Bacillus Thuringiensis

by Daisy on 11/13/2011

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I love cole crops.  Brassicas.

Of course I’m not alone.  Brassicas are considered the most important edible species out there.  Broccoli, turnips, rutabagas, mustard greens, kale, cauliflower, brussels sprouts, cabbage.  Yum.

And also of course, bugs love ’em, too.  Lepidoptera, unfortunately.  Pretty bugs.  Moths and butterflies lay eggs on the leaves of brassicas, and the larvae which hatch then nosh on said leaves.

Those pretty white moths flitting over the garden?  Cabbage moths.  Responsible for the waxy, yellow patches on the underside of brassica leaves, then the stripey green larvae munching holes on those same leaves.

I try scraping off the eggs, hand-picking the larvae, but they always get ahead of me.  I end up with this:

Brussels sprout leaf.

Enter Bt, or Bacillus thuringiensis.  It comes in a concentrate, which is mixed with water and sprayed on the plants.  It really works, too.  Here you can see a cabbage with lower, bug-eaten leaves, and newer, pristine leaves.

What happened was I let things get ahead of me early in the season, then finally got on the ball and started spraying.

And here’s that same brussels sprout plant, with the tatty old leaves below the nice, whole leaves above:

So, yes, it works.  But is there a cost?

As with everything, it seems, there is a downside.  On a big scale, whenever you start to get successful at killing a bug, the bug changes to adapt to the challenge.  Also, the other bugs, seeing an opening, start to take over.  Plus, some lepidoptera which aren’t targeted for eating your cabbages might be affected.  For example, if you planted parsley near your mustard greens and some overspray got on the parsley and a butterfly larvae ate it and died.  (The downside here is provided that, like me, you like butterflies more than parsley).

Overall, though, Bt is a pretty decent solution to the problem of growing brassicas where pests are a big problem.  Bt is considered safe for consumption by mammals, since it works by binding to a receptor which vertebrates (and all but a couple of invertebrates) do not have.  Without that receptor, no harm is done.

Every time I hear something like that I reserve a small measure of disbelief.  I’m a strong believer in the saying: If it seems too good to be true, it probably is.

So, with these caveats, it works in my garden.  I try to use it sparingly, directly on what I intend it for, and far from the parsley and milkweed that the butterflies love.

As I move more and more into permaculture, I hope one day I won’t need to use Bt or any other pest ridder.

Have you had any experience with Bt?  What do you do to protect your cole crops?

 



{ 8 comments… read them below or add one }

Jessica November 13, 2011 at 8:23 pm

There is another way. Find some thick white fabric interfacing (the non-woven polyester stuff). Draw your version of a cabbage moth on it, over and over again . Cut out your moths and attach one of them to a highly visible leaf on each of your brassicas (I pinned mine on, so the head of the pin looked a bit like the head of my moth). Most cabbage moths are territorial and won’t lay eggs on a plant if another one it already there. It kept mine larvae-free all summer. Only problem was that the chickens finally got to them and tore them up. Time to make new ones for the winter season! Maybe I’ll use tyvek this time.

Jennie November 14, 2011 at 9:24 am

That’s a great idea, Jessica! I’m definitely giving it a try next year!

Abigail November 14, 2011 at 9:46 am

Thanks for this information! I’ll be trying to grow cabbage and broccoli this coming year, and I know we have cabbage moths around here (my children love chasing them and calling them butterflies). They ate all my bok choy this past year. I’m just learning to garden and have so far succeeded in being pesticide free, but I was sad to not have any bok choy to saute. And thanks to Jessica for her idea! I’ve got plenty of interfacing to do that with!

Tomato Lady November 14, 2011 at 11:51 am

Jessica–I have never heard of this method! It sounds kind of fun! Sorry about your naughty chickens–maybe your “moths” were too lifelike. Thanks for the tip.

Anna November 14, 2011 at 12:38 pm

I just tack netting over the whole patch(es) of cole plants. Winged beast can’t land on the plants, winged beast can’t lay eggs on the plants. Simple and effective.
Disclaimer: My entire garden is about 16′ x 8′ so it doesn’t take much netting or work.

Cheryl November 14, 2011 at 1:05 pm

Another way to prevent cabbage moths from laying eggs on brassicas:

1. Go to your local Tractor Supply or other farm store and buy a roll of 9 gauge wire.
2. Borrow Hubby’s bolt cutters and cut pieces of wire long enough to make a hoop long enough to cover your fully grown brassicas plus enough to push into the soil about six inches on each side. Cut enough hoops to place one about every 4-5 feet of your brassica row.
3. Go to the fabric store and buy nylon net long enough to cover all the plants and hoops. If you watch for a sale you might be able to get it for as little as $.50/yd.
4. Lay the net over the hoops and bury the edges with a little soil so the wind doesn’t blow it away. You can also use some clothes pins to help secure the net.

You need the hoops to keep the net above the plants otherwise the moths could lay their eggs through the holes in the net.

We did this last summer and it worked really well. The rain and the sunshine both get through to the plants but the moths and other bugs can’t.

The hoops can be re-used with spun polyester row covers in the fall as a season extender for cool weather crops like lettuces, spinach, most other greens, carrots, and (of course) brassicas.

Sarah November 15, 2011 at 6:23 pm

I have never heard of Bt, but I am curious to know if it might also be beneficial in getting rid of squash bug larvae. I did not notice that they were laying eggs on the underside of the leaves of my spaghetti and butternut squash last year until it was far too late to save most of the squash. 🙁

Tomato Lady November 16, 2011 at 6:55 am

Sarah–You have my sympathy. I have almost given up on growing squash because of those pests. Unfortunately, the label of the Bt I use says it only controls “corn earworm, bollworm, armyworm, diamondback moth, green cloverworm, hornworms, loopers, melonworm, pickleworm, tomato fruitworm, tobacco budworm, salt marsh caterpillar, mimosa webworm, and imported cabbageworm.” So, no squash bugs.

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