Elevated temperature and drought interact to reduce parasitoid effectiveness in suppressing hosts

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.

Figure 1. Field site locations.

Markers indicate names and field site locations in Canterbury, South Island, New Zealand.

Figure 2. Temperature effects on each trophic level.

Relationship between mean daily temperatures and (A) aphid abundance (Z = 0.18, P = 0.857), (B) proportion of aphids parasitised (Z = 5.91, P<0.001), and (C) proportion of aphids hyperparasitised (Z = 1.71, P = 0.087). Dashed lines indicate non-significant effects. Note: data are site averages, whereas individual measurement dates were analysed, grouped by sites, in the mixed effects models.

Figure 3. Interactive effects of temperature and drought on aphid population growth.

Rate of aphid population growth in laboratory mesocosms under 3 treatments, calculated using aphid abundance in parasitoid (Diaeretiella rapae) treatments minus aphid abundances in the predator-free control to give the overall net predator effect (±SE). Temperature caused the greatest reduction in aphid population growth (Z = −4.87, P<0.001) followed by drought (Z = −5.92, P<0.001). In contrast, in the drought × temperature treatment (Z = 11.29, P<0.001) treatment, aphid population growth was positive.

Figure 4. Effects of temperature and drought on parasitoid fitness.

(A) Parasitoid longevity under drought, elevated temperature, drought and elevated temperature, and control treatments (±SE). (B) Number of offspring per female (dark bars) and percentage of successful adult emergence (light bars) (*temperature: Z = 2.29, P = 0.02, **drought × temperature: Z = −3.26, P = 0.001).