SATURATION DEFICIT TOLERANCE SPECTRA OF PHYTOPHAGOUS MITES AND THEIR PHYTOSEIID PREDATORS ON CASSAVA

In South America, phytophagous cassava mite populations reach high den sities during the dry season but occur in low numbers during the rainy season. Some of their phytoseiid predators, however, show the reverse pattern. We test the hypothesis that this different seasonal phenolog y can be attributed to distinct saturation deficit tolerance spectra. Eggs of various species c.q. strains of phytoseiids, originating from different climate zones, were screened for their tolerance to a range of saturation deficits and compared with the tolerance spectrum of tet ranychid species collected from cassava. In total 19 species/strains o f predatory mites were compared with six tetranychid species. The resp onse curves for all predators were sigmoidal with a narrow and specifi c region of saturation deficits where egg hatch success fell from 100% to 0%. Using probit analysis this region was characterized by the SD5 0, the saturation deficit at which only 50% of the eggs hatch. The res ponse curves for the herbivores were trapezoid. In contrast to the phy toseiids the latter do not differ in the range of saturation deficits tolerated, but in the magnitude of the response. It appeared that inte r-strain differences in the Phytoseiidae are sometimes larger than int erspecific differences. We demonstrate that there is a significant cor relation between population-specific SD50-values and long term average relative humidity in the collection site. The implications of this fi nding are discussed in terms of condition specific competition.