Commercial traps like the TrapNKill and others just use one poison. This poison diffuses from the landing pad (or air in the case of TrapNKill) into the water bath and so both adults and eggs and larvae are killed by it.
So why do I recommend two poisons? It’s much more hassle.
The answer is pesticide resistance. This is a real threat world-wide, where mosquito populations are rapidly becoming resistant to key pesticides used to control them. It’s a major headache, and countries that are on top of this problem regularly monitor pesticide resistance among their populations.
According to numerous research papers, pesticide resistance is increasing rapidly, and the pressures put on a mosquito population by a successful trap program are very large.
If you use just one pesticide, and a resistant female lands on the pesticide coated landing strip, she’s not too much affected by it, so she can lay eggs. The eggs also have the same mutation that allows the female to have resistance – and therefore, if the same pesticide is in the water bath, the larvae will be immune and go on to develop into adults. Other, non-resistant mosquitoes will die, leading to a rapid increase in the population of resistant mosquitoes.
So, use a different class of pesticide in the water bath – the chances of multiple-resistant strains is very small, and if the female can lay eggs, the eggs will die.
Kill dem mosquitoes dead!