White Cabbage Moths

It’s spring. The white cabbage moths have arrived, en masse. But they don’t seem to be doing much damage. So here are some of my speculations about why.

First thing is that most of the really susceptible crops are out now, in our bellies or the fridge, replaced with button squash and beans, corn and eggplants and capsicums – things that cabbage moths have no interest in. I don’t try to grow cabbages (European or Asian), or broccoli or broccolini, or cauliflowers or brussels sprouts, or anything much from that family (Brassicaceae) over spring or summer. The first seeds of them are planted in autumn and protected in a seedling raising area till the cabbage moths disappear for the winter. The last of them are harvested in spring aiming to have them all out and eaten, and their place in the garden taken by summer crops, before the cabbage moths get going in the spring. Timing saves a lot of work. And I wouldn’t have room for them anyway – the summer crops are a much better use of that soil, water, sun and attention.

I do have kale still bearing though, and some khol rabi, and about five red cabbages. So why aren’t they getting got? The red cabbages, and dinosaur kale, are much more resistant than green cabbages or broccoli or caulis. There are some studies that support the idea that the flavonoid that makes the red/purple/blue-ish colouring in the leaves (anthocyanin) is unpalatable to caterpillars (but good for people). It’s not 100% effective, but it does keep the numbers right down – just the odd one – and it makes the very green caterpillars easy to see. I check any brassicas as I go round the garden and pick off the odd caterpillar. I soak picked leaves in a sink full of cold water for a few minutes before cooking and they fall off – it’s an easy way to make sure I haven’t missed any, because fried caterpillars or caterpillar poo are foul and enough to ruin a dish.

The kale and cabbages and khol rabi are also all widely separated, one plant per bed, and disguised by their neighbours. The kale in the video, for example, is amongst a bed of broad beans. The white flowers with black markings on the broad beans might confuse the cabbage moths a bit – I do think hiding them amongst peas or broad beans or flowering rocket works better than amongst beetroots or carrots, and I know some people swear by the effectiveness of fake cabbage moths cut out of things like yoghurt containers as a deterrent. But more importantly I think, the moths have to identify a target plant’s scent in a pool of other smells . It means the cabbage moths spend a lot of time flying around searching, and while they are on the wing, they are vulnerable to predators.

Which brings me to the next point – lots of attractants for insectivorous birds – bird baths and nectar sources and perching spots – and un-attractants for cats that might deter the birds. And lots of attractants for predatory and parasitic insects like wasps. Most insect eaters are also nectar eaters – proteins from the insects and carbohydrates from the nectar. So having a nice range of flowering vegetables around serves multiple purposes. Even better if they have white flowers.

I also have a lot of nasturtiums growing down the shady south side of the house. Nasturtiums are a brassica, and probably act as somewhat of a decoy. I’m not too keen on annuals as decoy crops – I suspect they just breed up the target pest and aren’t there long enough to breed up the predators. Most pest species are faster breeders than their predators and there is a lag while the predator population builds up, and it’s in that lag that your garden is savaged. But perennials like nasturtiums keep a stable population of predators fed just well enough to stay ahead of any outbreak.

I don’t bother with netting – it would keep predators and pollinators out, the brassicas are not all together, and it’s too much work and expense. I’ve used Bacillus Thuringiensis, a bacteria that acts a bit like food poisoning for caterpillars but is harmless for humans, in the past, but again it’s more work and expense than I like, and in this rainy el Nino year it would have to be applied very often. But I’ve probably missed some other good strategies. What are yours?