Multigenerational Insecticide Hormesis Enhances Fitness Traits in a Key Egg Parasitoid, Trichogramma chilonis Ishii

Abstract

Hormesis for the intractable pests can be dreadful, but for natural enemies of pests, it is a puissant strategy in optimizing their mass rearing. We report multigenerational stimulatory effects of widely used insecticide, imidacloprid, on the demographic traits of an important egg parasitoid Trichogramma chilonis Ishii. The study investigated the consequences of sublethal (LC5), low lethal (LC30), and median lethal (LC50) concentrations, as well as a control, for five continuous generations (F1 to F5). The initial bioassay experiments revealed imidacloprid exhibiting the highest toxicity for the parasitoid with a LC50 of 2 µg·L−1, whereas LC5 and LC30 were 0.07 µg·L−1 and 0.6 µg·L−1, respectively. Among biological traits, compared to the F1 individuals, a substantial increase in the fecundity of T. chilonis was observed in the F5 individuals by 54.92% and 46.81% when exposed to LC5 and LC30, respectively (p < 0.00001). Further, there was a significant enhancement in the adult longevity as well as oviposition days of the F5 individuals at both these concentrations. Considering the population traits, along with gross reproductive rate (GRR), net reproductive rate (R0) was also enhanced by both LC5 and LC30 in F5 individuals than F1; whereas the intrinsic rate of increase (r) and finite rate of increase (λ) were enhanced only at LC30 upon comparing with control. On the other hand, LC50 exposure to T. chilonis did not result in notable differences in biological or population traits when compared across generations (F1 and F5). Low and sublethal concentrations of imidacloprid did not have a major influence on demographic traits of T. chilonis at initial generations of exposure but can induce hormetic effects in the subsequent generations. Overall, imidacloprid-induced hormesis stimulating the development of T. chilonis might be helpful under circumstances of mild exposure of imidacloprid in fields and could be leveraged for its mass rearing.

Functional response of an egg parasitoid,Trichogramma chilonisIshii to sublethal imidacloprid exposure

“…71 Moreover, both the lethal and sublethal insecticide concentrations that induce stress have an adaptive importance since they can persist across multiple generations either through transgenerational or multigenerational routes. 2,73,74 The continuous selection pressure of insecticides is costly and affects both biological and population parameters, thereby affecting the fitness costs of the bio-control agents over generations. Insecticide-induced stress often results in trade-offs in resource allocation among various physiological processes, thereby favoring one at the expense of the other in order to maintain fitness under stress.…”

“…Based on the outcomes of the concentration-mortality assay on the adults of T. chilonis, median lethal concentration (LC 50 , 2 μg L −1 ) and sublethal concentrations (LC 30 , 0.6 μg L −1 and LC 5 , 0.07 μg L −1 ) were used to assess the multigenerational effects of imidacloprid on the functional response of T. chilonis. 2 Analytical-grade acetone served as a control in the study. 2,48,49…”

“…The multigenerational effect (F 1 to F 5 ) of different concentrations (LC 5 , LC 30 , LC 50 ) of imidacloprid along with acetone (control) was carried out following the protocol of Ray et al 2 Briefly, 1 mL of a solution of different concentrations of the insecticide was applied on the inner walls of the glass tubes (Borosil®; height × diameter: 19.0 cm × 3.6 cm) and rotated until visible droplets were seen. To ensure complete evaporation, it was left at room temperature for 2 h. Further, adult and mated females of T. chilonis of equal age (< 24 h old) (n = 60) were introduced into the glass tubes for 8 h. Each tube was corked up with muslin cloth secured with rubber to allow the passage of air.…”

“…13,14 Moreover, the assessment of lethal and sublethal effects of insecticides on parasitoids and predators is a center of attention since it has a huge impact on the biological and demographic traits of the bio-agents. 2,[15][16][17][18] The effect can either be stimulating [example: multigenerational sublethal imidacloprid exposure enhanced the fitness traits on Trichogramma chilonis Ishii (Hymenoptera: Trichogrammatidae) 2 ] or reducing [example: transgenerational exposure to sublethal concentrations of chlorfenapyr, thiodicarb, and flupyradifurone reduced the biological traits of Trichogramma pretiosum Riley (Hymenoptera: Trichogrammatidae) 15 ]. In addition, insecticides also intervene in the predator/parasitoid and prey/host interactions.…”

“…Recently, the phenomenon of using bio‐control agents to reduce pest populations has gained momentum, but the predominance of chemical management in conventional agriculture remains 1–4 . The continuous selection pressure of insecticides leads to the development of resistance, either by up‐regulation of detoxification genes or because of mutations on the target site, thereby affecting the overall fitness parameters (biological and population) of the organism 5,6 .…”