Spinosad and the tomato borer Tuta absoluta: a bioinsecticide, an invasive pest threat, and high insecticide resistance

Abstract

The introduction of an agricultural pest species into a new environment is a potential threat to agroecosystems of the invaded area. The phytosanitary concern is even greater if the introduced pest's phenotype expresses traits that will impair the management of that species. The invasive tomato borer, Tuta absoluta (Meyrick) (Lepidoptera: Gelechiidae), is one such species and the characterization of the insecticide resistance prevailing in the area of origin is important to guide management efforts in new areas of introduction. The spinosad is one the main insecticides currently used in Brazil for control of the tomato borer; Brazil is the likely source of the introduction of the tomato borer into Europe. For this reason, spinosad resistance in Brazilian populations of this species was characterized. Spinosad resistance has been reported in Brazilian field populations of this pest species, and one resistant population that was used in this study was subjected to an additional seven generations of selection for spinosad resistance reaching levels over 180,000-fold. Inheritance studies indicated that spinosad resistance is monogenic, incompletely recessive and autosomal with high heritability (h(2) = 0.71). Spinosad resistance was unstable without selection pressure with a negative rate of change in the resistance level ( = -0.51) indicating an associated adaptive cost. Esterases and cytochrome P450-dependent monooxygenases titration decreased with spinosad selection, indicating that these detoxification enzymes are not the underlying resistance mechanism. Furthermore, the cross-resistance spectrum was restricted to the insecticide spinetoram, another spinosyn, suggesting that altered target site may be the mechanism involved. Therefore, the suspension of spinosyn use against the tomato borer would be a useful component in spinosad resistance management for this species. Spinosad use against this species in introduced areas should be carefully monitored to prevent rapid selection of high levels of resistance and the potential for its spread to new areas.

Figure 1. LC₅₀s for spinosad with successive selections for spinosad resistance of the tomato borer Tuta absoluta.

After 12 generations of spinosad selections, the selected line was split into two, one line maintaining selection and one line with interrupted selection.

Figure 2. Spinosad concentration-mortality curves (with observed data as symbols) for the (standard) spinosad susceptible strain, (selected) spinosad resistant strain, the F₁ progeny of the reciprocal crosses and the backcross progeny (pooled F₁ RC×spinosad-resistant) of the tomato borer Tuta absoluta.

Figure 3. Synergism of spinosad toxicity in spinosad-susceptible and -resistant strains of the tomato borer Tuta absoluta.

Figure 4. Relationship between detoxification enzyme activity and LC₅₀s for spinosad in spinosad-selected generations of the tomato borer Tuta absoluta.