Prof. Rongsong Liu, University of Wyoming


Delayed action insecticides and their role in mosquito and malaria control

There is considerable interest in the management of insecticide
resistance in mosquitoes. One possible approach to slowing down the evolution of
resistance is to use late-life-acting (LLA) insecticides that selectively kill only the
old mosquitoes that transmit malaria, thereby reducing selection pressure favoring
resistance. In this paper we consider an age-structured compartmental model for
malaria with two mosquito strains that differ in resistance to insecticide, using
an SEI approach to model malaria in the mosquitoes and thereby incorporating
the parasite developmental times for the two strains. The human population is
modeled using an SEI approach. We consider both conventional insecticides that
target all adult mosquitoes, and LLA insecticides that target only old mosquitoes.
According to linearised theory the potency of the insecticide affects mainly the
speed of evolution of resistance. Mutations that confer resistance can also affect
other parameters such as mean adult life span and parasite developmental time.
For both conventional and LLA insecticides the stability of the malaria-free equilibrium,
with only the resistant mosquito strain present, depends mainly on these
other parameters. This suggests that the main long term role of an insecticide
could be to induce genetic changes that have a desirable effect on a vital parameter
such as adult life span. However, when this equilibrium is unstable, numerical
simulations suggest that a potent LLA insecticide can slow down the spread of
malaria in humans but that the timing of its action is very important.