Mosquitoes are very sensitive to heat and light (indeed, their sensitivity to infra-red radiation helps them sense the presence of food, in the form of people).

The telltales are specific "lines" in the spectrum of infra-red light from the plume. These correspond to the frequencies of light given off when water molecules are energetically excited.

Nor is it possible simply to crank up the power of the source beam, because infra-red light at high intensity can cause eye damage.