Do reproductive activities compromise immunological competence as measured by phenoloxidase activity? Field and experimental manipulation in females of two damselfly species

Recent studies of female insects indicate that reproductive activities, such as mating and oviposition, can impair immune ability. Using the two tropical damselfly species Argia anceps Garrison and Hetaerina americana (Fabricius), egg production and phenoloxidase (PO) activity, a key enzyme in insect immunity, are measured in mating, ovipositing and perching females in December and March. Perching females of both species have fewer eggs compared with mating and ovipositing females, which suggests that perching females are not engaged in reproduction. There is seasonal variation in egg number for the three categories in H. americana but not in A. anceps, which can be interpreted in terms of adaptive changes in egg production depending on female–male interactions in the former species but not in the latter species. There is no difference in PO activity among mating, ovipositing or perching females within either species, although measurements in December and March indicate distinct seasonal changes. Juvenile Hormone (JH) is known to reduce the effectiveness of the immune system by favouring the use of resources for reproduction. A possible role for JH is examined in H. americana, using the JH analogue methoprene to manipulate hormone activity, revealing that PO activity is reduced in methoprene‐treated H. americana females. Thus, although the results of the present study are indicative of possible hormone‐driven changes in PO, there is not necessarily a down‐regulation of immune function (as determined by PO activity) during mating or oviposition. The results complement some recent studies countering the idea that reproductive activities reduce the immune ability in insects.