Influence of pesticide use on the natural occurrence of entomopathogenic fungi in arable soils in the UK: Field and laboratory comparisons

The spectrum and abundance of entomopathogenic fungi in agricultural soil receiving different pesticide applications were evaluated. Seven small field plots within a bailey crop were selected. Each plot had received a different pesticide treatment at slightly higher than the field late each year for the previous 12-19 yeats. The field plots received either benomyl (fungicide), triadimefon (fungicide), aldicarb (insecticide), chlorfenvinphos (insecticide), glyphosate (helbicide), all five of these pesticides ol no pesticides at all (control). Soil sampled from each plot was baited with Galleria mellonella larvae at either 18 or 26 degrees C. Five species of entomopathogenic fungi infected these larvae. Beauveria bassiana,vas the dominant species, and the only species for which infection levels were high enough to be analyzed statistically. Significantly fewer G. mellonella larvae became infected with B. bassiana in soil treated with benomyl than in other treatments. This deleterious effect was confirmed in in vitro experiments where benomyl inhibited fungal growth significantly. Chlorfenvinphos also reduced the proportion of G. mellonella becoming infected with B. bassiana slightly (although not statistically significantly). This effect was much greater in in vitro experiments in which chlorfenvinphos inhibited fungal growth significantly. B. bassiana-induced mortality was significantly greater in G. mellonella larvae placed in soil treated with triadimefon than in other treatments. In vitro studies, however, demonstrated that triadimefon inhibited rather than stimulated growth. A similar anomaly between field and laboratory observations was seen in the case of aldicarb. There was no significant difference in the number of G. mellonella larvae found infected with B. bassiana in soil treated with aldicaib than in the control soil. However, in the in vitro studies, aldicarb stimulated fungal growth at all concentrations except 10 times the field rate. In general, significantly fewer larvae became infected with B. bassiana at 26 degrees C than at 18 degrees C in all treatments. Pesticides may have a direct impact on the natural occurrence, infectivity and population dynamics of entomopathogenic fungi. They also affect other macro- and microorganisms in the soil which may interact with the entomopathogenic fungi. In combination with climate and soil variables, these effects are difficult to separate. Although any suppression of these fungi may be detrimental to their capacity as natural enemies, this study has demonstrated that pesticides used under field conditions are unlikely either to kill all the entomopathogenic fungi present in the treated area or to limit their recolonization. It seems likely, therefore, that pesticides have the potential to be used in conjunction with mycoinsecticides in integrated pest management systems.