Abstract
Climate change affects the abundance, distribution and activity of natural enemies that are important for suppressing herbivore crop pests. Moreover, higher mean temperatures and increased frequency of climatic extremes are expected to induce different responses across trophic levels, potentially disrupting predator-prey interactions. Using field observations, we examined the response of an aphid host-parasitoid system to variation in temperature. Temperature was positively associated with attack rates by parasitoids, but also with a non-significant trend towards increased attack rates by higher-level hyperparasitoids. Elevated hyperparasitism could partly offset any benefit of climate warming to parasitoids, and would suggest that higher trophic levels may hamper predictions of predator-prey interactions. Additionally, the mechanisms affecting host-parasitoid dynamics were examined using controlled laboratory experiments that simulated both temperature increase and drought. Parasitoid fitness and longevity responded differently when exposed to each climatic variable in isolation, compared to the interaction of both variables at once. Although temperature increase or drought tended to positively affect the ability of parasitoids to control aphid populations, these effects were significantly reversed when the drivers were expressed in concert. Additionally, separate warming and drought treatments reduced parasitoid longevity, and although temperature increased parasitoid emergence success and drought increased offspring production, combined temperature and drought produced the lowest parasitoid emergence. The non-additive effects of different climate drivers, combined with differing responses across trophic levels, suggest that predicting future pest outbreaks will be more challenging than previously imagined.
Highlights
Climate change is expected to affect the presence, physiology, abundance, and distribution of plants, insect herbivores and their natural enemies [1,2,3,4]
In contrast to aphid population growth, aphid primary parasitism (n = 947) increased as temperature increased, as did hyperparasitism (n = 501), though in the latter case the effect was marginally non-significant (Table 2, Figure 2B,C). Both primary and hyperparasitism rates increased as the season progressed (Table 2)
Parasitism rates were higher, the trend towards reduced aphid population growth should be interpreted with caution. Despite these apparent positive effects on parasitoids, adult parasitoid longevity was significantly reduced by temperature (Figure 4A), with adults in the warming treatment surviving for 55.867.7 hours, compared with 108.3615.9 hours in the control treatment, an average net reduction of approximately 2.2 days or 51%
Summary
Climate change is expected to affect the presence, physiology, abundance, and distribution of plants, insect herbivores and their natural enemies [1,2,3,4]. These changes can be underpinned by effects of climate on specific life-history parameters such as development times and voltinism, and can affect population densities. Drought stress is likely to have variable effects on plants and herbivores due to the different ways in which plant species respond to drought, as well as differences among herbivores in their sensitivity to plant quality [13] This uncertainty is further compounded by the potential for effects of drought on herbivores to depend on temperature or vice versa [14]
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