Increased temperature alters beetle and predator behavior to determine rates of predation and herbivory
Predators can limit prey abundance and/or levels of activity. The magnitudes of these effects are contingent on predator and prey traits that may change with environmental conditions. Predators help naturally limit herbivore (pest) numbers in many crops, though top-down pest suppression is often undermined by pest management strategies that also harm predators, such as applying pesticides. In lieu of chemicals, plant covers are often used in organic agriculture to block pest attacks, but the increased temperatures they can cause might stress or kill valuable predators. To explore the interactive effects of temperature and predator heat stress on suppressing pests and controlling herbivory, we performed a laboratory experiment at divergent temperatures that exposed an important pest species to two spider predator species that differ in their thermal tolerance. First, we assessed the thermal tolerance of the Cucumber Beetle (Diabrotica undecimpunctata) pest, wolf spider (Hogna helluo) predator, and nursery web spider (Pisaurina mira) predator in environmental chambers. Later, in the same chambers, beetles fed on squash plants for seven hours alone or with one of the spider predator species at ambient (22 C) and warmed (38 C) conditions. We recorded final herbivory and the location and behavior of predators and prey throughout.
Temperature had significant effects on beetle and spider activity/survival that together determined plant damage. The Cucumber Beetle pest and wolf spider predator were equally heat tolerant, having critical thermal maxima (CTM) above 40 C, but the nursery web spider had limited heat tolerance with a CTM of 34 C. At ambient temperatures, beetles rarely ventured into areas where they were vulnerable to predation, and both wolf and nursery web spiders killed an average of one out of three beetles over seven hours. In contrast, under warmed conditions beetles were more active at or near the surface where heat-tolerant wolf spiders killed an average two beetles, but the heat-intolerant nursery web spiders killed only one beetle on average. Beetle feeding and plant damage increased with temperature, but both decreased in the presence of either spider species. Ultimately, herbivory was highest at high temperatures with no predators, lowest at low temperature with predators, and moderate at high temperatures with predators or low temperatures with no predators. Our results show that the effect of predators on herbivore survival, feeding, and plant damage can be contingent on both predator and prey traits that vary, perhaps in opposition, across a single environmental gradient.