well over half the agricultural weeds in Canada are introduced species, many of them introduced without their chief insect enemies. Biological control, by the introdtiction of these insects, would reduce the aggressiveness of these weeds; but it should be regarded as a supplement to, and not a replacement for, mechanical or chemical control as it is most likely to succeed where these methods are ineffective or impractical. Thus the weeds chosen for biological control are either: resistant to chemical or mechanical control; serious pests of range or other low quality land where conventional methods of control are impractical; or resistant to control because they are continually spreading on to arable land from hedgerows and waste areas.