The article contains the following sentence: "In a randomized trial in Singapore, releases of Wolbachia-infected males cut mosquito populations by 85 percent compared to control areas and dengue infections by roughly 70 percent."
There are different things you can do to mosquitos with different techniques. The article discusses several different techniques. Some eliminate disease-spreading mosquitos, some eliminate the mosquitos. In Singapore, the latter was tried out. It is not the case that Singapore used chemicals.
lets say on the order of 1 billion, and let's assume the effectivity doesn't change when considering the 30% unaffected population
it would need to be applied to the surviving 30% repeatedly:
0.3 ^ N = 1 / 1B
so N = log(1B)/log(10/3) = ~17.2 times.
I predict nature protects itself with diversity: some of the male and female wild type will have natural aversion from, attraction to or neutral stance the modified Wolbachia carriers.
I predict that those mosquito family lines that happen to be more Wolbachia-carrier averse become enriched over time.
There is an energetic cost to maintaining a parasitic lifeform, and that energetic term will always favor immunity against the parasite.
The hosts originally had their normal wild type gene regulation.
After infection one can approximate the novel gene regulation to be a combination of the host wild type gene regulation, perturbed by the Wolbachia gene regulatory network.
The metabolic rate of the Wolbachia will be a function of genome variations in the wild type population. The population (even if fully infected) thus has access to a whole bunch of knobs (promoter region binding affinities) and can continuously shed the metabolic burden of the parasite by natural selection, since 2 infected mosquitos, with differing levels of Wolbachia metabolism / reproduction, will have more energy and amino acids available for flight, healinig / repair, eggs, ... so that over a number of generations the Wolbachia incidence rate falls back to equilibrium levels. Then its a matter of time until a dormant virus or a one of the many virus particles scattered in nature and a mosquito meet again.
There are different things you can do to mosquitos with different techniques. The article discusses several different techniques. Some eliminate disease-spreading mosquitos, some eliminate the mosquitos. In Singapore, the latter was tried out. It is not the case that Singapore used chemicals.