Teeney, I am not sure if the larva carry any disease. Personally I do not worry about this. From what I remember that mosquito carry disease if only they bite/suck blood from someone with infected agent and bite you.
Freezing is more difficult than simple freezing. Because the water and the ice with crystallize the larva and burst it (I think) and it became no good.
I am a cell biologist doing medical research before I went into teaching. As a cell biologist, I freeze cells, bacteria, virus, etc. We freeze them in a freezing media with about 70% DMSO (toxic to human). I am not sure about mosquito larva but I think they are big and the crystaline form around them with burst them and it become no good for fish.
However since you ask, I will try freeze it myself. I think harvest them then drain them to remove most of the water (pour them all into the net) and spoon them into zipcloc bag and freeze.