is unlikely to cause meaningful reductions in the abundance of nontarget arthropods (Fischhoff et al 2017)
We expected that Met52 and wolf spiders each had the potential for nonconsumptive effects on ticks, given the effects of M. brunneum and wolf spiders on behavior of other species
The interference observed in this study will require further testing in residential yards, where the patterns of spatial overlap in microhabitats between Met52, S. ocreata, and I. scapularis may differ from the microcosms
Summary
Intraguild interference can strongly mediate the effects of predators on prey. For example, due to interference between wolf spiders and carabid beetles, a doubling of carabids resulted in no impact on densities of herbivore pests of squash (Snyder & Wise, 1999). In Anopheles coluzzii, for example, exposure of larvae to the predatory backswimmer Anisops jaczewskii caused negative effects on life history traits that might reduce malaria transmission (Roux et al, 2015) Because of their importance as vectors of pathogens affecting humans, livestock, and wildlife, ticks have been the subject of extensive research on biocontrol. Met caused increased mortality in predatory rove beetles Dalotia coriaria and mites Stratiolaelaps scimitus and Gaeolaelaps gillespiei used to control Western flower thrips Frankliniella occidentalis (Saito & Brownbridge, 2016) These predators and Met were compatible biocontrol agents: the combination of Met and predators suppressed thrips to a greater degree than either predators or Met alone (Saito & Brownbridge, 2016). Models demonstrate that the conditions for coexistence of two consumers of a common resource hold for systems of predator–prey, host–parasitoid, and host–pathogen communities (Borer, Briggs, & Holt, 2007) Both wolf spiders and entomopathogenic fungi may have a combination of consumptive and nonconsumptive effects on ticks. We expected that Met and wolf spiders each had the potential for nonconsumptive effects on ticks, given the effects of M. brunneum and wolf spiders on behavior of other species
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