Susceptibility of Diabrotica virgifera virgifera (Coleoptera: Chrysomelidae) to the entomopathogenic fungus Metarhizium anisopliae when feeding on Bacillus thuringiensis Cry3Bb1-expressing maize

Abstract

Genetically engineered maize producing the insecticidal protein Cry3Bb1 from Bacillus thuringiensis (Bt maize) is protected against corn rootworms (Diabrotica spp.), which are serious maize pests in North America and Europe. The aim of the present study was to investigate the interaction of Bt maize (event MON88017) and the entomopathogenic fungus Metarhizium anisopliae for controlling the western corn rootworm, Diabrotica virgifera virgifera. Exposure to Cry3Bb1 expressed in Bt maize seedlings resulted in decreased weight gain in D. v. virgifera larvae but did not influence susceptibility to M. anisopliae. Adult beetles were not affected by Cry3Bb1 in their food, but mortality when feeding on maize leaves was higher than when feeding on silk. Adults were more susceptible to the fungus than larvae. The results indicate that the effects of Bt maize and M. anisopliae on D. v. virgifera are additive and that Bt maize does not interfere with the biological control provided by entomopathogenic fungi.

FIG. 1.

Weights of D. v. virgifera larvae (mean ± SE) at the start of the experiment (n = 118 to 133), 4 days after feeding on Cry3Bb1-expressing maize (event MON88017) or corresponding nontransformed maize (n = 118 to 133), and 4 days after treatment with spore suspensions of M. anisopliae (8 days on Bt and control maize) (n = 77 to 115).

FIG. 2.

Survival of D. v. virgifera larvae feeding on Cry3Bb1-expressing maize (event MON88017) or corresponding nontransformed maize seedlings after treatment with spore suspensions of M. anisopliae. Survival was analyzed until day 17 after infection, when the first adult beetle emerged (n = 118 to 133).

FIG. 3.

Survival of D. v. virgifera adults feeding on Cry3Bb1-expressing maize (event MON88017) or corresponding nontransformed maize leaves (A) or silk (B) after treatment with spore suspensions of M. anisopliae. The experiment was terminated 50 to 66 days after treatment with fungal spores (n = 33 to 36).

FIG. 4.

Cry3Bb1 concentration (mean + SE) in Bt maize (event MON88017) seedlings (n = 25) and roots (n = 5) sampled every 2 weeks in the greenhouse. Roots were divided into four sections from tip to base.