I'm just reading an interesting book, Thinking, Fast and Slow by the Nobel prize winning economist Daniel Kahneman. He has a chapter on "regression to the mean" and thinking more about it, I realized that that phenomenon might account for a fair chunk of the placebo effect. He has numerous examples - for example, the famous "Sports Illustrated curse" where an athlete ends up on the cover when he or she outperforms dramatically, and not surprisingly the outperformance is generally followed by a regression to the mean and the athlete reverts to their standard form.
Thus in say a depression trial, it's likely people sign up when they are particularly bad, and given that, they are likely to improve some over the course of the trial even though they are given a placebo. That improvement is all attributed to "the placebo effect" but I'm saying that it is likely only part placebo effect and part regression to the mean.
So anyhow, the insects would still exhibit regression to the mean, and so there would still be some "placebo effect."
That’s a pretty good attempt. However, there’s a more tangible (though perhaps less obvious) reason that an insect society would likely have a higher proportion of meritorious drug applications than a human society.