George WeigelThis is the kind of chewing damage cabbageworm larvae (adults are white moths) can do on broccoli.
Q: My neighbor and I are both having an infestation of white moths, about an inch wide. They’re eating up everything, it seems. While I try to keep a pesticide-free garden, I’m having to resort to a regular insecticide this season. I’ve had damage to basil, coneflowers and windflowers. My yellow loosestrife hardly bloomed this year. What, if anything can I do to prevent this next season?
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A: That sounds like bug damage, but the bug you see isn’t always the culprit. Those white moths are likely cabbageworm adults that turn into small green caterpillars that feed on a variety of plant vegetation.
If that’s the cause of the holes, you should be able to see the caterpillars lodged on the stems, leaves and buds. The best “organic” control is to simply squish them. Birds also do a decent job of cleaning them up eventually.
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If that’s the problem and it’s beyond your tolerance level, caterpillars are susceptible to a fairly soft pesticide called Bacillus thuringiensis (Bt). It’s a bacterium that makes caterpillars sick enough to stop eating while leading to virtually no collateral damage to non-targets.
A Bt spray every 5 to 7 days while moths are active should be enough to stop this damage.
But sometimes other less obvious bugs are causing the real damage… for example, beetles that feed at night. (Beetles and caterpillars are the two main bugs that cause chewing damage.) Sprays of Bt wouldn’t stop that damage, which usually causes people to think that for some mysterious reason, the Bt isn’t killing their caterpillars.
You’d need a broader insecticide, such as the bacterium Spinosad or the long-used chemical Sevin, to stop beetle damage.
This all starts to get complicated pretty fast and usually sorts people into two main classes. One group goes for the most deadly product they can find, figuring that something that kills everything makes things simply again in a hurry. The other group is concerned about tradeoffs with killing birds and polluting water and decides it’s better to just live with holy plants.
I’m in the latter group. Besides the expense and difficulty of using pesticides accurately, I don’t want to go down the path of killing off potentially beneficial wildlife that would help bring the problem under control eventually. Once we start trying to kill off some things, too often it sets up a cycle where we then have to continue spraying to prevent everything from spiraling out of control again.
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What I see happen often enough in stay-out-of-the-way approaches is that bug damage waxes and wanes as the balance between pests and predators changes from year to year.
In cases where unacceptable damage happens year after year, I try a different plant. Like that twist on the old cigarette commercial, I’d rather switch than fight…
If you’re a fighter, start with Bt. And if that doesn’t work, try Spinosad.
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