It all depends on the size of the bug/spider, of course, but these brothers won’t take something like that lightly. Mosquitoes are a little easier to deal with. As far as bugs go, they’re fairly frail. One good boot, even a teeny one like Dean’s, is enough to kill it off. (Bonus – check out this animation from @ask-tinysam: here) Lucky for them, they’re less of a target next to a regular sized human who has a lot more blood to offer. That would be a bite they’d feel for a long time afterwards.
If Jacob ever, ever, twitches the brothers off his shoulders, he better be ready to catch them! And the scolding would never end for him after that. Never.
Spiders they’ll need to take out with their knives if they encounter any. Worst comes to worst, they find a way out of there without setting the spider off. Better not get stuck in any webs like Sam managed in Schism!