Anything with a citrus scent will reply the blighters (hence citronella candles). Diluted lemon or bergamot oil is a good start. It's also why the Avon Skin So Soft works - the army uses it, which is a pretty good endorsement. When I go on holiday I take an oil burner and some lemon oil to scent the room for 10 minutes before going to bed and daub some round the window - haven't been bitten once since doing this. Much better than those plug-in insecticides... and more environmentally friendly.