Resistance build up against chemicals has been known and confirmed for many years.

Example:

Extract of Dr. Uton Muchtar Rafei’s speech, Regional Directors office of WHO/South East Asia - August 1997:
“ The reappearance of several mosquito vectors in most Member States has been attributed to the failure of chemical control as a result of the mosquito vectors developing resistance to insecticides. In the present scenario, mosquitoes have developed resistance to all the major groups of insecticides, including biocides “

Preventine MLC ^® is a product with no such chemical content

Breeding cycle:
In order to produce millions of mosquitoes, each female needs to fill her blood-sack before she lays often 1000+ eggs during her short life.