Effects of eugregarine parasites on adult longevity in the polymorphic damselfly Mnais costalis Selys

Abstract. 1. The relationship between the abundance of midgut parasites (eugregarine trophozooites) and the survival of hosts (field‐collected damselflies, Mnais costalis ) was investigated under laboratory conditions. 2. Males of M. costalis are morphologically and behaviourally polymorphic, typically existing as clear‐winged non‐territorial sneaks and orange‐winged territorial fighters (the latter are larger in size). The survival cost of eugregarine infection for the two morphs was compared. 3. Orange‐winged males showed shorter longevity compared to clear‐winged males when they were fed at levels lower than the natural feeding rate, or when they were deprived of food. In contrast, morph longevity did not differ when they were fed until satiation every day. 4. Within each morph, the survival of damselflies was negatively correlated with the parasite abundance except when damselflies were fed until satiation. 5. The results suggest that the abundance of eugregarine parasites exerts a substantial cost, which is associated with the maintenance of alternative mating strategies, together with the higher developmental cost and energy expenditure of the fighter morph.