Every year at around this time, I am reminded that growing any of the brassicas (kale, cabbage, cauliflower, sprouts etc) is basically a war between you and all the creatures that want to eat the plants. I know that doesn’t sound very calming or nature-loving, but there you go.
Even though kale is one of the easiest of the brassicas to grow, it’s still vulnerable to caterpillars, particularly the tender cavolo nero or Italian kale that I love above all the other kales. Sometimes I think that “brassica” must be Latin for “oh my god what’s eating my fecking kale?!”
The primary enemy is the cabbage white butterfly, a white butterfly which dances prettily around the veg patch from spring until autumn. It lays eggs on the leaves of the plants and those eggs turn into larvae (caterpillars) that feast on the leaves. An infestation of caterpillars can completely strip a brassica plant of leaves if given the chance.
Fighting back
As an organic grower, there are a limited but thankfully effective array of weapons at your disposal to deal with the cabbage white. A physical cover to prevent the butterfly from landing on the leaves is the best of all. I use a net called bionet, which I drape over the plants and pin down with bricks or stones at the edges. But be careful – it needs to be really well secured, for the cabbage white is a crafty opponent and has literally nothing else to be doing with its time other than trying to find gaps so it can flutter in to lay eggs on the leaves.
The second weapon is to inspect the underside of the leaves for the eggs (little clusters of skittle-shaped yellow eggs) or later in the season to pick off (or wash off with a hose) the caterpillars themselves. The caterpillars of the small cabbage white butterfly are green, while those of the large white are yellow and black. This clearing off of caterpillars can be an increasingly futile effort if they’ve got really established. I find I start the season with great intentions to keep checking the leaves but I become less careful later (ironically, since this is when vigilance is most needed).
If you want to turn the war really nasty (but still keeping away from chemical interventions) there’s a bio-insecticide approach you could take – a bacteria called Bacillus thuringiensis – which comes in powder form and is added to a watering can (see above). A garlic spray can also be somewhat effective and, let’s be honest, is a little less brutal.
Equally, planting lots of flowers around your veg patch will provide a heady source of nectar for beneficial insects (such as wasps) and insect-eating birds.
For further information, visit www.GIY.ie
Bacillus thuringiensis (Bt) is a naturally occurring bacteria, common in soils, that causes disease and starvation in certain insects, notably caterpillars. Discovered in the 1900s, it has been available as a product for use in organic growing since the 1960s and multiple tests have shown it to be safe for use on crops.
It is a good example of a natural, targeted pesticide that is lethal to a specific range of insects, but not to other beneficial insects, animals, birds or humans.
Bt works because of its active ingredient – a crystal protein which messes with the digestive system of the insects and starves them to death. It is the genes of this active ingredient that have been genetically modified into some crops such as corn, causing some controversy somewhat unfairly; for Bt itself is an organic control.
Bt generally comes in powder form and is added to a watering can or sprayer to be sprayed on to the leaves of brassica plants. The timing of the spraying is crucial – Bt is a stomach poison for insects, so the caterpillars have to actually eat it for it to work.
The time to spray is after the eggs have hatched in to caterpillars but before they start to pupate (turn into a cocoon). Make sure to spray on the underside of leaves too. Spray the plants early in the morning and evening, since the product is susceptible to sunlight. Bt is generally safe, but you should still follow all the safety directions on the label.