Research concerning the biodiversity-disease relationship over the past 20 years has emphasized how changes in the diversity of hosts in a community, many of which are anthropogenically induced, influence the risk of disease transmission into a focal species of host (Halliday & Rohr, 2019;Johnson & Thieltges, 2010). Tests of the dilution effect hypothesis, which suggests that increased biodiversity (of both host and nonhost species) reduces infection risk within host communities, have been central to this research focus (Civitello et al., 2015;Keesing et al., 2006). The results of such tests have shown that increases in biodiversity can dilute the risk of transmission of infective stages of important human (e.g.,
1. Climate effects on plant defences, and subsequently insect herbivores, have received considerable attention. However, climate also directly affects the physiological condition of insects that may influence their ability to combat plant defences.2. This study tested the effects of body fat and water content on the ability of mountain pine beetles, Dendroctonus ponderosae Hopkins, to tolerate monoterpenes, the primary defences of their host pine trees. To manipulate their physiological condition, we exposed beetles to one of three environmental treatments: cold (4 °C, 30% RH), humid (21 °C, 70% RH) or dry (21 °C, 30% RH). Beetles had the highest fat and water content in the cold treatment, followed by lower fat but the same water content in the humid treatment, and the lowest of both in the dry treatment. Following environmental treatments, beetles were exposed to 0, 312.5 or 625 ppm vapours of monoterpenes (−)‐α‐pinene or (R)‐(+)‐limonene.3. Survival during both environmental and monoterpene treatments was highest for the cold treatment, less for the humid treatment, and lowest for the dry treatment. Fat and water content positively predicted survival. In response to increasing monoterpene concentration, survival, body condition and water content, but not fat, declined.4. These results indicate that water content of insect herbivores was important for the realised toxicity of plant defences, presumably due to the process of excreting detoxified compounds. Energetic costs of detoxification were less evident. Dehydration of insects has not been widely considered in insect–plant interactions, but effects of climate change on humidity may become increasingly important for these interactions.
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