I used to use high concentration DEET but have always been a little concerned about side effects. Consumer Reports did a story about insect repellents in May and said research shows a higher concentration of DEET has not been proven any more effective than a 30% concentration, and the higher concentrations have been associated with rashes, disorientation, and seizures. Picardin (20% concentration) and oil of lemon eucalyptus (30% concentration) are considered to be safer choices. I bought REPEL brand of oil of lemon eucalyptus at REI and used it on a June trip out of Agnew Meadows. It worked very well for flies and mosquitoes. It comes in a 4 ounce spray bottle, so it weighs about the same. To save a couple ounces of weight if only needed for a few days, you could do what I did: pour half into another clean container and save for later.
