Maintenance of water balance in the twice-stabbed ladybird beetle, Chilocorus cacti (Coleoptera: Coccinellidae), despite heavy infestation by the mite, Hemisarcoptes cooremani (Acari: Acariformes)

Abstract

Large numbers (up to 400/elytron) of heavily sclerotized deutonymphs of the mite, Hemisarcoptes cooremani, attach to the elytral hypodermis of desert-inhabiting beetles, Chilocorus cacti, for transport and feeding. A potential physiological benefit to beetles of tolerating attached mites in dry habitats may be a reduction of the surface area-to-volume ratio of exposed evaporatory tissue (decreased water loss), especially during flight. In this study, infested beetles were observed to have identical water balance characteristics as nonparasitized beetles, indicating that mites provide no extra, mechanical advantage to the beetle in terms of increased water conservation. The beetles were not distinguished by having a low water content, high tolerance for dehydration, or ability to absorb water vapor from the atmosphere. Their ability to retain water (low net water loss rate that compares favorably to values for diapausing beetles that maximize water conservation during quiescence), however, helps adapt them to a dry environment, supports the behavior of reflex bleeding, and probably aids in the toleration of their interaction with Hemisarcoptes.