The active ingredient in Derris dust is Rotenone, a natural compound that is found in a number of tropical and subtropical plants in Central and South America. It is the strongest and longest lasting of the natural insecticides and is relatively safe for humans. It is very effective against cabbage butterfly and I regularly use it on Brassica  seedlings (cabbage, cauliflower, broccoli). However it is water soluble, so it's best not to use it if rain is expected.

Here in Australia Derris dust is readily available at nurseries.  Remember that it is a poison, so make sure you follow the safety instructions on the labels of any Derris product.

Pyrethrum is a natural insecticide that comes from the White Chrysanthemum daisy. It is effective against a wide range of insects and, in lower doses, will act as an insect replant. I have found it very effective at killing aphid infestations. It does come in a powder form but I have only ever used it as a spray. Like Derris dust it is readily available at nurseries.

The problem with insecticides such as Pyrethrum and Derris is that they are broad spectrum insect killers. Meaning that they kill all insects, not just the ones actually eating your vegetables.

Bacillus thuringiensis is a bacteria that only attacks a limited range of leaf eating caterpillars, leaving predatory insects unharmed. It is able to do this because it is effectively a stomach poison. If a caterpillar eats part of a leaf that has been sprayed with Bacillus thuringiensis it will also ingest some of the bacteria. The caterpillar usually stops eating within hours and is dead within two or three days.

While there are a range of caterpillars that this bacteria will attack I use it mainly for killing cabbage butterfly, which specializes in attacking plants of the Brassica family (cabbage, cauliflower, broccoli etc...)  While it is slower acting then Derris dust it seems to last longer on the plants and leaves predatory insects unharmed to do their work.

Bacillus thuringiensis is sold under the label of Dipel. Unfortunately, here in Australia, it is only supplied by one company (Yates). The smallest size it is sold in is way too big for the average backyard vegetable gardener as each package supplies enough for several years of spraying. But I get around this by getting together with some gardening friends and sharing the sachets and the cost.  One sachet is plenty for me and I have a large garden.

In treating Brassica plants against cabbage butterflies I generally use Derris dust on the young seedlings in their seedling boxes and when they are first planted out. But once established in the garden I switch to Bacillus thuringiensis. The instructions on the Dipel packet say spray weekly, but I generally find that two sprayings is enough. Though note that I only grow Brassica plants as an Autumn/Winter crop. If you are growing them as a Spring/Summer crop (when cabbage butterflies are more active) you might have to spray more often.