I've seen various articles/posts to say that decoy artificial white butterflies, such as the one I just made, are effective in preventing attack of brassicas by the white butterfly Pieris rapae. The thesis is that the insect is highly territorial and will fly off if it sees a competitor. But are these reports just publication bias or does it really work? I'm looking for controlled studies.
They do not help at all. The presence of another butterfly will often attract these, and you will often see numbers of them visiting the same plants. Even when finding a spot to lay eggs. They avoid a spot that has another butterfly on it (because of caterpillar food competition), but it really doesn't stop them from taking advantage of the plants. My neighbor used decoys similar, and reported absolutely no difference.
He said the plants near the decoys took the same hit as the ones further off. And that didn't surprise me at all. I think a better option would be to cover the plant(s) with floating row cover or similar.