Can my landlord blame me for bedbugs?

3 weeks ago we had the outside of the house sprayed/bug proofed. Within the last week myself and my wife have been eaten alive with bedbug bites, my landlord sent someone round to confirm the bedbugs and is now charging us a $1200 treatment cost claiming they were brought in by us. We have lived here since July 2010. Can he do this? and is it possible that we brought them in?