Beyond redundant kill: A fundamental explanation of how insecticide mixtures work for resistance management

Aedes aegypti control in breeding sites through an insecticidal coating with dual effect: Laboratory trials and safety assessment

Ground water tanks are known to be preferred Aedes aegypti oviposition places providing opportunities for adult and larvae control. Therefore, a dual-effect insecticidal coating (IC) (alphacypermethrin/ pyriproxyfen) with a slow-release mechanism and safe for users could be applied within Aedes spp. breeding sites, representing a promising option. Bioassays were designed to determine the mortality and sterilizing effects on gravid mosquitoes exposed to IC. The effect of inhibition of emergence was evaluated in eggs, larvae and pupae exposed in different containers. For the water safety assessment concentrations of active ingredients were determined by reverse phase high performance liquid chromatography (RP-HPLC) and the health risk was calculated. The IC applied to the interior walls of water-holding containers showed efficacy against Ae. aegypti in terms of high gravid-female mortality (81% at 24 h, p < 0.01), sterilizing effect (inhibition of oviposition by 63%, p < 0.01) and emergence inhibition (100% in eggs, L3 and L4; 97% in pupae). The offspring rate was reduced [only 0.15 (38/250) new adults emerged per exposed gravid females as against 11.90 per unexposed female (2976/250) at baseline]. Emergence inhibition was recorded up to 12 months and adult mortality >80% up to 6 months. The use of water stored in treated containers, either for washing or drinking, is not expected to pose a health risk to users. IC applied to domestic water containers has dual and complementary action that reduces Ae. aegypti densities (immature and adult stages). This represents baseline information for a cluster randomized efficacy trial in Colombia.

Insecticide mixtures reduce the selectivity enhanced by pyrethroid resistance in a predatory lady beetle

Insecticide mixtures provide various modes of action in a ready-to-use formulation, broadening the range of managed pest species and delaying the development of insecticide resistance. Nonetheless, the insecticides used in the mixture may change the selectivity status for natural enemies obtained when using an individual formulation. Eriopis connexa (Germar), a lady beetle resistant to the broad-spectrum pyrethroid λ-cyhalothrin (EcViR), was exposed to insecticides in both individual and mixture formulations. The insecticides λ-cyhalothrin, chlorantraniliprole, sulfoxaflor, λ-cyhalothrin+dinotefuran, λ-cyhalothrin+thiamethoxam, and λ-chlorantraniliprole+thiamethoxam were tested. Survival of larvae and survival and reproduction of adults were assessed when enclosed with dry insecticide residues obtained at the maximum insecticide recommended rate. Furthermore, concentration-mortality curves were also established for larvae and adults exposed to insecticides with mortality rates exceeding 40%. For 30 days, the number of adults who survived exposure at the larval and adult stages and were still living and reproducing was recorded. The toxicity of the neonicotinoids thiamethoxam and dinotefuran present in the mixture prevailed over the physiological selectivity obtained through the resistance to λ-cyhalothrin. The combination index revealed that adding neonicotinoids to λ-cyhalothrin or chlorantraniliprole was very harmful to the lady beetle. On the other hand, the combination of λ-cyhalothrin with chlorantraniliprole or sulfoxaflor allowed EcViR larvae and adults to survive more than 80%. Therefore, incorporating neonicotinoids into the insecticide formulation nullified the physiological selectivity achieved by E. connexa resistance to λ-cyhalothrin, reducing the potential effectiveness of E. connexa in pest management as an augmentative biological control agent.

Developmental toxicity of formulated insecticide mixture containing imidacloprid and beta-cyfluthrin in fish: Insights using zebrafish

Insecticides are critical in controlling pests and disease vectors. However, there is still a lack of ecotoxicological studies using commercial formulations of insecticides containing active ingredients. The study aimed to evaluate the developmental toxicity of a commercial insecticide mixture (imidacloprid [IMI] + beta-cyfluthrin [β-CYF]). Mortality, hatching rate, spontaneous contraction, heartbeat, morphological changes, reactive oxygen species (ROS), skeletal development, and locomotor behavior of zebrafish were analyzed. Embryos were exposed to imidacloprid (IMI) and β-cyfluthrin (β-CYF) in the following ratios: 0.001 mg IMI·L-1 + 0.000125 mg β-CYF·L-1 (C1); 0.01 mg IMI·L-1 + 0.00125 mg β-CYF·L-1 (C2); 0.1 mg IMI·L-1 + 0.0125 mg β-CYF·L-1 (C3); 1.0 mg IMI·L-1 + 0.125 mg β-CYF·L-1 (C4); 10.0 mg IMI·L-1 + 1.25 mg β-CYF·L-1 (C5) for 144 h. The results showed a mortality of 50 % of organisms in the C5 concentration. Embryos exposed to C1 and C3 showed tachycardia and hatched early compared to the negative control, indicating cardiotoxic and embryotoxic effects. The two highest concentrations tested (C4 and C5) induced evident morphological changes (yolk sac and pericardial edema, and spine alterations), and skeletal toxicity (absence of cartilage and bone formation), along with decreased larval swimming behavior. Also, the formulated insecticide (C1) increased ROS levels in zebrafish larvae. Results showed that the formulated insecticide containing IMI and β-CYF induces several toxic effects on developing zebrafish, indicating its environmental risk to aquatic organisms.

Insecticide mixtures-uses, benefits and considerations

Insecticides remain an important tool for the control of many insect pests. There has long been an interest in insecticide mixtures (in-can and tank-mix) as a means to provide the needed efficacy and/or spectrum to control many insect public health, crop pests or crop pest complexes. This aspect has become more important since insecticides developed in the last 30 years tend to be narrower in spectrum with many primarily focused on either sap-feeding or chewing insect pests. Insecticide mixtures are also seen as an important approach to insect resistance management (IRM) with certain requirements for optimal implementation. Additionally, insecticide mixtures can also address certain agronomic, commercial and intellectual property needs and opportunities. This perspective will review some of the drivers and considerations for insecticide mixtures and their potential uses. © 2024 Society of Chemical Industry.

Theory versus practice: are insecticide mixtures in Arizona cotton used for resistance management?

BACKGROUND

Resistance management in pesticide use is critical, yet grower practices, especially pesticide mixing motivations, diverge from theoretical frameworks. This study analyzes 30 years of Arizona cotton growers' practices and pest manager insights to understand mixing trends.

RESULTS

Growers predominantly mix pesticides for spectrum or efficacy, not resistance management. This highlights a gap between theory and practice, emphasizing the complexity of real-world dynamics. A shift over time towards selective insecticides and integrated pest management (IPM), supported by extension education, has reduced reliance on broad-spectrum insecticides and increased opportunities to conserve the natural enemies of key pests. This reduced the frequency of insecticide use, a mutual goal of both IPM and resistance management. The availability and adoption of selective products with diverse modes of action, along with the resulting increases in biological control and refuges, likely has delayed or prevented resistances without emphasis on using mixtures specifically for resistance management. In a disrupted system exclusively dependent on broad-spectrum insecticides (1991-1995), 75% ± 5% of cotton area was sprayed with mixtures of these materials. With the availability of selective insecticides, few broad-spectrum products are used today and mixtures of insecticides are sprayed on only 36% ± 3% of the cotton area (2015-2020).

CONCLUSION

Although mixing has theoretical relevance, it is diminishing in stable systems with diverse modes of action and adherence to moderation principles. Arizona cotton guidance prioritizes multi-crop refuges over mixtures for resistance management. Integrated research and education, targeting professional pest managers, are pivotal in advancing resistance management without mixtures specifically designed to prevent or mitigate resistance. © 2024 Society of Chemical Industry.

The impact of insecticide decay on the rate of insecticide resistance evolution for monotherapies and mixtures

BACKGROUND

The problem of insecticide decay following their deployment in public health applications is frequently highlighted as an issue for sustained disease control. There are additional concerns that it also increases selection for insecticide resistance. Despite these concerns insecticide decay is largely absent from models evaluating insecticide resistance management strategies.

METHODOLOGY

The impact of insecticide decay is investigated using a model which assumes a polygenic basis of insecticide resistance. Single generation evaluations are conducted that cover the insecticide efficacy and insecticide resistance space for insecticides when deployed as monotherapies or mixtures to mechanistically investigate how insecticide decay impacts selection for resistance. The outcome is the between generation change in bioassay survival to the insecticides. The monotherapy sequence and mixture strategies were compared in multi-generation simulations incorporating insecticide decay, with the outcome being the difference in strategy lifespan.

RESULTS

The results demonstrate that as insecticides decay, they can apply a much greater selection pressure than that imposed by newly deployed, non-decayed insecticides; this applies to both monotherapies and mixtures. For mixtures, selection for resistance is highest when both insecticides have reduced decayed efficacies; this also occurs if reduced dosages are deliberately used in mixtures. Insecticide decay was found to reduce the benefit of mixtures compared to sequential monotherapies, especially when reduced-dose mixtures are used.

CONCLUSIONS

Insecticide decay is often highlighted as an important consideration for mixtures and these results indicate its absence in previous modelling studies may be over-inflating the performance of full-dose mixtures.

IN SUMMARY

as insecticides decay, they can impose increasing selection pressures for resistance with reduced ability to control the target insect populations. The optimal frequency with which decaying insecticides should be replenished is an important policy consideration.

Resistance Management for Cancer: Lessons from Farmers

One of the main reasons we have not been able to cure cancers is that treatments select for drug-resistant cells. Pest managers face similar challenges with pesticides selecting for pesticide-resistant insects, resulting in similar mechanisms of resistance. Pest managers have developed 10 principles that could be translated to controlling cancers: (i) prevent onset, (ii) monitor continuously, (iii) identify thresholds below which there will be no intervention, (iv) change interventions in response to burden, (v) preferentially select nonchemical control methods, (vi) use target-specific drugs, (vii) use the lowest effective dose, (viii) reduce cross-resistance, (ix) evaluate success based on long-term management, and (x) forecast growth and response. These principles are general to all cancers and cancer drugs and so could be employed broadly to improve oncology. Here, we review the parallel difficulties in controlling drug resistance in pests and cancer cells. We show how the principles of resistance management in pests might be applied to cancer. Integrated pest management inspired the development of adaptive therapy in oncology to increase progression-free survival and quality of life in patients with cancers where cures are unlikely. These pest management principles have the potential to inform clinical trial design.

Effects of Bacillus thuringiensis Treatment on Expression of Detoxification Genes in Chlorantraniliprole-Resistant Plutella xylostella

Detoxification genes are crucial to insect resistance against chemical pesticides, yet their expression may be altered by exposure to biopesticides such as spores and insecticidal proteins of Bacillus thuringiensis (Bt). Increased enzymatic levels of selected detoxification genes, including glutathione S-transferase (GST), cytochrome P450 (CYP450), and carboxylesterase (CarE), were detected in chlorantraniliprole (CAP)-resistant strains of the diamondback moth (DBM, Plutella xylostella) from China when compared to a reference susceptible strain. These CAP-resistant DBM strains displayed distinct expression patterns of GST 1, CYP6B7, and CarE-6 after treatment with CAP and a Bt pesticide (Bt-G033). In particular, the gene expression analysis demonstrated significant upregulation of the CYP6B7 gene in response to the CAP treatment, while the same gene was downregulated following the Bt-G033 treatment. Downregulation of CYP6B7 using RNAi resulted in increased susceptibility to CAP in resistant DBM strains, suggesting a role of this gene in the resistant phenotype. However, pretreatment with a sublethal dose of Bt-G033 inducing the downregulation of CYP6B7 did not significantly increase CAP potency against the resistant DBM strains. These results identify the DBM genes involved in the metabolic resistance to CAP and demonstrate how their expression is affected by exposure to Bt-G033.

Biological lessons for strategic resistance management

Biological resistance to pesticides, vaccines, antibiotics, and chemotherapies creates huge costs to society, including extensive morbidity and mortality. We simultaneously face costly resistance to social changes, such as those required to resolve human-wildlife conflicts and conserve biodiversity and the biosphere. Viewing resistance as a force that impedes change from one state to another, we suggest that an analysis of biological resistance can provide unique and potentially testable insights into understanding resistance to social changes. We review key insights from managing biological resistance and develop a framework that identifies seven strategies to overcome resistance. We apply this framework to consider how it might be used to understand social resistance and generate potentially novel hypotheses that may be useful to both enhance the development of strategies to manage resistance and modulate change in socio-ecological systems.

Insecticide resistance management strategies for public health control of mosquitoes exhibiting polygenic resistance: A comparison of sequences, rotations, and mixtures

Malaria control uses insecticides to kill Anopheles mosquitoes. Recent successes in malaria control are threatened by increasing levels of insecticide resistance (IR), requiring insecticide resistance management (IRM) strategies to mitigate this problem. Field trials of IRM strategies are usually prohibitively expensive with long timeframes, and mathematical modeling is often used to evaluate alternative options. Previous IRM models in the context of malaria control assumed IR to have a simple (monogenic) basis, whereas in natural populations, IR will often be a complex polygenic trait determined by multiple genetic variants. A quantitative genetics model was developed to model IR as a polygenic trait. The model allows insecticides to be deployed as sequences (continuous deployment until a defined withdrawal threshold, termed "insecticide lifespan", as indicated by resistance diagnosis in bioassays), rotations (periodic switching of insecticides), or full-dose mixtures (two insecticides in one formulation). These IRM strategies were compared based on their "strategy lifespan" (capped at 500 generations). Partial rank correlation and generalized linear modeling was used to identify and quantify parameters driving the evolution of resistance. Random forest models were used to identify parameters offering predictive value for decision-making. Deploying single insecticides as sequences or rotations usually made little overall difference to their "strategy lifespan", though rotations displayed lower mean and peak resistances. Deploying two insecticides in a full-dose mixture formulation was found to extend the "strategy lifespan" when compared to deploying each in sequence or rotation. This pattern was observed regardless of the level of cross resistance between the insecticides or the starting level of resistance. Statistical analysis highlighted the importance of insecticide coverage, cross resistance, heritability, and fitness costs for selecting an appropriate IRM strategy. Full-dose mixtures appear the most promising of the strategies evaluated, with the longest "strategy lifespans". These conclusions broadly corroborate previous results from monogenic models.