The effect of cucurbitacin E glycoside, a feeding stimulant for corn rootworm, on biocontrol fungi: Beauveria bassiana and Metarhizium anisopliae

Corn rootworms compulsively feed on cucurbitacins (bitter compounds found i n many cucurbits), while most other pest insects are repelled by them. Seve ral hypotheses have been proposed for this activity, but the results have b een equivocal. One recent hypothesis suggested that cucurbitacin may provid e protection against soil borne fungal entomopathogens, both in the adult c orn rootworm and in the eggs laid in the soil. Any antifungal activity woul d preclude the use of this feeding stimulant to enhance the activity of fun gal pathogens used in biocontrol. To test this hypothesis, we exposed two f ungal pathogens of corn rootworm, Beauveria bassiana and Metarhizium about 0.05% cucurbitacin E glycoside. The extract inhibited the growth of both fu ngi. However, when the extract was sterilized by passing through a 0.45 mic ron filter, this inhibitory activity disappeared. Purified cucurbitacin E g lycoside did not inhibit the growth of either fungus. Four Bacillus isolate s (identified as Bacillus subtilis var. globigii and B. amyloliquefaciens) were isolated from this extract which were able to inhibit the growth of bo th fungi. All of these bacteria excreted the inhibitory activity into the m edium. Thus, the inhibition of fungal growth attributed to cucurbitacin may actually be due to bacterial incidentally associated with cucurbits.