Pretty White Butterfly? Beware!

If you see this small white butterfly in your garden, it’s a call to action! The Cabbage White Butterfly; also known as the Imported Cabbageworm (Pieris rapae), is a pest in your vegetable garden. It’s a shame really, since they are so delightful to watch fluttering around.

You’ll start seeing the butterflies around May or June, and they will be around all Summer. It lays eggs on the underside of leaves that hatch in 4-8 days and the larvae (cabbage worm) matures in 2-3 weeks. In that 2-3 weeks the cabbage worm has a voracious appetite and can cause significant damage in a short time – I had a bed of lettuce wiped out in just a week by horde of worms when I wasn’t paying attention. (or swarm? Not sure what you call a lot of worms?!).

Lettuce eaten by a horde of worms. Notice their black pellet-like droppings, that is a tell-tale sign of the guilty culprit.

The worms start out tiny, barely the size of a grain of rice and grow very quickly. They are the same color as leaves and often difficult to see.

It is most associated with causing damage to broccoli, cauliflower, and other cabbage family plants. It also is a nuisance for any leafy green like kale, bok choy and lettuce.

An entire bed of bok choy destroyed by cabbage worms. A hallmark sign that this is caused by cabbage worms - they eat the delicate leafy portion and leave behind the stems.

As always, it’s best to be proactive to prevent damage in the garden. The same holds true for the cabbage worm. But sometimes life is busy and it’s hard to get in front of everything. I always lose a portion of my crop to their damage, and sometimes it’s a lot. Today’s maintenance in the garden I filled a bucket with trimmings of destroyed leaves. My final harvest was pretty small. So I’m writing this today not only as an alert for you, but as a reminder to myself!

The first instance that you see the Cabbage White Butterfly flying around, starting the next week, I recommend that you preemptively spray Bt (Bacillus Thuringiensis). And do this weekly throughout the summer – be sure to follow the instructions on the bottle. Shake it well before and during application and coat both the top and bottom of leaves as best you can. BT compounds are easily degraded by sunshine, spray the garden in the early morning or late evening.

Bt spray is available at nearly all local nurseries and hardware stores in the garden supply area. It’s a good idea to have a bottle on hand in your supplies at the start of Summer so there is no delay when it needs to be applied.

Alternatively, you can use a row cover that will keep the butterflies out. The row cover will need to be in place all summer – be sure it’s secured on all sides so the butterfly can’t get in.  And only apply to plants/beds of crops that don’t require pollination. For example, if you have lettuce planted around or under a trellis of cucumbers, you don’t want to also cover the cucumbers that would prevent bees from reaching the blossoms. If you are able to use row covers it has the additional benefit of providing diffused light and shade on the hottest days.

Fabric row cover secured on all sides keeps out all flying insects.

If you happen to be like me, I only start to realize there is a problem after there is significant damage. In this case, trim off all damaged leaves, harvesting any portion that is edible. If you find a worm in the head of cauliflower or broccoli, or on any greens leaves, just pluck it off. The vegetables are still edible, though they may not be as attractive as you’d like.

Cabbage worm damage on Lacinato kale

Once the plant is trimmed, spray with Bt. It also helps to give the plants a dose of liquid fertilizer to help it recover growth after the heavy trimming. (Fish emulsion works great for greens).

Bt is a biological organic method. It’s a naturally occurring soil-borne bacteria that affects the worm’s digestive system, but is harmless to birds, earthworms, beneficial insects, such as bees and ladybugs, and humans. (You can apply up to and on the day of harvest.) It must be ingested by the worm when it eats the plant, it won’t kill the eggs or butterfly. Once ingested the worm immediately stops feeding and will die within 2-3 days.

With weekly applications you have a better chance of interrupting the life cycle before the worm horde takes over!