Differences in immune ability do not correlate with parasitic burden in two Zygoptera species (Calopterygidae, Coenagrionidae)

Differences in phenoloxidase (PO) and hydrolytic enzymes (HE) activity, two key components in insect immune ability, were investigated in Hetaerina americana and Argia tezpi, to see if they are correlated with patterns of gregarine and mite infection. The prediction was that the sp. with the more robust immune responses would show a less intense parasitic burden. Fully mature adults of both sexes were used. No clear pattern was found: H. americana had higher PO activity while A. tezpi had higher HE activity but the latter sp. had a higher parasitic load for both parasites. Several possible explanations are discussed. However, it seems most likely that either the immune responses measured may be traded-off with other non-immune functions in which both spp. differ in investment or that both immune components may be traded-off with each other.