Evans died in 2002, at age 83. If he retired around age 65, that would have been around 1967, so if he was referring to the state of the field when he was a professor that would have been in nearly 50 years ago.

"Life on a Little Known Planet" was first published in 1968, which fits in with that. The latest edition was revised and updated in 1993, which was still quite a while ago.

I don't recall for sure, but I believe the passage that mentioned only two parasitic wasp experts in the US was talking about the time when his graduate student was interested in specializing in that area, which could have been any time before he retired.

(I can't check because unfortunately I have it as an audio book. Great for listening in the car, but terrible for searching to find and reread a particular passage!)