Firstly because cactaceae as a group of plants are quite separate from most other groups. It was fairly safe to say that insects feeding on cactus would not eat any other type of plant and secondly, we fairly well knew that the insects wouldn't be able to live on most of them, simply because the cactoblastis was a gregarious internal borer. To begin to test it on wheat and oats and things like that was simply ridiculous, but we had to do it, just to prove it was safe to use them. Our results showed without a doubt that these insects had a group of plants which they could live on and nothing else. And that's usually what is found right throughout the insect kingdom.
Organisms keep to one group of plants for their feeding.
They did crawl over some and they made a few small marks where you could see that they had made an attempt to feed on such things as potatoes and melons, and tomatoes, etc. They never even began to breed on them. To me that was overwhelming evidence that they had a particular group of plants to live on and would not live on any others. We also found they were confined to certain types of cacti. There are quite a lot of groups of cactus that they will not feed on.