Certain cabbage caterpillars are so voracious, they can wipe out a cabbage crop almost overnight, too fast even for some organic remedies to help. What to do? Nudge nature into action and let her take the anxiety out of raising brilliant brassicas.

Garden allies

Earlier this week as I was clearing my winter cabbages, I found and saved all the pupae of the native white butterfly parasite wasp, Cotesia glomerata. This little parasitoid keeps a lid on destructive cabbage caterpillars in my garden.

Parasitoids are mostly wasps, they spend part of their life cycle inside a host insect of a different species. With the cabbage white butterfly (Pieris rapae), the cabbage white butterfly parasite wasp (Cotesia glomerata) lays eggs in them and they never survive to reproduce. This parasitoid homes in on the scent a caterpillar feeding on cabbage emits and with surgical precision inject eggs into each host.

As the larvae develop they take care not to feed on vital organs, enabling the caterpillar to continue feeding briefly. To see the developing larvae inside a caterpillar you need to shine a light through its body. You’ll easily recognise the outline of the tiny larvae feeding inside.

When the larvae are ready to pupate, they guide their host to stop feeding and to seek somewhere sheltered, high and dry, where they burst through the caterpillar to form an irregular, silken cluster of pupae.

Cabbage pests

Apart from the ubiquitous cabbage white butterfly, the diamondback moth (Plutella xylostella) is a smaller and more destructive pest of cabbage. Adult moths are less frequently noticed. Their caterpillars feed on their own, away from other caterpillars. Telltale signs of feeding is the way they leave the waxy outer coating of the leaf, consuming the juicy tissue in between, creating squiggly pathways as they tunnel, feeding inside the leaves. The damage is sometimes mistaken for leaf-miner attack.

Then there is the large cabbage moth, aka the cabbage cluster caterpillar (Crocidolomia pavonana). This is worse still, and their caterpillars feed as a team. Farmers dread this pest because the caterpillars are voracious feeders and capable of wrecking a crop overnight.

Harvest your allies before composting cabbage waste

Always check cabbage leaves and spent cabbage plants before they get put in the compost heap. That way you can set aside the wasp pupae. I cut off each cluster of pupae, placed them in an open container in a shaded and well ventilated spot – on the sideboard – where they can complete their metamorphosis into adult wasps.

In this video, tiny adult wasps have hatched at the daybreak of a warm spring morning. They briefly linger together – some are stretching out their unfurling wings, allowing the membranes to fully expand and strengthen. They meander a bit, looking a bit like they could do with a coffee, before grooming themselves and zooming off on their maiden flight to reproduce.

During this calm period before they take off, I carried their container outside and placed it amongst the spring cabbages. I’m growing ‘Couve Tronchuda’, a Portuguese non-hearting cabbage that adores a subtropical Brisbane spring. Here’s hoping a few fertile females find some pest caterpillars amongst my heritage cabbage. Food for their offspring, and hopefully they will continue their valuable work.

Conventional control isn’t always ideal

The conventional way of treating cabbage caterpillar pests is to pick them off by hand or to spray BT (Bacillus thuringiensis, an organic, bacterial bio-control) but in my biodiverse garden even BT is rarely required, thanks to the beneficial wasps that live here.

Yes, you can net crops with pest excluding mesh. It works, however, it prevents the dynamic equilibrium between the pest and the crop-saving parasitoid that has established itself in my food garden. And those parasitoid wasps can jump the fence and help your neighbours’ cabbages as well.

There will always be plenty of caterpillars to pester cabbage crops, there’s no risk of them becoming extinct, that is why they are known as pests.

Cotesia glomerata is one of fifty wasp species recorded at my food garden. There are more, but small wasps like this one are hard to see. The interaction between pests and parasitoids is an interesting lesson where you can learn two things:

1) How and why biodiversity preserves life, and

2) How to apply the least toxic solution in horticulture.

You can even go one step further by growing an edible trap crop of landcress. That was what my grandmother used to do, but that is a different and complimentary gardening story.

So fly, my pretties!

Jerry Coleby-Williams
Director, Seed Savers Network
Patron, National Toxics Network
11th September 2021