Just because a sample does not find insects does not mean that you can't keep sampling. Plenty of insects if you stop mowing the law for a while. The real problem is that people has become blind about it. Decline of entomologists lead to a perception that there aren't insects anymore. Pale children collecting pokemon in a dark room is "good", children out collecting insects is "bad". This is (gasp) animal cruelty!, the new religion. We punished curiosity, so nobody cares. Nobody young seems interested on entomology anymore.