How to control cabbage worms
UPDATED: 5-14-2013 (See update at the bottom under the header "Overheating Row Covers")
If you see little white butterflies near your brassicas (broccoli, cauliflower, brussel sprouts, and cabbage), beware of the cabbage worm! They can also damage your lettuce, but I have found that my brassicas are more susceptible.
As soon as spring hits, you typically start seeing these butterflies, and not soon after you will see damage to the leaves of your brassicas. The damage is from the larvae of the cabbage butterfly, the cabbage worm. You will start to see good sized holes in the leaves. This is a pretty easy sign to tell that you have this common pest.
I have never had a year without my “friend” the cabbage worm. Last year, as soon as I noticed any damage to my brassicas, I started checking my plants for worms, and removed them to give to the chickens for an extra special treat. I also checked the base of the stem, and removed all the eggs, which tended to be greenish and slimy, or dark green little balls. The problem with handpicking is that it is time consuming, and it is very easy to overlook the worms because they are the exact same color as your broccoli. You run the risk of consuming a few worms here and there, not that it would be bad for you, but it's not all that appetizing.
The cabbage worms can be controlled with a spray. I will not be spraying, but if you are going proceed with this option, I would recommend that you use bacillus thuringiensis or BT for short. BT is a bacterium that is used as a biological pesticide. It is safe to spray right up to the harvest date. The good thing about BT is that you spray the plant, and it only harms those insects that are eating your plant. The bad thing about using BT, is that you have to spray often to get good control, and you are spending time and money to do it.
Another option, is using a floating row cover. The floating row cover will prevent those little white butterflies from ever laying the eggs on my brassicas to begin with. It is important that there are no eggs or larvae present at the time of covering your plants, as this could exacerbate the problem as you have just locked out any potential helpful predator.
I would only use a row cover for plants that do not need to be pollinated, as you do not want to lock out the pollinators you need for good fruit set. Brassicas make good candidates for row covers as you do not need pollinators for fruit set, and typically you only harvest once, or maybe just a handful of times.
There are certain row covers that are for frost protection, and insect protection, and there are others only for insect protection. If the row cover says it is for frost protection and insect protection, it will in fact perform those functions, but in the summertime it will get way too hot under the row cover. You will need to open up and ventilate the covers on warm days. I don't think this is very practical at all. I purchased a row cover from Gardens Alive that is a lightweight insect protection cover only. Gardens Alive said this cover will not overheat the plants, but it will also not stop frost.
I installed my row cover yesterday and it took a lot longer than I thought it would, but I think that is because it was windy, and I had never installed one before. In the future it will go much quicker I think. I put some short stakes in between my broccoli plants because I did not want the row cover on top of the plants. I know it is made to rest on the plants, but the plants are young, and I didn't feel like it was good for them. I staked one side down with small sod staples, and I folded up the other side and I held this side down with some logs and branches. My thinking is that as the plants grow taller, I can unfold the one side easily and let more slack out for the plants to have space to grow.
Overheating Row Covers
So I installed my row covers over my broccoli, and after a week of temperatures in the low 70’s with rain, I checked on my plants. I was horrified to find that ALL of my broccoli plants were burned from overheating, while my uncovered broccoli plants were looking great. The Gardens Alive marketing was completely untrue in regard to this row cover. After pulling the cover off, my plants are starting to recover. I called Gardens Alive, and they were very good about giving me a refund for the row cover.
I’ve finally come to the conclusion that I will grow my broccoli slightly different going forward to avoid these issues. My brassicas that I started in my greenhouse in December, ended up being really nice this spring. I had a nice harvest of broccoli and kale that I have been harvesting for the past 2 months. I am going to stop trying to grow broccoli to harvest in the summer. I am going to plant my broccoli in the greenhouse in December or January, and transplant them out in mid to late March. In this scenario, I should be able to harvest everything before June 1st. This would help me to avoid the cabbage worms all together. The key to this is to use large pots for your transplants, as they will be fairly large when you move them out. I am planning to use 1 gallon pots for this. I will still of course be companion planting my broccoli with onions to help mask the brassica smell from the cabbage worms.