Does cypermethrin affect enzyme activity, respiration rate and walking behavior of the maize weevil (Sitophilus zeamais)?

The impact of lethal and sub-lethal exposure of emamectin benzoate on populations of Spodoptera litura (Lepidoptera: Noctuidae) under laboratory conditions

Emamectin benzoate is an avermectin bio-insecticide commonly used for managing several insect pests including Spodoptera litura (Fabricius) (Lepidoptera: Noctuidae), a major polyphagous pest of many cultivated crops. The current study was conducted to evaluate the effects of emamectin benzoate on the fitness of S. litura populations exhibiting differential susceptibility to insecticide. The selection process and all the bioassays were carried out using 6-day-old 2nd instar larvae of S. litura. A field-collected population of S. litura was divided into two groups: one selected with emamectin benzoate for eight generations (EB-Sel) and the other kept unexposed (Unsel-Lab) to insecticide in the laboratory. An increase in resistance ratio from 1.71-fold in the F1 generation to 22.54-fold in the F8 generation of the EB-Sel population was observed compared to the Unsel-Lab (F8) population. The EB-Sel and Unsel-Lab populations were treated with their respective lethal and sub-lethal concentrations which resulted in an extended development period, decreased larval survival, and adult emergence along with increased morphological abnormalities in adults. Significant reductions were observed in both male and female longevity, fecundity, egg hatching, net reproductive rate (R0), intrinsic rate of increase (rm), and finite rate of increase (λ) in EB-Sel and Unsel-Lab populations. Higher concentrations of the insecticide also reduced the relative fitness (Rf) of S. litura larvae, with maximum effect at LC50 of the EB-Sel population where the Rf value was 0.32 compared to the Unsel-Lab population. Both populations have been affected by emamectin benzoate exposure, however, the impact was more pronounced in the EB-Sel population indicating fitness costs. Our results suggested the fitness cost linked to emamectin benzoate resistance in S. litura which might favor managing insecticide resistance by reducing the frequency of resistant alleles by removing selection pressure. Consequently, our research provides significant insights to devise better pest management strategies for S. litura.

Fitness costs of resistance to insecticides in insects

The chemical application is considered one of the most crucial methods for controlling insect pests, especially in intensive farming practices. Owing to the chemical application, insect pests are exposed to toxic chemical insecticides along with other stress factors in the environment. Insects require energy and resources for survival and adaptation to cope with these conditions. Also, insects use behavioral, physiological, and genetic mechanisms to combat stressors, like new environments, which may include chemicals insecticides. Sometimes, the continuous selection pressure of insecticides is metabolically costly, which leads to resistance development through constitutive upregulation of detoxification genes and/or target-site mutations. These actions are costly and can potentially affect the biological traits, including development and reproduction parameters and other key variables that ultimately affect the overall fitness of insects. This review synthesizes published in-depth information on fitness costs induced by insecticide resistance in insect pests in the past decade. It thereby highlights the insecticides resistant to insect populations that might help design integrated pest management (IPM) programs for controlling the spread of resistant populations.

Acute and Chronic Toxicity of Neem Oil to the Endoparasitoid Palmistichus elaeisis (Hymenoptera: Eulophidae)

Palmistichus elaeisis Delvare and LaSalle 1993 (Hymenoptera: Eulophidae) and neem oil are two control alternatives for the integrated management of defoliating lepidopterans. The aim of this study was to evaluate the acute and chronic toxicity of neem oil compared to the synthetic insecticide deltamethrin, on the endoparasitoid P. elaeisis, in generations F0, F1, F2, and F3. Females of P. elaeisis were exposed to neem solutions at concentrations of 0, 1.87, 3.75, 7.50, 15.00, 30.00, and 60.00 mg ml-1, to determine the dose-response relationship and estimate the neem LC50. The sublethal effects on the parasitoid P. elaeisis in generations F0 to F3 were evaluated with these same concentrations of neem and 0.033 mg ml-1 of deltamethrin. The neem LC50 was estimated at 3.92 mg ml-1. The LC50 for P. elaeisis is 3.83 times lower than that recommended by the neem manufacturer for pest control, demonstrating high acute toxicity to this natural enemy. The chronic toxicity of both the commercial dose and those below it to P. elaeisis caused low sublethal effects. The correct concentration of neem oil in pest control is important, and its use should be performed with caution in integrated pest management programs using the endoparasitoid P. elaeisis to avoid causing interference between the two pest control techniques.

Characterization of the pyrethroid resistance mechanisms in a Blattella germanica (Dictyoptera: Blattellidae) strain from Buenos Aires (Argentina)

The use of chemical insecticides is the main control method for Blattella germanica worldwide. The prolonged and frequent use of insecticides produced the selection of insecticide-resistant individuals. The German cockroach is one of the most widespread urban pests in Argentina. In the last decades, resistance monitoring studies in this country demonstrated that there is a high prevalence of pyrethroid-resistant populations of B. germanica in the field. In this work, we studied the resistance mechanisms of a field-collected strain of B. germanica at toxicological, enzymatic, and molecular levels. A resistance ratio of 100 was obtained for the resistant strain when it was exposed to β-cypermethrin. The pretreatment with specific synergists (piperonyl butoxide and triphenyl phosphate) led to a significant increase in the toxicity of the pyrethroid, suggesting an involvement of oxidases and esterases in the detoxification of this insecticide. Moreover, esterase and oxidase activities in the resistant strain were 1.5-fold and 2-fold higher respectively, compared to the susceptible individuals. On the other hand, the voltage-gated sodium channel gene of the resistant cockroaches did not show nucleotidic substitutions in the domain II which are associated to knockdown resistance in this species. These results suggest that the main mechanism of resistance of the studied cockroaches' strain is metabolic, mainly due to an increase in the activity of oxidase and esterase enzymes. The results of this work in addition to other reports found in literature show that the extended use of a single active principle for cockroach control promotes the development of resistance leading to control failure in the field. In contrast, integrated pest management strategies include the use of different control tools in addition to chemical insecticides, which delay the appearance of resistance increasing the efficacy of pest control.

Influence of Bacillus thuringiensis and avermectins on gut physiology and microbiota in Colorado potato beetle: Impact of enterobacteria on susceptibility to insecticides

Gut physiology and the bacterial community play crucial roles in insect susceptibility to infections and insecticides. Interactions among Colorado potato beetle Leptinotarsa decemlineata (Say), its bacterial associates, pathogens and xenobiotics have been insufficiently studied. In this paper, we present our study of the survival, midgut histopathology, activity of digestive enzymes and bacterial communities of L. decemlineata larvae under the influence of Bacillus thuringiensis var. tenebrionis (morrissoni) (Bt), a natural complex of avermectins and a combination of both agents. Moreover, we estimated the impact of culturable enterobacteria on the susceptibility of the larvae to Bt and avermectins. An additive effect between Bt and avermectins was established regarding the mortality of the larvae. Both agents led to the destruction of midgut tissues, a decrease in the activity of alpha-amylases and alkaline proteinases, a decrease in the Spiroplasma leptinotarsae relative abundance and a strong elevation of Enterobacteriaceae abundance in the midgut. Moreover, an elevation of the enterobacterial CFU count was observed under the influence of Bt and avermectins, and the greatest enhancement was observed after combined treatment. Insects pretreated with antibiotics were less susceptible to Bt and avermectins, but reintroduction of the predominant enterobacteria Enterobacter ludwigii, Citrobacter freundii and Serratia marcescens increased susceptibility to both agents. We suggest that enterobacteria play an important role in the acceleration of Bt infection and avermectin toxicoses in L. decemlineata and that the additive effect between Bt and avermectin may be mediated by alterations in the bacterial community.

Diversity and convergence of mechanisms involved in pyrethroid resistance in the stored grain weevils, Sitophilus spp

Target-site mutations and changes in insect metabolism or behavior are common mechanisms in insecticide-resistant insects. The co-occurrence of such mechanisms in a pest strain is a prominent threat to their management, particularly when alternative compounds are scarce. Pyrethroid resistance among stored grain weevils (i.e., Sitophilus spp.) is an example of a long-standing concern, for which reports of resistance generally focus on a single mechanism in a single species. Here, we investigated pyrethroid resistance in maize and rice weevils (i.e., Sitophilus zeamais and S. oryzae), exploring potential knockdown resistance (kdr) mutations in their sodium channels (primary site for pyrethroid actions) and potential changes in their detoxification and walking processes. Resistance in pyrethroid-resistant rice weevils was associated with the combination of a kdr mutation (L1014F) and increases in walking and detoxification activities, while another kdr mutation (T929I) combined with increases in walking activity were the primary pyrethroid resistance mechanisms in maize weevils. Our results suggest that the selection of pyrethroid-resistant individuals in these weevil species may result from multiple and differential mechanisms because the L1014F mutation was only detected in Latin American rice weevils (e.g., Brazil, Argentina and Uruguay), not in Australian and Turkish rice weevils or Brazilian maize weevils.

Impact of Age on the Biological Activities of Sitophilus zeamais (Coleoptera: Curculionidae) Adults on Stored Maize: Implications for Food Security and Pest Management

The present work was undertaken in three successive years in three different regions of Nigeria to study the impact of age on the postharvest biological performance of maize weevil Sitophilus zeamais Motschulsky (Coleoptera: Curculionidae) adults infesting maize grain. Eight different age groups of S. zeamais adults which ranged from 1 to 56 d were used. The study assessed important parameters and was carried out using standard methods at mean temperature and relative humidity of 28.2°C and 87.5%, respectively. Results indicated that age affected bionomics of S. zeamais adult infesting maize grain in storage. The minimum age threshold for prolific oviposition in the females was 8 d. Weevils in the age groups 8-14 and 15-21 d showed highest fecundity. There was no effect of age on progeny emergence and development time for weevils younger than 36 d. Consequent upon infestation, only S. zeamais adults of ≤35 d old caused great damage and loss to grains in storage. The susceptibility of infesting weevil to cypermethrin was not greatly influenced by weevil age except at an elderly age of ≥43 d. Observations signal that weevil adults are physiologically fittest at ≤14 d old but indicate that adults of ≤35 d old pose the highest food security threat. The S. zeamais ecotype studied showed resistance to the pyrethroid insecticide at 24 h evaluation period and that also has implications for food security.

Cypermethrin Formulation (Ustad-10 EC) Induces Genotoxicity via Apoptosis, Affects Nutritional Physiology, and Modulates Immune Response in Silkworm Philosamia ricini (Lepidoptera: Saturniidae)

Cypermethrin is a pyrethroid insecticide with high insecticidal activity, low mammalian toxicity, and biodegradability. The present study aimed to determine the acute toxicity and evaluate the secondary toxic effects of a commercial formulation of cypermethrin on silkworm Philosamia ricini Hutt of Northeast India. The potential genotoxicity of cypermethrin on silkworm hemocyte was examined by comet assay, caspase activation, and annexin V affinity assay. Alteration in nutritional physiology and histoarchitecture of the gut region was evaluated. Additionally, immunotoxicological effect of cypermethrin was studied by phenoloxidase (PO), lysozyme assay, and abundance of circulating hemocytes. The LC50 value at 24-, 48-, 72-, and 96-h exposure period was recorded as 185.96, 105.34, 72.42, and 58.41 µg/liter, respectively. Approximately sevenfold increase in mean comet tail length was observed at 24 h posttreatment with sublethal concentrations of cypermethrin. Cypermethrin also induced apoptosis and activated caspase reaction in silkworm hemocytes. Moreover, a significant decrease in digestive enzyme activity was observed at higher concentrations of cypermethrin. In cypermethrin-exposed groups, alteration in histoarchitecture was also observed in the form of ruptured microvilli and thin, deformed, fused mucous layer. The PO enzyme and lysozyme enzyme activity was also altered with sublethal concentration of cypermethrin. Total hemocyte count was reduced to 10587.10, 10052.30, 9234.30, and 8842.60 per mm3 with 10, 20, 30, and 40 µg/liter, respectively. The results offer new insights into the negative consequences of very low concentrations of cypermethrin formulations on nonmulberry silkworm of Northeast India.