I use 10% sucrose solution to feed lab mosquitoes. Until now, I mix sucrose in autoclaved water and use it directly for feeding mosquitoes. Is it necessary to autoclave the sucrose solution itself before use?
I wouldn't autoclave this solution - sucrose will partly break down into glucose and fructose. A portion of the sugars will also caramellize and color the solution yellow to brown.
If you need the solution to be sterile, I would filter sterilize it, although this can be hard because of the higher viscosity of the sugar solution. If you haven't seen any negative effects of preparing the solution fresh with autoclaved water, I don't think there is any need to change your protocol.
I found a standard protocol for the same here : http://vosshall.rockefeller.edu/assets/file/Vosshall%20Lab%20Mosquito%20Rearing%20SOP%20DEC%2012-2014.pdf
Take a look at page no.8. They suggest to autoclave the sucrose solution.