Don, that is what I've thought different times as well and sometimes its leaned that way, canola on canola for instance causing more flea beetles or lygus but none of that flew this year, not even the having sprayed last year aspect as we had them just as bad if not worse on what had been wheat last year. The neighbor as well, he never sprayed anything last year, never had any canola on canola this year and was totally shocked as he was finding over 100 bugs and some of his fields are near here while others are a number of miles away.
I know berthas can invade a whole area as they lay their eggs and then we are in for a rude awakening when someone screams BERTHA !, go out into the field and see them chewing away. I hate it though, we never used to have to spray very often and perhaps its because there is more canola grown now then their used to be but also because of the chemical taken off the market that worked so well on the canola for the flea beetles which now forces us into spraying in the spring sometimes.