IMHO spraying is a waste of time. On quite a few occasions I am somewhat amused that, after spraying, a few little bugs come flying past my head totally oblivious to supposedly having been exterminated. I suppose you could argue that it probably kills some and so lessens the risk but with all the cr*p imported on ships, the ineffectiveness of sprays it almost certainly poses more of a risk to passengers then the benefits of killing a few bugs.