Plutella xylostella(L.) is the most important pest of Brassicaceae worldwide, with a recent estimate of US$ 4–5 billion expenditure for the control of this insect. A case of very high resistance of this pest to chlorantraniliprole was recently associated with reduced efficacy in a Brazilian field ofBrassicaspp. Although diamide resistance has been characterized, the fitness of insects due to such resistance has yet to be examined. Therefore, in this study, biological parameters were assessed in both susceptible and resistant strains ofP. xylostellasubjected to sublethal chlorantraniliprole concentrations. The field strain showed high resistance to chlorantraniliprole (RR₅₀=27,793-fold), although resistance rapidly decreased in the first generations, showing instability. The exposure of susceptible and resistant larvae to their respective LC₁, LC₁₀, and LC₂₅values led to an increased duration of the larval and pupae phases and reduced weight in both strains; however, no significant differences in pupal viability across the treatments were observed. The resistant insects presented significantly lower larval weight and fecundity and higher larval and pupal periods, hatchability, and male longevity when not exposed to chlorantraniliprole, suggesting a fitness cost associated with resistance. In addition, resistant females showed a significantly higher egg-laying period and longevity at LC₂₅, whereas the males lived longer at LC₁. Chlorantraniliprole negatively impacted the biological parameters of both strains tested, although these effects were more relevant to the resistant insects. ResistantP. xylostellashowed negative and positive biological trade-offs when compared with the susceptible individuals in both the absence and presence of chlorantraniliprole. Despite the important role that these trade-offs may play in the evolution of resistance to chlorantraniliprole, practical applications still depend on such information as the dominance of fitness costs and resistance.

Tuta absoluta (Meyrick), one of the most important tomato pests worldwide, is heavily controlled by the application of insecticides. Diamide insecticides represent a new class of products recently registered to control T. absoluta . After 6 yr of use, control failures have been reported in populations of this pest, suggesting a hypothetical resistance development. Therefore, a resistance survey was performed using nine populations of T. absoluta that were collected in open fields, including from areas with reports of a reduced efficacy of diamides in the Northeast and Central regions of Brazil. Initial surveys with diagnostic and label doses proved the reduced efficacy of diamides against most populations. The LC50 values of chlorantraniliprole varied from 0.0044 (Brasília) to 1,263 (América Dourada) mg AI liter−1 (the resistance ratios [RR50] ranged from 1.0- to 288,995-fold), whereas the LC50 values for cyantraniliprole and flubendiamide, respectively, varied from 0.015 (Brasília) to 281 (América Dourada) mg AI liter−1 and from 0.038 (Guaraciaba do Norte) to 3,018 (Gameleira 1) mg AI liter−1. The resistance ratios (RR50) ranged from 1.0- to 18,423-fold for cyantraniliprole and from 1.0- to 80,413-fold for flubendiamide. The log LC50 values of pairwise diamides were strongly and significantly correlated, which denoted cross-resistance among them. Very high resistance to diamides in T. absoluta was observed in this study, suggesting that strategies to mitigate resistance and thereby control the pest must not include only insecticides. Other control tactics must be carefully implemented over time to increase the life span of diamides, including rotational practices with other molecules.