You planted out these beautiful broccoli plants, and seemingly almost overnight one day the plants are almost completely gone. The leaves have been stripped back to just the stems. What happened? What do you do? Your plants were likely eaten by cabbage worms.
Now saying cabbage worms is a bit inaccurate. These caterpillars feed on any member of the Brassica family. Though they prefer broccoli, cabbage, cauliflower, and brussel sprouts, they will also feed on mustards, turnips, radishes, kale and other greens you might have in their garden. They might even be caught munching on your tomatoes if they get desperate.
But where did they come from?
Since cabbage worms will feed on any member of the Brassica family, they came from around. They aren’t picky eaters. There are plenty of wild weeds around almost any neighborhood they can live on.
As you probably know, caterpillars turn into moths and butterflies, so it is easy for them to relocate from the weeds to your garden. When you planted broccoli (one of their favorite foods) the cabbage worm moth in your neighborhood took notice and moved to your yard. You might have even noticed white or brown moths fluttering around near your broccoli plants prior to having a problem. They were busy laying eggs all over your plants.
And then….
Your plants are full of holes. Immediately. Well, actually not quite immediately, but it doesn’t take long. Shortly after the moths lay eggs eggs a zillion tiny little caterpillars emerged and started munching on your plants. So many of them can hatch at the same time, and they eat so fast that they can seemingly devastate a broccoli plant almost overnight.
Now there are two different types of cabbage worms commonly seen. A slightly fuzzy green cabbage worm and one with yellow stripes called the cross striped cabbage worm. In our experience, the cross striped one appears in greater numbers and is more devastating. This is because the green cabbage worm eggs are laid individually, and the cross striped cabbage worm eggs are laid in clusters of up to 25 eggs.
Cross Striped Cabbage Worms
Cross striped cabbage worms also tend to leave unpleasant webs in your heads of broccoli where they have been feeding. The good news about these worms on a fall crop is that they seem to disappear quite quickly as the weather cools. They however can be a huge ongoing problem on spring crops. They start out very small and like to feed on the undersides of leaves, so they can do a lot of damage before they becomes highly visible.
Green Cabbage Worms
The green cabbage worm is less of a menace in our opinion. Because eggs of this species are laid individually you will typically only have 1 worm on a leaf rather than dozens. They will also often appear on the tops of leaves, so are visible and easy to pick off and destroy. This makes them a bit easier to get ahead of. Unfortunately these caterpillars are much more cold tolerant. They can still be out there munching on our broccoli heads in December–long after there has been a hard freeze.
Not to worry though
In our opinion, they are one of the easiest garden pests to control, and even if your plants have been severely eaten, they can recover and go on to produce a nice crop once you have controlled the worms. Our 2023 crop is proof of that.
That fall we were in the midst of a drought, and it was a very hot fall. Both of these things contributed to slow growing, stressed plants. Coupling that with the continued high pest pressure that we had been struggling with all summer was a bad combination. By the end of September, this resulted in our Brassica crop looking like the first picture below. These poor plants were covered in both types of cabbage worms and Harlequin bugs.
These pictures are from September 25th and November 4th. Despite being eaten down to almost nothing, these plants recovered and produced a beautiful crop in just over a month’s time.
So how do you control cabbage worms?
The best way to control cabbage worms is by growing your Brassicas underneath insect barrier. This is particularly true if you live in an area that has a lot of cabbage worms. Unlike other crops, like tomatoes and zucchini, your Brassicas do not require pollination for you to get a crop. This means that you can cover the plants with insect barrier at planting and leave it over them until harvest.
This is what we typically do in our garden, and we have no cabbage worm problems. If the moths cannot lay eggs on your plants, you will not have problems with worms eating them. In our experience this is much easier and more effective for home gardeners than spraying your plants.
In the fall of 2023 we did not put insect barrier over our Brassica crop because there had been so many Harlequin bugs all over our garden (another Brassica pest we will cover in a future blog post). In hind site we probably should have covered them anyways as they were absolutely demolished by cabbage worms.
What is insect barrier?
Insect barrier is a very lightweight fabric that allows light and water to go through it. If you take care of it, you can likely use it for several seasons. We like to bend metal wire hoops over our plants to suspend the insect barrier above them. We then anchor the sides down with soil and bricks. You don’t want the insect barrier to be touching the plants. This can sometimes allow the moths to lay eggs on your plants through the fabric. After we put the insect barrier on at planting, we only remove it to weed or to harvest the crop. Watering can be done through the cloth.
If you decide to use insect barrier it is imperative that you put it on immediately after planting. It is also important that you remove it only briefly to weed or harvest. Cabbage moths don’t take long to lay eggs, so if you leave the insect barrier off all afternoon or overnight you might end up with a crop of worms under your insect barrier.
Also, equally as important is to thoroughly check over purchased plants. If you buy plants that have been kept outdoors prior to you purchasing them, they may already have cabbage worm eggs on them. Check the plants over thoroughly before you plant them–particularly on the undersides of leaves. You should also monitor them after planting to make sure no worms hatch out in the first week after planting them under the insect barrier.
If you live in an area with low cabbage worm pressure.
You can also stay ahead of cabbage worms by taking a few minutes every day to pick off and squish any worms or eggs you see. Since we don’t spray our vegetable garden with any chemicals organic or otherwise, this is the route we took in 2023 to salvage our devastated crop. Picking worms daily coupled with some rain and cooler weather had our plants growing new healthy leaves in a couple weeks time. This is much more time consuming than covering plants with insect barrier, but it can work if you are diligent about doing it. This method works better in more northern climates where the cross striped cabbage moth isn’t prevalent.
Our least preferred method
And of course there is also the option of spraying your plants. This can leave you with less than desirable results. Most common insecticides will result in inadequate control of cabbage worms. Do not spray your plants with these as they can be harmful to the parasitic wasps that are the natural enemies of cabbage worms.
You can spray the plants with BT (Bacillus thuringiensis) early in the season to control the cabbage worms. Most insecticides that do kill cabbage worms will only be effective if the worms are still very small. If you have large worms on your plants spraying is not effective and you will need pick the worms off manually. This is why we suggest simply growing the plants under insect barrier. Then there is no concern about correctly timing pesticide applications or pesticide failure.
We hope this guide helps you to grow a beautiful worm free broccoli crop this spring!
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