Saturation deficit tolerance spectra of phytophagous mites and their phytoseiid predators on cassava

Abstract

Frank M. Bakker, Ma_rtha E. Klein, Nora C. Mesa and Ann R. Braun, 1993. Saturation deficit tolerance spectra of phytophagous mites and their phytoseiid predators on cassava. Exp. Appl. Acarol. 17: 97-113. In South America, phytophagous cassava mite populations reach high densities 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 phenology 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 tetranychid species collected from cassava. In total t9 species/strains of predatory mites were compared with six tetranychid species. The response curves for all predators were sigrnoidal with a narrow and specific region of saturation deficits where egg hatch success fell from 100% to 0%. Using probit analysis this region was characterized by the SDs0, the saturation deficit at which only 50% of the eggs hatch. The response curves for the herbivores were trapezoid. In contrast to the phytoseiids the latter do not differ in the range of saturation deficits tolerated, but in the magnitude of the response. It appeared that inter-strain differences in the Phytoseiidae are sometimes larger than interspecific differences. We demonstrate that there is a significant correlation between population-specific SDs0values and long term average relative humidity in the collection site. The implications of this finding are discussed in terms of condition specific competition.