This worm is the larva of a small white moth known scientifically as Pieris rapae (or commonly as the cabbage white moth). The female moth lays eggs on the undersides of leaves of specific host plants which include those in the family Brassicaceae (mustard family). Cabbage, Brussels spouts and cauliflower are very susceptible to damage from this moth's larvae in that the worms hungrily devour the plant's foliage leaving behind skeletonized leaves that can no longer serve any purpose to the plant. They also leave behind lots and lots of dark green poo. So gross. As if skeletonized leaves and diminished plant health aren't enough of a reason to rid your plants of these worms! Who wants to pick through layers of worm poo to harvest their veggies? And let me just tell ya, it will start to pile up. So here's what you do...
|If you look closely at the center of this photo you'll see a lovely dark green cabbageworm turd!|
Go out and buy yourself some "BT", which is Bacillus thuringiensis. It is a naturally occurring bacteria that kills larvae of some caterpillars including cabbageworms, tomato hornworms and tent caterpillars. The best part is that it is considered organic and is safe for humans and animals. I bought this small bottle of liquid BT from a local landscape supply store.
You mix a small amount of it with water in a spray bottle and douse down your plants, including the worms. The BT paralyzes the insect's digestive system which causes it to stop feeding and it eventually dies. The Colorado State University extension office has a great fact sheet on BT here.
BT can also be found as a powder that you dust onto the plants and worms. The store where I bought this bottle didn't have the powder form and I'm left wondering if the powder would have been easier to use. I ended up mixing up too much and it went to waste, as the directions say you must use the mixture within a week of adding water, otherwise the bacteria lose effectiveness. The powder most likely would have prevented waste.
I found that the BT worked great. I didn't see any sign of cabbageworms within a day or two after application, but they eventually came back. We noticed a lot of the white Pieris rapae moths flying around the property all summer, so I knew I would eventually have more larvae. That means you just have to keep spraying the BT every couple of weeks, which is no big deal. I just wish the mixture would hold its potency longer so I could use the same batch through a few applications. I'll have to keep an eye out for the powder form once this bottle runs low.
|Pieris rapae moth larvae|