Bioinsecticidal activity of cajeput oil to pyrethroid-susceptible and -resistant mosquitoes.

Mosquito-borne diseases are a significant threat to human health. The frequent and repetitive application of insecticides can result in the selection of resistant mosquito populations leading to product failures for reducing community disease transmission. It is important that new interventions are discovered and developed for reducing mosquito populations and, in turn, protecting human health. Plant essential oils are promising chemical interventions for reducing mosquito populations. The myrtle family, Myrtaceae, has numerous species to be studied as potential bioinsecticides. Here, we combined toxicological, biochemical, and neurophysiological approaches to provide evidence for cajeput oil and terpene constituents to elicit bioinsecticidal activity to pyrethroid-susceptible and -resistant Aedes aegypti. We show cajeput oil terpenes to enhance cAMP production, increase ACh levels, inhibit in vivo and in vitro AChE activity, and disrupt spike discharge frequencies of the mosquito CNS. This study presents the first report on the bioinsecticidal activity of cajeput oil terpenes to pyrethroid-susceptible and -resistant mosquitoes and provides comparative data for the octopaminergic system as a putative molecular target for the bioinsecticides with implications for resistance management.

Age- and sex-related ABC transporter expression in pyrethroid-susceptible and -resistant Aedes aegypti.

Resistance mechanisms to synthetic insecticides often include point mutations and increased expression of genes encoding detoxification enzymes. Since pyrethroids are the main adulticides used against Aedes aegypti, which vectors pathogens such as Zika virus, understanding resistance to this insecticide class is of significant relevance. We focused on adenosine triphosphate (ATP)-binding cassette (ABC) transporters in the pyrethroid-resistant Puerto Rico (PR) strain of Ae. aegypti. We investigated the expression patterns of six ABC transporters previously characterized as differentially expressed in insecticide-challenged mosquitoes, or increased mRNA expression in pyrethroid-resistant Ae. aegypti, by comparing PR to the Rockefeller (Rock) susceptible strain. No constitutive differential expression between strains was detected, but expression differences for these genes was influenced by sex and age, suggesting that their role is independent from resistance in PR. Instead, ABC transporters may be induced after insecticide exposure. Challenging mosquitoes with deltamethrin, with or without ABC transporter modulators, showed that Rock and PR responded differently, but a contribution of ABC transporters to deltamethrin toxicity is suspected. Moreover, the effect of dexamethasone, which enhanced the inhibition of nerve firing by deltamethrin, was observed using a Drosophila central nervous system preparation, showing synergy of these two compounds through the potential inhibition of ABC transporters.

Vapor delivery of plant essential oils alters pyrethroid efficacy and detoxification enzyme activity in mosquitoes.

The use of synthetic insecticides to limit the spread of mosquito-borne disease faces a number of significant challenges, including insecticide resistance, concerns related to the environmental impact of widespread insecticide use, as well as slowed development of new insecticide chemistries. One important alternative to broadcast insecticides is the use of personal protection strategies to limit contact with vector species, including the use of spatial repellents that can employ synthetic pyrethroids or botanical products to effect control. A currently underexplored area of research involves the investigation of botanical products for their potential to serve as insecticide synergists when delivered as a vapor. This study describes the development of an assay that facilitates the screening of essential oils delivered as a vapor for enhancement of deltamethrin efficacy in both pyrethroid-susceptible and -resistant strains of the vector mosquito species Aedes aegypti. Deltamethrin efficacy was significantly increased following exposure to cinnamon (Cinnamomum cassia), tagetes (Tagetes bipinnata), and sage (Salvia officinalis) oils, while efficacy was significantly decreased following exposure to amyris (Amyris balsamifera) oil. These effects appeared to be mediated by changes in cytochrome P450 activity. This work demonstrates that some plant-derived essential oils delivered as a vapor are capable of increasing the efficacy of deltamethrin similar to classical synergists such as piperonyl butoxide, supporting the use of a real world delivery method instead of traditional contact exposure studies.

Association of age, sex, and pyrethroid resistance status on survival and cytochrome P450 gene expression in Aedes aegypti (L.).

Aedes aegypti is a vector of viruses that negatively impact human health. Insecticide resistance complicates mosquito control efforts, but understanding the mechanisms of resistance can help to improve management practices. This study examined different factors that could influence the interpretation of toxicity bioassays and gene expression studies in A. aegypti, including sex and age, in the context of resistance to pyrethroids. Bioassays using a pyrethroid-resistant strain, Puerto Rico (PR), and a pyrethroid-susceptible strain, Rockefeller (Rock), of A. aegypti were conducted with females and males of three age groups to determine differences in mortality induced by deltamethrin. Overall, strain was the only factor with a significant effect on the LD50. Enzyme assays showed that cytochrome P450 monooxygenase activity in PR was constitutively higher than in Rock, and that pretreatment with the cytochrome P450 inhibitor piperonyl butoxide (PBO) followed by a topical application of deltamethrin (LD25) significantly increased mortality in both strains. Evaluation of the expression levels of seven CYP9J genes previously reported to be involved in pyrethroid resistance revealed that CYP9J10, CYP9J19, and CYP9J28 were more highly expressed in PR than in Rock at all ages of females and males, indicating that they may be essential for resistance. The expression of CYP9J24, CYP9J26, CYP9J27, and CYP9J32 was higher in PR males compared to other groups, including PR females. Significant differences in expression between sexes and strains were also observed as a result of age.