In early fall, garden forums are always full of photos from new gardeners showing their fall veggie transplants full of holes and inquiring what is happening. The plants in question are likely in the brassicacea family (which encompasses many fall garden crops such as broccoli, cauliflower, kale, mustard greens, and collard greens to name just a few), and the clear culprit is cabbage worms. There are various types of cabbage worms, but all result in a garden full of hole-ridden leaves if they're left unchecked. And if you want to grow brassica family plants, it's important to know how to deal with these dudes.
When encountering any pest in the garden and deciding how to handle it, the first course of action should always be to observe.
Here are some organic solutions for dealing with cabbage worms:
Protect your plants. Cabbage worms are the larvae of moths, so covering your plants with insect netting can prevent the moths from landing on your plants and laying their eggs. This is our preferred method because it's proactive rather than reactive.
Check your leaves regularly. Even with row covers, though, the moths can find their way in sometimes. And it only takes a few egg sacs to end up with a lot of cabbage worms. That's why it's always a good idea to make a routine of checking the underside of your plant leaves and wiping off the inevitable yellow egg sacs you'll find. It's a good way to prevent the cabbage worms from hatching on your plants in the first place.
3. Handpick the caterpillars and pull them off. Once the eggs hatch and you notice that your broccoli and kale plants are getting chomped, you'll want to find the caterpillars and pull them off your plants.
4. Apply BT. This isn't something we personally do very much, but we understand that some preventative measures can be impractical for large gardens and people want a product-based solution that is organic. BT, which stands for Bacillus thuringiensis, is a naturally occurring soil bacterium that kills cabbage worms. There are various formulations and applications that come in powders or sprays. You'll want to coat the plant leaves, especially the undersides, at the first sign of an infestation and repeat the application every 7-10 days or so. While this biological insecticide doesn't harm birds, humans, or most other animals, we don't like to use it because it doesn't just kill cabbage worms. It is toxic to many other types of butterflies, moths, and pollinators. To minimize unintended harm, avoid broad-spectrum formulations, always read the product labels, and apply BT in the late afternoon or evening when fewer pollinators are out and about.
5. Embrace the seasons. It's also worth mentioning that brassica-family plants are meant to be grown in cool weather. You can avoid a lot of unnecessary stress by planting your plants in the appropriate season and saying goodbye to them once that season has passed. Once the temperature heats up in Central Texas, we are done with kale and cabbages and the increased pest pressure can be a signal that it's time to move on to warm weather crops. It's an uphill battle trying to plants alive when their season has passed.
Also, don't forget that healthy plants grown in diverse ecosystems are much more resilient and hold up better to pest pressure and disease overall. Aim for nutrient-rich, well-draining soil and interplant an array of herbs and flowers that attract birds and beneficial insects.