Natural enemies delay insect resistance to Bt crops

Abstract

We investigated whether development of resistance to a Bt crop in the presence of a natural enemy would be slower than without the natural enemy and whether biological control, in conjunction with a Bt crop, could effectively suppress the pest population. Additionally, we investigated whether insecticide-sprayed refuges of non-Bt crops would delay or accelerate resistance to the Bt crop. We used a system of Bt broccoli expressing Cry1Ac, a population of the pest Plutella xylostella with a low frequency of individuals resistant to Cry1Ac and the insecticide spinosad, and a natural enemy, Coleomegilla maculata, to conduct experiments over multiple generations. The results demonstrated that after 6 generations P. xylostella populations were very low in the treatment containing C. maculata and unsprayed non-Bt refuge plants. Furthermore, resistance to Bt plants evolved significantly slower in this treatment. In contrast, Bt plants with no refuge were completely defoliated in treatments without C. maculata after 4-5 generations. In the treatment containing sprayed non-Bt refuge plants and C. maculata, the P. xylostella population was low, although the speed of resistance selection to Cry1Ac was significantly increased. These data demonstrate that natural enemies can delay resistance to Bt plants and have significant implications for integrated pest management (IPM) with Bt crops.

Figure 1. Plutella xylostella populations on Bt plants (A) and on non-Bt refuge plants (B) in greenhouse cages (Means ± SEM).

Bt+R: 75% Bt and 25% non-Bt refuge plants; Bt+R+Cm: 75% Bt and 25% non-Bt refuge plants with C. maculata; Bt+SR: 75% Bt and 25% spinosad-sprayed non-Bt refuge plants; Bt+SR+Cm: 75% Bt and 25% spinosad-sprayed non-Bt refuge plants with C. maculata; Bt Only: 100% Bt plants only, Non-Bt+Cm: 100% non-Bt refuge plants with C. maculata. Different letters within the same generations denote significant differences (P<0.05, one-way ANOVA, Games-Howell test for G3 and G6 in A, G1, G3 and G5 in B, other comparsions by Tukey HSD test).

Figure 2. Coleomegilla maculata population on Bt plants (A) and on non-Bt refuge plants (B) in greenhouse cages (Means ± SEM).

Bt+R+Cm: 75% Bt and 25% non-Bt refuge plants with C. maculata; Bt+SR+Cm: 75% Bt and 25% spinosad-sprayed non-Bt refuge plants with C. maculata; Non-Bt+Cm: 100% non-Bt refuge plants with C. maculata. Different letters within the same generations denote significant differences (P<0.05, one-way ANOVA, Tukey HSD test).