Interaction of Beauveria bassiana and Metarhizium anisopliae with chlorpyrifos ethyl and spinosad in Spodoptera frugiperda larvae.

Background: The high natural resistance of the fall armyworm (FAW) to entomopathogenic fungi and the speed at which it develops resistance to chemical insecticides make it difficult to control in several crops where it is a key pest. The aim of the study was to improve mortality by combining a 50% lethal concentration of chlorpyrifos ethyl or spinosad with the entomopathogenic fungus Beauveria bassiana (Bb88) or Metarhizium anisopliae (ETL).

Results: Three main results were obtained. (i) Both insecticides improved the performance of the fungus when the insecticide and fungus were applied simultaneously and when spinosad preceded fungi. In control larvae treated with fungi, <1% of larvae showed fungal sporulation, but under simultaneous application of the fungus with chlorpyrifos or spinosad, 31 and 47% did so, with 68 and 93% of cadavers showing sporulation, respectively. (ii) Synergistic mortality was observed when Bb88 and spinosad were applied simultaneously, which resulted in 34% more dead larvae than the spinosad control (44%). Finally, (iii) antagonism occurred when Bb88 was applied before chlorpyrifos, when Bb88 and chlorpyrifos were applied simultaneously, and when ETL was applied before chlorpyrifos, which reduced larval mortality by 27, 31 and 19%, respectively.

Conclusion: The synergistic mortality and improved fungal performance observed here support the hypothesis that combinations of low insecticide doses and entomopathogenic fungi can enhance integrated pest management programs and reduce the environmental impact of insecticides. Antagonism shows that some particular combinations and application sequences may produce disadvantages in pest control. © 2018 Society of Chemical Industry.