I think it was DDT. I don't believe there has been a substantial change in the weather since the '50's according to most reports. I think every year we progress from the last DDT spraying, the more ticks and chiggers we experience. Although, you would think that the populations would have reached an equalibrium since then. Just a thought, I am not a scientist and have not looked into it too deeply. Perhaps Sue has some insight.
Jeff
DDT is extremely persistent, but I seriously doubt it has any bearing on the tick populations. It is VERY toxic to fish and birds, and if it was within their reach from environmental spraying, these populations would be a lot different. DDT persists in the fat of animals (including humans), where it goes and stays there. Forever. It stays in the environment basically forever, too, the half life is about 15 years in the soil. It doesn't move around though, and as organic matter is deposited on the soil, it gets buried and stays there. It associates very tightly with soil, in fact, so it won't go anywhere. It won't run off or wash away. Therefore the ticks (or the birds, deer, humans, etc) are generally not exposed to it.
I think the tick population increasing is simply a product of a shift in the ecological balance of the area. It may or may not be due to the increase in the average temperature, that is obviously the topic of lots of debate. Their ecological niche is expanding, whatever the cause. The most plausible explanation to me is the increase in deer populations, as well as rodent hosts, both of which they need to reproduce.
Ticks species have definitely been expanding their ranges in the US. This is a study on the changes in the ranges of
Ixodes scapularis, the black-legged (deer) tick.
http://www.ehponline.org/members/2002/110p635-640estrada-pena/estrada-pena-full.html