The uptake of tritiated water by a beetle, Chilocurus cacti (Coleoptera: Coccinellidae), from a mite, Hemisarcoptes cooremani (Acari: Acariformes)

Understanding the origin and evolution of symbiotic interactions between extant species plays an important role in comprehending the nature of their ecological relationships. The dynamics of such complex evolutionary relationships must be examined from multiple perspectives, using both field and laboratory methods.

Phoresy is a form of symbiotic interaction that results in dispersal. It is a coevolved interaction that benefits the dispersed organism but does not affect the phoretic host. In this thesis, I address a purportedly phoretic interaction between the deutonymphal stage (subadult) of a mite Hemisarcoptes cooremani (Astigmata: Acariformes) and the adult stage of the beetle Chilocorus cacti (Coleoptera: Coccinellidae), two potentially important biological control gents of scale insects.

Past radiolabeling studies indicated that the deutonymph of cooremani acquired tritiated water from adult C. cacti, challenging the validity of the paradigm that this relationship, and many like it among the Astigmata, is phoretic. Passage of materials from one organism to another can be indicative of a parasitic relationship. This conclusion may be an incomplete assessment of a fuller relationship in which materials are freely passed among symbionts (mutualism). To probe this Hemisarcoptes-Chilocorus relationship, I conducted a tritiated radiolabel study, similar to the previous test, to determine whether materials were being passed from the deutonymphs to the beetles.

The explicit research hypothesis was that tritiated water could be passed from the deutonymphs to the beetles during the symbiotic interaction. To test this hypothesis, deutonymphs of cooremani were radiolabeled and then allowed to attach to C cacti. After remaining attached for 48-Hours, the beetles were tested for tritium concentrations. The results of this experiment demonstrated that beetles acquired tritium from the deutonymphs. Both the elytra and the body of the beetle showed significantly higher levels of tritium when mites were attached than when they were not. These results have interesting implications for understanding the complex relationship between Hemisarcoptes and Chilocorus. I propose that we now tentatively redefine this relationship as mutualistic. However, this judgment must be held in reserve until verified by examination of the potential fitness advantages conferred on both members of this interaction.