The impact of sublethal permethrin exposure on susceptible and resistant genotypes of the urban disease vector Aedes aegypti.

BACKGROUND: In urban environments, some of the most common control tools used against the mosquito disease vector Aedes aegypti are pyrethroid insecticides applied as aerosols, fogs or residual sprays. Their efficacy is compromised by patchy deployment, aging residues, and the evolution and invasion of pyrethroid-resistant mosquitoes. A large proportion of mosquitoes in a given environment will therefore receive sublethal doses of insecticide. The potential impact of this sublethal exposure on the behaviour and biology of Ae. aegypti carrying commonly reported resistance alleles is poorly documented. RESULTS: In susceptible insects, sublethal exposure to permethrin resulted in reductions in egg viability (13.9%), blood avidity (16.7%) and male mating success (28.3%). It caused a 70% decrease in the lifespan of exposed susceptible females and a 66% decrease in the insecticide-resistant females from the parental strain. Exposure to the same dose of insecticide in the presence of the isolated kdr genotype resulted in a smaller impact on female longevity (a 58% decrease) but a 26% increase in eggs per female and a 37% increase in male mating success. Sublethal permethrin exposure reduced host-location success by 20-30% in all strains. CONCLUSION: The detrimental effects of exposure on susceptible insects were expected, but resistant insects demonstrated a less predictable range of responses, including negative effects on longevity and host-location but increases in fecundity and mating competitiveness. Overall, sublethal insecticide exposure is expected to increase the competitiveness of resistant phenotypes, acting as a selection pressure for the evolution of permethrin resistance. © 2021 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

The presence of knockdown resistance mutations reduces male mating competitiveness in the major arbovirus vector, Aedes aegypti.

BACKGROUND: The development of insecticide resistance in mosquitoes can have pleiotropic effects on key behaviours such as mating competition and host-location. Documenting these effects is crucial for understanding the dynamics and costs of insecticide resistance and may give researchers an evidence base for promoting vector control programs that aim to restore or conserve insecticide susceptibility. METHODS AND FINDINGS: We evaluated changes in behaviour in a backcrossed strain of Aedes aegypti, homozygous for two knockdown resistance (kdr) mutations (V1016G and S989P) isolated in an otherwise fully susceptible genetic background. We compared biting activity, host location behaviours, wing beat frequency (WBF) and mating competition between the backcrossed strain, and the fully susceptible and resistant parental strains from which it was derived. The presence of the homozygous kdr mutations did not have significant effects on blood avidity, the time to locate a host, or WBF in females. There was, however, a significant reduction in mean WBF in males and a significant reduction in estimated male mating success (17.3%), associated with the isolated kdr genotype. CONCLUSIONS: Our results demonstrate a cost of insecticide resistance associated with an isolated kdr genotype and manifest as a reduction in male mating success. While there was no recorded difference in WBF between the females of our strains, the significant reduction in male WBF recorded in our backcrossed strain might contribute to mate-recognition and mating disruption. These consequences of resistance evolution, especially when combined with other pleiotropic fitness costs that have been previously described, may encourage reversion to susceptibility in the absence of insecticide selection pressures. This offers justification for the implementation of insecticide resistance management strategies based on the rotation or alternation of different insecticide classes in space and time.

Identifying the fitness costs of a pyrethroid-resistant genotype in the major arboviral vector Aedes aegypti.

BACKGROUND: Effective vector control measures are essential in a world where many mosquito-borne diseases have no vaccines or drug therapies available. Insecticidal tools remain the mainstay of most vector-borne disease management programmes, although their use for both agricultural and public health purposes has resulted in selection for resistance. Despite this, little is known about the fitness costs associated with specific insecticide-resistant genotypes and their implications for the management of resistance. In Aedes aegypti, the primary vector of dengue, chikungunya, and Zika, the best-characterised resistance mechanisms are single-point mutations that protect the voltage-gated sodium channel from the action of pyrethroids. METHODS: We evaluated the fitness cost of two co-occurring, homozygous mutations (V1016G and S989P) by back-crossing a resistant strain of A. aegypti from Timor-Leste into a fully susceptible strain from Queensland. The creation of the backcross strain allowed us to isolate these kdr mutations in an otherwise susceptible genetic background. RESULTS: In comparison to the susceptible strain, the backcrossed colony exhibited longer larval development times (5 days, P < 0.001), 24% fewer mosquitoes reached the adult stage (P = 0.005), had smaller wing lengths (females, P = 0.019 and males, P = 0.007) and adult female mosquitoes had a shorter average lifespan (6 days, P < 0.0006). CONCLUSIONS: These results suggest specific and significant fitness costs associated with the double homozygous V1016G/S989P genotype in the absence of insecticides. The susceptibility of a population may recover if the fitness costs of resistant genotypes can be emphasised through the use of insecticide rotations and mosaics or the presence of untreated spatial or temporal refuges.

Roles of biogenic amines in regulating bioluminescence in the Australian glowworm Arachnocampa flava.

The glowworm Arachnocampa flava is a carnivorous fly larva (Diptera) that uses light to attract prey into its web. The light organ is derived from cells of the Malpighian tubules, representing a bioluminescence system that is unique to the genus. Bioluminescence is modulated through the night although light levels change quite slowly compared with the flashing of the better-known fireflies (Coleoptera). The existing model for the neural regulation of bioluminescence in Arachnocampa, based on use of anaesthetics and ligations, is that bioluminescence is actively repressed during the non-glowing phase and the repression is partially released during the bioluminescence phase. The effect of the anaesthetic, carbon dioxide, on the isolated light organ from the present study indicates that the repression is at least partially mediated at the light organ itself rather than less directly through the central nervous system. Blocking of neural signals from the central nervous system through ligation leads to uncontrolled release of bioluminescence but light is emitted at relatively low levels compared with under anaesthesia. Candidate biogenic amines were introduced by several methods: feeding prey items injected with test solution, injecting the whole larva, injecting a ligated section containing the light organ or bathing the isolated light organ in test solution. Using these methods, dopamine, serotonin and tyramine do not affect bioluminescence output. Exposure to elevated levels of octopamine via feeding, injection or bathing of the isolated light organ indicates that it is involved in the regulation of repression. Administration of the octopamine antagonists phentolamine or mianserin results in very high bioluminescence output levels, similar to the effect of anaesthetics, but only mianserin acts directly on the light organ.