Flowering Brassicas and Cabbage Butterflies

[Walk]

Earlier this year we had no cabbage butterflies and the spring broccoli was absolutely clean for the first time ever. Later, as the season progressed and the drought and heat set in, we started to see the cabbage butterflies arriving. With the lack of caterpillars in the spring, I became complacent and didn't check the fall collards and kale until it was too late and they were completely infested. But I noticed that the broccoli that was flowering and producing seed was still without caterpillars. I talked to a couple of other seed savers in our region and they had noticed the same thing, one also saving broccoli for seed and the other on kale. They both had serious infestations on their fall crops while the earlier planted seed crops remained larva-free. Now I'm curious as to whether there is a different compound in the flowering plants that changes the scent or makes it repulsive? Or is it that the butterflies choose younger plants so their larva will hatch on a plant that's not on its way to shutting down the food source? Has anyone else noticed something like this or read of it?

[raymondo]

I have brassicas going to seed most years in my garden but I haven't been watching for this. One thing I have noticed is that the grey cabbage aphids prefer the flowering plants, especially the flower stems. I'll watch the cabbage whites more carefully in future. It is quite possible that flowering triggers the production of a chemical in the plant that the butterflies find less attractive. I've noticed, for example, that coriander (cilantro) in flower has an unpleasant bitterness to it.