Studies on the integrated control of the Russian wheat aphid, Diuraphis Noxia (Kurdjumov) (Hemiptera: Aphididae), using entomopathogenic fungi combined with sublethal doses of insecticides.

Abstract

The Russian wheat aphid (RWA), Diuraphis noxia (Kurdjumov) (Hemiptera: Aphididae), is one of the most damaging aphid pests of wheat grown under dryland conditions. Host plant resistance is considered as the most, cost-effective, and ecologically attractive method of control for this cereal pest. However, it is a slow process to breed wheat varieties for resistance to RWA, and resistance-breaking biotypes continue to pose a serious threat to wheat producers in the country. Currently, there are four RWA biotypes that have been recorded in South Africa. Entomopathogenic fungi (epf), such as Beauveria bassiana, have the potential to suppress RWA as shown by Hatting et al. (2004), whereby B. bassiana together with host plant resistance managed to provide about 60% level of control of this pest. Nevertheless, that was not a satisfactory level of control. Furthermore, hyphomycetes are known to kill their target insects more slowly than chemicals. One approach to increase their efficiency, and therefore insect mortality is by combining epf with sub-lethal doses of chemical insecticides. The interaction observed from this combination can be synergistic, antagonistic or neutral. Synergistic effects would allow for reduced insecticide use, minimized environmental pollution, the preservation of natural enemies and a slowing of the development of insecticide resistance. The hypothesis for the synergistic interaction is that the insecticide acts as a stress inducer, making the insect pest more susceptible to fungal infection. Given the development of new resistance-breaking biotypes of the RWA and many reported cases of insecticide resistance, this study aimed to enhance the virulence of selected entomopathogenic fungal strains through synergism with sub-lethal doses of chemical and botanical insecticides. The first objective was to find several virulent entomopathogenic strains of B. bassiana and M. anisopliae against D. noxia. The virulence of three Beauveria bassiana and three Metarhizium anisopliae strains were evaluated against D. noxia biotype RWASA1. B. bassiana Isolate SGI921, was the most pathogenic strain, and was used for the subsequent studies. Four insecticides registered for RWA control were screened in vitro for compatibility with the selected fungal strain, SGI921, with special emphasis on germination, radial vegetative growth and sporulation intensity. All the tested insecticides (active ingredients: chlorpyriphos, dimethoate, demeton-S-methyl and acetamiprid) reduced germination, radial vegetative growth and sporulation intensity in various degrees in a concentration-dependent manner. Mospilan was found to be compatible with the selected strains and, at its highest concentration, it seemed to have stimulated vegetative growth. The last objective was to determine the effect of combining sub-lethal doses of pyrethrum-based insecticides with either a commercial formulation of Beauveria bassiana, Eco-Bb®, and Beauveria bassiana Strain R444 in both in vivo and in vitro trials. In the laboratory bioassays 10% Pyrol was able to enhance the sporulation intensity of R444. In glasshouse trials 10% Pyrol enhanced the efficacy of B. bassiana Strain R444 by increasing RWA mortality and reducing the mean time to mortality. At a concentration of 10%, Mospilan killed the aphids before either of the fungi could infect the aphids, i.e., within 48 hours post fungal inoculation. Mospilan concentrations below 10% will be further investigated to establish a minimal sub-lethal dose that does not kill the RWA but make the aphids more susceptible to epfs. The interaction of Pyrol with the unformulated strain of B. bassiana Strain R444 will be tested in other biotypes of Russian wheat aphids. Since vegetable oils are also known to enhance efficacy of entomopathogenic fungi, future research will also evaluate the interactions of the pyrethrum extract and canola oil, in order to enhance the synergistic effects of the oil in the Pyrol and B. bassiana interaction.