There are two types of caterpillar that love to eat brassicas - you will find them among your cabbages and broccoli from July to September (even longer if you grow in polytunnels).
The Small Cabbage White Butterfly larva is probably the most destructive of all of the caterpillars, devouring cabbages and other cruciferous crops all over the world.The adult male has one spot on the wing and the female has two - both have a pale yellow underside to the wings. The caterpillars are green and difficult to spot among green leaves, especially when they lie along the ribs. The Large White is a larger version of the small white with much more pronounced black markings. The caterpillars are patterned and larger and much easier to spot.
There is another caterpillar that does a lot of damage - the caterpillar of the codling moth The caterpillars feed inside the fruits of apple (Malus), quince (Cydonia oblonga), pear (Pyrus) and other wild and cultivated fruit. The adults of the codling moth emerge mainly in July and August, sometimes with a second generation in September and October.
We supply a paste of tiny, tiny, worm-like creatures called "nematodes". The nematodes (Steinernema carpocapsae), attack the pest by entering natural body openings. Once inside, they release bacteria that stops the larvae from feeding, quickly killing it and breaking the pest's lifecycle. We offer a three-pack treatment - so that you can make three repeat applications at 5 to 7 day intervals. In the case of cabbage caterpillars in your brassicas you should target the caterpillars directly, and spray on the nematodes between July and September. Each application covers 40sq metres of ground. If you are treating codling moth then you should spray the tree, main branches and surrounding soil in the Autumn, to kill the overwintering caterpillars. Each application covers up to 20 trees.