Nothing can ruin a trip more than swarms of mosquitoes.  Swarms so big that they fly into your mouth and ears and eyes.  Retreat into the tent is the only way to keep one's sanity.  Usually we try to plan our trips before the big hatch or after.  The best time is after a frosty evening which knocks the buggers down for a few days.  Going in the fall with cooler night weather in exchange for less bugs is a good trade as far as I am concerned. 

However, if you are going in June or July and expect mosquitoes you can use the traditional repellents.  You might also try taking garlic pills and vitamin B pills for a couple of days before the trip.  They change your blood a bit and make you less attractive. 

 

different type website - zen backpacking

 

 

Bigger pests such as bears and mountain lions may poise threats but you can do a lot to minimize the threat.  We have had many encounters with black bears in Yosemite.  In the Olympics, they are more timid but in Yosemite, they are pests.  We now always use bear canisters to haul and store our food.  We once had a mountain lion snarl in Eastern Oregon and that was a bit unnerving.  I have a friend who claims that with the decrease in hunting of bears and mountain lions, they are losing their fear of man.  If this is true, one must be even more vigilant and use preventative measures to minimize threats.