It had nothing to do with it. The trees were almost all dead long before DDT was banned.
I’m also not sure what the connection to elms is.
As for Elm trees, there were still plenty of Elm trees around in 1971. Check out this article from the New York Times in 1971: Dutch Elm Disease Spreads Westward
Here is an excerpt from that 1971 NYT article: "...Forestry officials in Minnesota reported this week that there had been a 30 per cent increase in elm deaths this summer and the disease had spread into six new counties. Here in Chicago, where there are between 750,000 and one million elms in the parks, along the streets and in yards of homes, city foresters responsible for parkway trees expect to have to remove more than 50,000 dead elms by the end of the year. Last year 50,098 had to be cut down along the streets alone..." There is a connection to Elm trees. Dutch Elm Disease was a fungus, and it was spread by a species of beetle called the Elm Bark Beetle. The only thing keeping Dutch Elm Disease at bay by 1971 for the remaining trees was dusting with DDT that was particularly effective against that beetle. That is the connection with DDT. Anyway, it was my mistake, sorry for the confusion.