As the seasons change, and particularly when they are as odd as this past month has been, plants can come under increasing stress, and insects can attack.
One of the most common attackers at this time of year is the White Butterfly Caterpillar. (Sometimes referred to as the Cabbage White Butterfly Caterpillar.)
The first thing that will make you realise that your garden has been visited by White Butterfly is the holes in the leaves, sometimes described as shot-holes (the leaves look like you’ve taken a shot gun to them). This is the caterpillar at work.
The holes are normally spread randomly across the leaves, with large chunks taken from the edges of the leaves in particular. Sometimes the damage can reach the heart of the plant – particularly devastating on a nearly ready cabbage! Adults, eggs and pupae don’t cause any damage.
Caterpillars can attack at any time in northern parts of the country, but in most areas they occur from spring until late autumn.
White butterfly are partial to a brassica diet – i.e. especially your cabbages and cauliflower. They will also attack many other plants, from radishes to some ornamentals, and even weeds.
Recognising the White Butterfly
Adult White Butterfly’s are moth-like insects that you will commonly see at this time of year, flitting around the garden, particularly near your vege patch. Their wings, of which there are four, are a dirty white/cream colour, over a dark greyish body. The female has two pairs of spots on the wings which are closest to the insects’ head, while the male only has one pair of spots. The rear set of wings has one pair of spots on both sexes.
Eggs are laid on the underside of the leaves and are laid singly. The initial creamy-white colour of the egg changes to orange just before it hatches and a tiny caterpillar emerges.
The caterpillar is dull green in colour, and is covered with minute hairs, which give it a velvety soft look. When the caterpillars are nearly mature, an orange stripe can sometimes be seen along its back.
The next stage of the White Butterfly Caterpillars life – the pupa – are very rarely seen by most home gardeners, making it a difficult stage to use for identification or control.
There are 2 main ways of controlling the White Butterfly – by hand, or by chemical control.
If you have the time, and prefer not to use chemicals, then the best way to eliminate the caterpillars is by searching each damaged plant carefully by hand. Turn over each leaf, closely inspecting each nook and cranny, then pull off each caterpillar and squash them. It would pay to cast your eye over neighbouring plants as well to ascertain any other places that the caterpillars may have crawled to.
Each female can lay up 400 eggs, so it’s well worth the time and effort. While you are searching for the caterpillars, keep an eye out for eggs, and remove these as well.
The best chemical control to use is Derris Dust. This is a powder that is ready for use, and you just dust the plants thoroughly with it. It is an organic insecticide and should be applied to the underside of the leaves in particular. You may need to apply a couple of dustings in order to continue killing each generation as they hatch.
Derris Dust has a withholding period of one day; this means that the plant is safe for human consumption after the withholding period has passed – in this case one day after application.