Pyriproxyfen for mosquito control: female sterilization or horizontal transfer to oviposition substrates by Anopheles gambiae sensu stricto and Culex quinquefasciatus

Abstract

Background: The use of gravid mosquitoes as vehicles to auto-disseminate larvicides was recently demonstrated for the transfer of pyriproxyfen (PPF) by container-breeding Aedes mosquitoes and presents an appealing idea to explore for other disease vectors. The success of this approach depends on the female's behaviour, the time of exposure and the amount of PPF that can be carried by an individual. We explore the effect of PPF exposure at seven time points around blood feeding on individual Anopheles gambiae sensu stricto and Culex quinquefasciatus fecundity and ability to transfer in laboratory assays.

Method: Mosquitoes were exposed to 2.6 mg PPF per m2 at 48, 24 and 0.5 hours before and after a blood meal and on the day of egg-laying. The proportion of exposed females (N=80-100) laying eggs, the number of eggs laid and hatched was studied. Transfer of PPF to oviposition cups was assessed by introducing 10 late instar insectary-reared An. gambiae s.s. larvae into all the cups and monitored for adult emergence inhibition.

Results: Exposure to PPF between 24 hours before and after a blood meal had significant sterilizing effects: females of both species were 6 times less likely (Odds ratio (OR) 0.16, 95% confidence interval (CI) 0.10-0.26) to lay eggs than unexposed females. Of the few eggs laid, the odds of an egg hatching was reduced 17 times (OR 0.06, 95% CI 0.04-0.08) in Anopheles but only 1.2 times (OR 0.82, 95% CI 0.73-0.93) in Culex. Adult emergence inhibition from larvae introduced in the oviposition cups was observed only from cups in which eggs were laid. When females were exposed to PPF close to egg laying they transferred enough PPF to reduce emergence by 65-71% (95% CI 62-74%).

Conclusion: PPF exposure within a day before and after blood feeding affects egg-development in An. gambiae s.s. and Cx. quinquefasciatus and presents a promising opportunity for integrated control of vectors and nuisance mosquitoes. However, sterilized females are unlikely to visit an oviposition site and therefore do not transfer lethal concentrations of PPF to aquatic habitats. This suggests that for successful auto-dissemination the optimum contamination time is close to oviposition.

Figure 1

Schematic diagram showing the pyriproxyfen-exposure times for Anopheles gambiae s.s. and Culex quinquefasciatus . Blue arrows show treatment groups exposed before a bloodmeal, red arrows show treatment groups exposed after a bloodmeal. Control females were exposed to acetone at 0.5 hours before bloodmeal. Time of egg-laying was in Anopheles gambiae s.s. 72 hours after a bloodmeal (6 day old females) and in Culex quinquefasciatus 144 hours after a bloodmeal (9 day old females). All treatment groups and control were tested in parallel, 20 individual females at a time, repeated 4–5 times (rounds).

Figure 2

Median adult emergence rates from late instar larvae introduced into oviposition cups. Results for pyriproxifen exposed Anopheles gambiae(A) and Culex quinquefasciatus(B) from cups in which eggs were laid (1) and for cups in which no eggs were laid (2). Blue box plots show treatment groups exposed before a bloodmeal, red box plots show treatment groups exposed after a bloodmeal.