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Responses to artificial selection on body mass in the maize weevil Sitophilus zeamais (Coleoptera: Curculionidae) were investigated to determine whether changes in body mass are associated with insecticide susceptibility, rate of population growth, and metabolic rate. Two strains of the maize weevil differing in susceptibility to pyrethroid insecticides were subjected to bidirectional selection on body mass. The susceptible strain responded to selection resulting in individuals with lower or higher body mass, but the resistant strain responded significantly only to selection for lower body mass . The resistant strain selected for low body mass increased its level of deltamethrin resistance in 44 × . In contrast, selection for low body mass in the susceptible parental strain led to increased deltamethrin susceptibility (50 × ) and selection for high body mass increased deltamethrin resistance (4 × ). Thus, the correlated response of insecticide resistance to selection for body mass differed between strains, a likely consequence of their distinct genetic background. Regardless, body mass was positively correlated with fitness (reproductive output) (r = 0.79; P < 0.001), while such correlation with respiration rate was significant only at P = 0.07 (r = 0.44). Therefore, the association between body mass and deltamethrin resistance is population-dependent in the maize weevil, and the confluence of deltamethrin resistance and high body mass in a given strain will likely favour its energy metabolism and lead to the mitigation of fitness costs usually associated with insecticide resistance. The genetic background and selection history of insecticide resistant populations should not be neglected since they may favour the confluence of insecticide resistance with mitigation mechanisms of its associated fitness costs limiting the tactics available to their management. [ABSTRACT FROM AUTHOR]

Phenotypic plasticity contributes to the adaptative evolution of populations exposed to new or altered environments. Feeding plasticity is a component of phenotypic plasticity not usually considered in insect strains adapted to insecticide-altered environments, but which may either accentuate or mitigate insecticide resistance. This is a concern in the pyrethroid-resistant strains of the maize weevil Sitophilus zeamais Motsch. (Col., Curculionidae), and the reason for this study. A pyrethroid-susceptible and two pyrethroid-resistant strains of maize weevil were subjected to free-choice and no-choice tests with maize grains sprayed with increasing doses of the pyrethroid, deltamethrin. The insects from the pyrethroid-resistant strains exhibited higher feeding avoidance with increased deltamethrin doses than insects from the susceptible strain when subjected to free-choice tests. The strains of maize weevil physiologically resistant to pyrethroids were also behaviourally resistant to deltamethrin – an additional management concern. The resistant strains avoid deltamethrin-sprayed grains and are less nutritionally affected by this compound, with divergent responses from the susceptible strain with increased doses of deltamethrin. Furthermore, the higher relative growth rate and consequently higher efficiency of food conversion observed in the insecticide-resistant strains were significant even without insecticide exposure, indicating that these traits are stimulus-independent and may persist even without further insecticide selection, potentially limiting the options available for their management. [ABSTRACT FROM AUTHOR]

ABSTRACT The assessment of pesticide effects in arthropods historically have relied heavily on acute lethal effects, Although the sublethal responses to such compounds are sometimes neglected, stimulatory effects associated with low doses of compounds toxic at higher doses, such as pesticides, have been widely reported in recent years and recognized as a general toxicological phenomenon. Evidence of such stimulatory response has also been reported among mites and a few insect pestspecies exposed to pesticides and recognized as a one of the potential causes underlying pest resurgence and secondary pest outbreaks. However, fitness parameters and its implications were seldom considered in these studies and natural enemies are not usually target of attention. Here, we reported the stimulatory effect of sublethal doses (ranging from 0.02 to 172.00 ppb in addition to the control) of the pyrethroid permethrin topically applied to third instar nymphs of the spined soldier bug, Podisus distinctus (Stâl) (Heteroptera: Pentatomidae). The parameters estimated from the fertility tables of insects exposed to the increasing doses of insecticide indicated a slight increase in the mean survival time for doses ⩾0,20 ppb and a peak in the net reproductive rate at 1.72 ppb. This trend is coincident and correlated with the intrinsic rate of population growth (n = 18, r = 0.78, P = 0.0001), which also shows a peak at 1.72 ppb, leading to higher reproductive values of insects exposed to this dose. The phenomenon is consistent with insecticide-induced hormesis, for which the potential implications are discussed. [ABSTRACT FROM AUTHOR]

A common assumption in models of insecticide resistance evolution is the association between resistance and fitness costs in the absence of insecticides. There is empirical evidence of such associations, but their physiological basis (and mitigation) is little investigated. Pyrethroid-resistant populations of the maize weevil Sitophilus zeamais (Coleoptera: Curculionidae) offer this opportunity. Pyrethroid resistance in this species was initially observed in five Brazilian states by 1995, but the phenomenon apparently decreased and did not spread to other regions, probably due to the occurrence of a fitness disadvantage in resistant individuals in the absence of insecticides. The present investigation aims to verify whether differences in respiration rate and fat body morphology are related to differences in rate of development in Brazilian populations of S. zeamais resistant to insecticides, and thereby provide evidence for the existence (or not) of a physiological fitness cost acting against insecticide resistance in maize weevils. This may occur due to a possible energy trade-off between insecticide resistance and other physiological processes associated with development and reproduction. To achieve this, studies of the rate of development, respiration and fat body cytomorphology are carried out in one insecticide-susceptible (from Sete Lagoas) and two resistant populations (from Jacarezinho and Juiz de Fora) of S. zeamais. The resistant population from Jacarezinho shows that higher body mass is associated with higher energy reserves (larger trophocyte area) for development and reproduction, as well as for insecticide resistance. However, the resistant population from Juiz de Fora does not appear to have large enough energy allocation for insecticide-resistance expression and development and/or reproductive performance, suggesting a trade-off between resistance and other life history traits. [ABSTRACT FROM AUTHOR]

The selectivity of malathion, carbaryl, permethrin, and deltamethrin to Supputius cincticeps (Stal) (Het., Pentatomidae) and its prey Spodoptera frugiperda (J. E. Smith) (Lep., Noctuidae) (fall armyworm) was evaluated using leaves dipped in insecticide dilutions. Based on LC50 values, malathion was the least toxic insecticide to the fall armyworm followed by carbaryl, permethrin and deltamethrin which was the most toxic compound to this insect pest. Malathion was also the least toxic insecticide to the predaceous bug S. cincticeps, while carbaryl and permethrin showed intermediate toxicity and deltamethrin was the most toxic insecticide to this species. Malathion and carbaryl were more toxic to the predator than to the fall armyworm showing low selectivity in favour of the first. However, the pyrethroids showed the opposite effect with deltamethrin being the most selective insecticide in favour of S. cincticeps. Nymphs of the fifth instar and adults of the predator were more tolerant to the pyrethroids than third-instar nymphs. The other insecticides showed similar toxicity to all three developmental stages of the predator. Résumé Sélectivité des insecticides pour Supputius cincticeps (Say) (Het., Pentatomidae) et sa proie Spodoptera furgiperda (J. E. Smith) (Lep., Noctuidae) La sélectivité du mulathion. carbaril. pennétrine et deltamétrine pour Supputius cincticeps (Say) (Het., Pentatomidae) et sa proie Spodoptera frugiperda (J. E. Smith) (Lep., Noctuidae) (chenille du cornet) fut évaluée à l'aide de feuilles imprégnées d'insecticides. Le malathion s'est avéré l'insecticide le moins toxique pour la chenille du cornet, suivi du carbaril. permétrine et deltamétrine, cette dernière étant l'insecticidc le plus toxique pour cet insecte-fléau. basé sur les valeurs du CL50. Le malathion a égalemenl été l'insecticide le moins toxique pour la punaise prédatrice S. cincticeps, alors que le carbaril et la permétrine ont affiché une toxicité intermédiaire et la deltamétrine, une grande toxicité pour cette espèce. Les insecticides malathion et carbaril furent plus toxiques pour la punaise prédatrice que pour la chenille du cornet, affichant une basse sélectivité en faveur du premier. Toutefois, les pyrétroïdes ont présenté un effet contraire avec la deltamétrine, celle-ci étant l'insecticide le plus sélectif en faveur du S. cincticeps. Les nymphes du 5e stade et punaises adultes ont été encore plus tolérantes aux pyrétroides que les nymphes du 3o stade. Les autres insecticides ont révélé une toxicité similaire pour les trois stades de développement du prédateur. [ABSTRACT FROM AUTHOR]