Residual activity of spinosad applied as a soil drench to tomato seedlings for control of Tuta absoluta.

BACKGROUND: Tuta absoluta (Lepidoptera: Gelechiidae) is difficult to control by means of foliar insecticides, partly because of the endophytic feeding behavior of its larvae. The biopesticide spinosad is applied as a foliar spray for control of T. absoluta and has systemic properties when applied as a soil drench to the growing medium of tomato plants. The aims of this study were to determine the: (i) instar-dependent tolerance of larvae to spinosad; (ii) efficacy of spinosad drench application for the control of larvae; (iii) residual period of systemic activity of spinosad in leaves and fruit after drenching; and (iv) effect of spinosad drenching on tomato plant growth parameters. RESULTS: The estimated LC50 value (Lethal Concentration at which 50% of the larvae died) differed between instars. The LC50 for second-instar larvae (0.41 ppm) to spinosad was significantly lower than that for third- (0.64 ppm) and fourth-instar (0.63 ppm) larvae. The LC80 value (Concentration at which 80% of the larvae died) for fourth-instar larvae (2.48 ppm) was 2.6- and 1.7-fold higher than that for the second- and third-instar larvae, respectively. The spinosad concentration recorded in leaves at 25 days after treatment (DAT; 0.26 µg g-1 ) was significantly lower than that in leaves sampled at 3, 10 and 15 DAT. High larval mortalities were, however, recorded for the duration of the experiment, which lasted 25 days (equivalent to one T. absoluta generation). CONCLUSION: Systemic spinosad effectively controlled T. absoluta larvae over a prolonged period. However, drenching this insecticide violates the recommendation of the Insecticide Resistance Action Committee to avoid treating consecutive insect generations with the same mode of action and can therefore result in the evolution of insecticide resistance. © 2023 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Susceptibility of Tuta absoluta (Lepidoptera: Gelechiidae) Pupae to Soil Applied Entomopathogenic Fungal Biopesticides.

Management of Tuta absoluta (Meyrick) (Lepidoptera: Gelechiidae) in greenhouses and under open-field tomato cultivation relies on an integrated approach, largely targeting the egg and larval stages of the pest. However, little to no research has been done on the efficacy of EPFs for control of the pupal stage. The aims of this study were to determine the susceptibility of T. absoluta pupae to Beauveria bassiana and Metarhizium anisopliae spores applied as soil drench treatments, and the possible effects of these treatments on fecundity and fertility of moths. The lethal concentrations (LC50 and LC80) of the respective products were estimated in dose-response bioassays by exposing pupae in a soil substrate to different concentrations of EPF products. Emerging moths were paired in different combinations, according to EPF exposure treatments after which fecundity and fertility of females were recorded. Pupae in the soil were effectively controlled by all EPF products in both bioassays as well as in a growth tunnel experiment. The LC50 value of the B. bassiana oil formulation was significantly lower than that of other treatments. The fecundity of females that were subjected to the B. bassiana oil formulation was significantly lower than that of the control treatment. This study showed the potential of soil drench applications of both B. bassiana and M. anisopliae for control of T. absoluta pupae.

Introgression of a cry1Ab transgene into open pollinated maize and its effect on Cry protein concentration and target pest survival.

In Africa, the target pests of genetically modified Bt maize are lepidopteran stem borers, notably Busseola fusca (Lepidoptera: Noctuidae). Gene flow between Bt maize hybrids and open pollinated varieties (OPVs) that do not contain the Bt trait is highly likely in areas where both types of maize are cultivated. Consequently, introgression of the cry1Ab transgene into local OPVs will result in unknown patterns of Cry1Ab protein expression in plants during follow-up seasons when recycled seed of OPVs is planted. Too low concentrations of Cry protein in such plants may result in selection for resistant alleles and accelerate resistance evolution. The aim of this study was to determine the effects of introgression of the cry1Ab transgene into an OPV, on Cry protein concentration levels and pest survival. Bt transgene introgression was done by crossing a transgenic donor hybrid containing the cry1Ab gene with a non-Bt OPV as well as with a non-Bt near-isogenic hybrid. F1 and F2 crosses as well as back crosses were done yielding 11 genotypes (treatments). Cry1Ab protein concentrations in leaf tissue of these crosses were determined by means of ELISAs. All crosses that contained the transgene had similar or higher Cry1Ab concentrations when compared to the Bt parental hybrid, except for the Bt x OPV F1-cross that had a significantly lower Cry1Ab concentration. Survival B. fusca larvae were evaluated in assays in which larvae were reared for 14 days on whorl leaf tissue of the different treatments. Larval survival did not differ between any of the maize plant treatments which contained the Bt gene. Results suggest that Bt transgene introgression into OPVs may produce plant progenies that express Cry1Ab protein at sufficient concentrations, at last up to the F2 seed, to control B. fusca larvae. Resistance evolution is however not only influenced by the frequency of pest individuals that survive exposure to the Cry proteins but also by factors such as genetics of the pest and recipient OPV, pest biology and migration behaviour.