Regulation of the external mycoflora of the giant Madagascar hissing-cockroach,gromphadorhina portentosa, by its mite associate,gromphadorholaelaps schaeferi, and its implications on human health

Abstract

The giant Madagascar hissing-cockroach,Gromphadorhina portentosa, and its mite associate,Gromphadorholaelaps schaeferi, constitute an intimate commensalistic symbiosis. While the mite’s very survival is dependent by feeding on cockroach saliva and associated organic debris, the degree that the cockroach benefits from this association is unclear. We investigated the mite’s potential role at regulating surface fungi on the exoskeletons of this insect. Numbers of fungal isolates that resulted were compared between captive-bred cockroaches with and without mites. The mycoflora of both groups consisted of common molds (Alternaria sp.,Aspergillus sp.,Cladosporium sp.,Geotrichum sp.,Mucor sp.,Penicillium sp.,Rhizopus sp.,Trichoderma sp.). The presence of mites reduced the number of isolates by 1/2 in mature females, 1/3 in males, and 1/4 in sixth (final) instar nymphs. Fungus levels continued to drop when mite-free cockroaches were artificially supplemented with mites. A direct correlation was detected between mites and the reduction in the quantity of surface molds up to 20 mites per cockroach. The addition of more mites above 20 per cockroach, even 4x more, had a minimal, but still reducing, effect. Mites regulated all types of fungi, not just a select few taxa. We propose that mites reduce the mycoflora not because they consume fungi, but because mites and molds compete for the same resources in an ecological niche, saliva and organic debris that accumulate in between cockroach’s legs. Cockroaches reared in captivity do not apparently benefit by the removal of surface molds by mites, lending support for a commensalistic symbiosis. This cockroach species has been linked to severe allergic reactions in children, in part, because it harbors antagonistic molds. GivenG. schaeferi’s regulatory role at suppressing fungi, these mites could conceivably impose a small indirect, albeit beneficial role to humans by reducing the amount of fungal inoculum (conidia) that might otherwise be inhaled.