This study assesses the pathogenicity of Metarhizium anisopliae isolate ICIPE 51 against different nymphal instars and adults of Rhopalosiphum padi and Metopolophium dirhodum and investigates effects of fungal infection on fecundity and intrinsic rate of aphid increase. To obtain different developmental stages, adult aphids were inoculated onto fresh leaf discs, reproducing parthenogenetically. Rearing was carried out to ensure different developmental stages were obtained at the same time so that treatments could be performed concomitantly. Concentrations of 1.0 x 106, 3.0 x 106 and 1.0 x 107 conidia/ml were used for each developmental stage. Mortality was recorded daily for 10 days. For fecundity, treated aphids were transferred to a leaf in an assay cell, one aphid per cell and observed for 7 days. New born nymphs were removed after counting. Five to seven day adults were significantly more susceptible than nymphs of other developmental stages. No significant difference in susceptibility was observed within each stage in the first three days. Thereafter, susceptibility increased steadily to maximum levels of 71 and 57% for five to seven day old adults and 0 - 2 day old nymphs, respectively. M. dirhodum was significantly less fecund than R. padi at all concentrations. Fecundity and intrinsic rate of increase among both aphid species declined progressively over time. Thus, maximum fecundity of 3 and 3.5 nymphs/aphid among M. dirhodum and R. padi respectively was recorded during the first day as compared to less than 1 nymphs/aphid/day in each species from the sixth day. These results indicate that susceptibility of R. padi and M. dirhodum to entomopathogenic fungal control increases with aphid maturity and that both species are significantly more fecund in early adulthood, suggesting the stage as ideal for biopesticide management intervention.
Key words: Metarhizium anisopliae, Metopolophium dirhodum, Rhopalosiphum padi, fecundity, intrinsic rate of increase.