We have been working with our yellow lab the last couple of weeks to retreive. We bought a plug and read a book and have been training her on leash to retreive. She does really well but once in a while she tries to take the plug and goes and chew it. How do I correct this behaviour properly? We don't really want her mutilating the bird once she picks it up. Any suggestions would be appreciated. Thanks.

Just out of curiosity, what book did you buy that refers to bumpers as plugs?

Put a check cord on your pup for the time being. Toss out the bumper and encourage her to return to you. As she comes back reel in the check cord so she can't slow down to chew the bumper. Give her verbal encouragement and praise as she is coming back to you. Also don’t immediately take the bumper from her mouth when she gets back to you. Let her hold the bumper for a shot period and continue to praise her before asking her to drop it. Only do a few retrieves at a time, three or four. You want to quit before she gets bored and is still excited to do more retrieves.