Is it possible that the local insect populations got wise to these malaise traps that the researchers were using in western germany?
Apparently the way that a malaise trap works is that it waits for insects to fly into it's wall, hopes that they crawl up the wall, and then hopes that they crawl right into a bottle filled with ethanol.
Now I do not know anything at all about insects and how they might learn or adapt to various situations.
But I know that they can adapt just like anything else, and maybe some of them starting saying "no I will not crawl up that screen into the bottle of alcohol" after 27 years and got the word out to other local insects?
Am I crazy for thinking that might be the case?