Immune Priming, Fat Reserves, Muscle Mass and Body Weight of the House Cricket is Affected by Diet Composition.

Some insect species are capable of producing an enhanced immune response after a first pathogenic encounter, a process called immune priming. However, whether and how such ability is driven by particular diet components (protein/carbohydrate) have not been explored. Such questions are sound given that, in general, immune response is dietary dependent. We have used adults of the house cricket Acheta domesticus L. (Orthoptera: Gryllidae) and exposed them to the bacteria Serratia marcescens. We first addressed whether survival rate after priming and nonpriming treatments is dietary dependent based on access/no access to proteins and carbohydrates. Second, we investigated how these dietary components affected fat reserves, muscle mass, and body weight, three key traits in insect fitness. Thus, we exposed adult house crickets to either a protein or a carbohydrate diet and measured the three traits. After being provided with protein, primed animals survived longer compared to the other diet treatments. Interestingly, this effect was also sex dependent with primed males having a higher survival than primed females when protein was supplemented. For the second experiment, protein-fed animals had more fat, muscle mass, and body weight than carbohydrate-fed animals. Although we are not aware of the immune component underlying immune priming, our results suggest that its energetic demand for its functioning and/or consequent survival requires a higher demand of protein with respect to carbohydrate. Thus, protein shortage can impair key survival-related traits related to immune and energetic condition. Further studies varying nutrient ratios should verify our results.

Effect of juvenile hormone on senescence in males with terminal investment.

Senescence, a decline in survival and reproductive prospects with age, is controlled by hormones. In insects, juvenile hormone (JH) is involved in senescence with captive individuals, but its effect under natural conditions is unknown. We have addressed this gap by increasing JH levels in young and old wild males of the damselfly Hetaerina americana. We assessed survival in males that were treated with a JH analogue (methoprene), which is known to promote sexual activity, and an immune challenge, which is known to promote terminal investment in reproduction in the studied species. We replicated the same procedure in captivity (to control for environmental variation), where males were deprived of any activity or food. We expected old males to show the lowest survival after being treated with JH and immune-challenged, because the effect of terminal investment on senescence would be exacerbated by JH. However, this should be the case for wild animals, but not for captive animals, as the effects of JH and immune challenge should lead to an increase in high energetic-demanding activities only occurring in the wild. Old animals died sooner compared with young animals in both the wild and captivity, confirming that males are subject to senescence. In wild but not captive animals, JH decreased survival in young males and increased it in old males, confirming that JH is sensitive to the environment when shaping animal senescence. Immune challenge had no effect on survival, suggesting no effect of terminal investment on senescence. Additionally, contrary to the expected effects of terminal investment, with an immune challenge, recapture rates increased in young males and decreased in old males. Our results show that male senescence in the wild is mediated by JH and that terminal investment does not cause senescence. One explanation is that animals undergoing senescence and terminal investment modify their feeding behaviour to compensate for their physiological state.