Antioxidants, oxidative stress and reactive oxygen species in insects exposed to heat

In response to high temperatures, insect metabolic rates increase, favoring the release of higher amounts of reactive oxygen species (ROS). These ROS need to be counteracted by antioxidants to avoid oxidative stress, which can lead to cell damage and death. In this manuscript, we review evidence in insects showing the effects of high temperatures on ROS production, the antioxidant defenses reported in insects in response to high and extremely high temperatures and the extent to which they contribute to preventing oxidative damage. Endogenously produced antioxidants can be enzymatic or non-enzymatic and are involved in heat responses in at least seven insect orders. Our review indicates that evidence is very limited for the effect of high temperature on ROS production, but it clearly shows that at least one antioxidant is upregulated during short-term heat exposure. However, the effects of antioxidants in effectively reducing oxidative damage in biomolecules are still poorly supported by evidence. Dietary-dependent antioxidants show strong potential for coping with heat stress, but evidence is limited, although numerous plants produce antioxidant compounds and a great number of insect species feed on plants. The role of antioxidants in heat acclimation and adaptation is promising but evidence is still very limited in insects. Antioxidants also protect from other prooxidant conditions such as pesticide exposure, nutrient stress, or new biotic interactions, which often act in combination. Potential trade-offs between antioxidant use to different functions could define insect survival and pace of life in response to multiple stressors, including high temperatures. Our literature review indicates that there is only limited evidence of the role of antioxidants in preventing oxidative damage caused by heat, opening the possibility that ROS production might be mitigated by the action of uncoupling proteins or degradation of mitochondria. Finally, we conclude by proposing promising research avenues to gain a deeper understanding of the role of ROS and antioxidants in the oxidative balance of insects exposed to mild and extreme heat.