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Bacillus thuringiensis: For small caterpillar pests use Bt

Updated on August 1, 2014
Swallow Tail on zinnia
Swallow Tail on zinnia | Source

Cabbage worms are one of the few major pest problems I worry about in my vegetable garden. I love my kale, broccoli, collard, cauliflower, Brussels sprouts, Kohlrabi and all the other cruciferous vegetables. It’s fun to try new ones too like sharaku. My favorite types of vegetables have a great deal of value in my diet. Most of these are high in zinc and iron and several other important minerals. They are excellent fiber sources. They grow from the time the ground is no longer frozen until and sometimes past the time that the ground freezes. For the most part they are easy to grow too. There are some problems with these vegetables a future blog will explore.

No, the largest part of my concern is with cabbage worms. They are the laval stage of the cabbage butterfly. This is that common smallish white butterfly with a small dark grey or black spot on the smaller bottom wing on each side. The butterflies are everywhere in the summer. I enjoy seeing them. I don’t however enjoy finding little green worms in my evening salad. I hate trying to clean these vegetables to make sure they have all been found. I hate the fact that in just an evening they can completely eat their way through my entire cruciferous vegetable collection. Aaaaaaarrrgh

This is where Bacillus thuringiensis comes in. You will usually see this on labels as Bt. This bacterium is naturally occurring and can be purchased in concentrated formulas. This bacterium harms the cabbage worm by compromising the cell lining of the digestive tract of several types of invertebrates. That’s the fancy way of saying that the particular sharp crystals produced by this bacterium during its reproduction cycle are broken down by the digestive juices. Part of these broke down crystals are absorbed by the cells lining the digestive tract. This material causes the cells to cease absorbing nutrients and then passing these nutrients on to the rest of the organism. The inactive cells then become pores. The worm literally starves to death. What is really interesting is that this bacterium occurs naturally and is commonly found in caterpillars intestinal tracts. The difference is in the concentration level. By spraying your crops you greatly increase the worm’s exposure.

Bt has been used for many years to treat mosquitoes as well. This is sprinkled in standing water. Sometimes these are compressed into discs so that they can be tossed here and there in larger bodies of water. Bt works the same way on mosquito larva as it does with cabbage worms and other caterpillars.

Cabbage butterfly on corn leaf
Cabbage butterfly on corn leaf | Source

There are only slight problems when a person or other animal with a backbone eats this bacterium. For a person, intestinal distress can happen. One would have to drink quite a bit of concentrate to do any real damage. You will definitely want to keep this out of the reach of inquisitive young children. That makes it fairly safe for us to use. Moderate and recommended diluted amounts are very safe.

The ideal environment for Bt is in shaded cool moist areas including a bit of the top of the soil and of course water. Hot, dry sunny locations are harmful to its existence. When treating your vegetables you should be sure that enough moistens the top of the soil. It helps that some of this is on the underneath side of the leaves so that there is some extra protection from direct sun light. I mix my concentration to a medium recommended level. I generally spray late afternoon or early evening to maximize the potential of my efforts. I find that it is most economical to use a compression sprayer. You just want the spray to run off and then move on to the next plant. A drench would be an over kill method of delivery. A deep soak could also be harmful to soil inhabitants not generally known to interact with this bacterium.

I like two different products. Each has a different concentration level which will affect the dosage. I have included a link to both from at the end of this blog. I find that as long as the infestation is not too severe I can usually treat once a week. When the worms are particularly heavy then I try and treat twice a week.

It is important to remember that cabbage worms are an integral part of the world. Without them there would be a hole in the biodiversity necessary for a healthy planet. I find that I can protect my cabbage and still promote a sustainable garden by planting a sacrifice row. This row is an untreated row that has some of each of my different cabbage types represented. This untreated row is planted just for the cabbage butterfly to have a place to raise its young. There are other predators that rely on the cabbage worm for their own life cycle. This area then is for them as well. I have more than enough for myself anyway. Saving a bit for others is important for us all.


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