Sublethal effects of experimental warming on host–parasitoid interactions remain consistent across host plants

Abstract

Rising global temperatures can affect host plants and insects in a variety of ways. Species interactions can be disrupted as temperatures increase, particularly when members of different trophic levels show differential responses to thermal stress. In agricultural systems, parasitoids are frequently used as biological control agents to reduce crop damage by herbivores, so understanding the effects of thermal stress on multiple trophic levels is imperative for effective management. We experimentally tested the effects of increased temperatures (ambient: 23–30°C; hot: 27–34°C) on the multi-trophic interaction involving three varieties of cultivated Brassica oleracea (L.) (Brassicaceae) (cabbage, kale, or broccoli), the caterpillar pest Plutella xylostella (L.) (Lepidoptera: Plutellidae), and its primary parasitoid Diadegma insulare (Cresson) (Hymenoptera: Ichneumonidae). We found that performance metrics of both the caterpillar and parasitoid were consistent among host plants. We found decreased development time and reduced pupal weights at higher temperatures in both the caterpillar and the parasitoid, but no effect of temperature on mortality of either the herbivore or its parasitoid. Development time was more accelerated in the caterpillar than the parasitoid, and pupal weights declined more in parasitoids than caterpillars. Caterpillars behaviorally mitigated temperature stress by pupating beneath leaves, resulting in sublethal effects for both hosts and parasitoids. Kale plants growing under higher temperatures had higher foliar nitrogen, but caterpillar performance data suggested that this was due to increased chemical defenses rather than increased nutrition. Together, these results suggest that biological control of P. xylostella could be disrupted by climate change, especially as lower pupal weights are correlated with reduced fecundity, which could reduce parasitoid population densities; however, as the effects of increased temperature are largely consistent across host plants, similar management practices can be used on different host plants.