Every time I plant brassicas, I do so with a mix of excitement and dread – while I am looking forward to harvesting the heads and leaves, I know that voracious and destructive green caterpillars are also going to enjoy this crop.
Be prepared with some organic controls for Cabbage White Butterfly (Pieris rapae), so that when they inevitably arrive you are ready to take action!
If you are out in the garden most days, it can be simple enough to just pick off the grubs as you see them. Keep in mind that they can start being destructive even while very small. But the sheer numbers of caterpillars appearing may lead you to use both preventative methods and sprays to control them.
However, if the sheer numbers of caterpillars appearing becomes overwhelming – then consider these organic methods to control them.
First Things First – Identification
It is important to know what you are dealing with. The adult Cabbage White Butterfly is a small white butterfly with black spots on the wings. They hover and dip over the plants in the garden as they fly around, which means they are either seeking an egg laying position (females), or looking for a mate (males).
It is the caterpillars emerging from the eggs laid on the underside of the leaves that cause the damage. They strongly prefer the brassica or cruciferous vegetables – cabbage, kale, broccoli, cauliflower, etc. – so must be something about the mustard oils that they need.
The caterpillars are smooth and a dark greyish green colour, but can also have variations of brown and grey. When older they can have a distinct yellow line down their back.
They can be hard to find, but if you have holes in the leaves and tell tale droppings – then look under the leaves and in the centre of the plant.
The goal here is to stop the butterflies from laying their eggs in the first place, either by discouraging them from landing – or by physically preventing them from accessing the leaves.
Mess With Their Tiny Little Minds – Decoys
The cabbage white butterfly is reputed to be territorial, meaning that it will seek out an area to lay eggs away from their comrades, this makes sense in terms of them wanting their offspring to have a food source with little competition. I keep hearing about using decoys to trick the butterflies into thinking that your patch is already occupied, techniques such as
- tying strips of plastic to string, so it flutters
- leaving largish segments of eggshells lying about
- cutting white plastic into butterfly shapes, and placing on bamboo stakes
Does This Work?
I am not entirely sure, and here is why. I have tried the string lines of white plastic bows method – and I can’t say that the cabbage white butterflies seemed to avoid the garden.
Overall I am not convinced about the decoy method. I’ve seen plenty of Cabbage Whites fluttering about the Broad Bean patch, and Broad Bean flowers look a lot like Cabbage Whites. They have white flowers with black spots, and I would think that if the butterflies were that concerned about look-alikes they would clear out!
That is no scientific argument by the way, so if you think decoys are working in your garden please let me know.
Don’t Let Them In – Physical Barriers
By using a mesh, the butterflies can be prevented from landing on the leaves and laying the eggs. This can be achieved either by using a netting fabric, or metal enclosures.
The challenge is to cover each plant, if you know you need this solution at the start, then it is fairly simple to plant a row that is designed for a tunnel covering. There are also some excellent frame and netting systems available for purchase.
If you have your brassica plants dotted around the patch then each needs to be covered individually. I have started using paper bins for small seedlings, just remember to remove them often to check on the plant’s health.
Curtain netting is inexpensive, and the nylon fabrics hold up well in wet weather. Also, the discount/variety shops often have products such as pop-up laundry hampers and food covers that can make effective screens – just look for a fine mesh.
There are two organically acceptable sprays available, the Bacillus thuringiensis (also known as bt) type sold under the brand Dipel, and a spinetoram type sold under the brand Success.
I prefer Success as it is faster acting, and holds up well when it rains.
Remember to spray under the leaves and over the stems – the spray is only effective when the leaves are covered.
** Always read the label before using, follow the mixing instructions, discard any unused sprays and clean your spraying equipment thoroughly, and use the recommended personal protective equipment **
If using sprays, please spray either early in the morning or late in the afternoon when bees are not feeding on flowering plants.
Natural Control Methods
The caterpillars can be parasitised by a wasp, with the young emerging soon after. If you see a caterpillar grub surrounded by tiny yellow cocoons wrapped in a webbing – this is a sign that the wasps have already emerged from the caterpillar and moved on to their next stage of development.
Gross But Effective – How Wasps Use Caterpillars
The female wasps lay their eggs into the caterpillars, which is still living, as they hatch and mature the wasp larvae bore out of the caterpillar and spin yellow silk cocoons around themselves. The host caterpillar attached to this cluster of yellow cocoons slowly dies.
Wasps emerge from these cocoons and the cycle continues.
If you are able to remove a few caterpillars by hand, and leave the rest to the wasps – then you can easily manage this pest without introducing sprays. You may not be able to do this in a single season, building a healthy organic garden with a population of beneficial insects takes time. It is important to have many varieties of flowering plants throughout the year to support insects like wasps.
Do you have a lot of green caterpillars in your vegetable garden? What are your best organic controls for Cabbage White Butterfly?