Archive Tip of the Week
TIP OF THE MONTH
October 2009
About Bacillus thuringiensis
(Bt to You and Me)
Bacillus thuringiensis is a naturally occurring, soil-dwelling bacterium that’s been formulated for use as an insecticide. There are several strains available to gardeners, the most popular of which is Bacillus thuringiensis kurstaki, which targets one pest and one pest only – the larval form of moths and butterflies that prey on plants (better known as caterpillars.)
Bt acts by producing protein crystals that react with the cells lining the guts of caterpillars. The proteins basically paralyze the digestive system, causing the caterpillars to stop feeding. As a result, the poor critters die of starvation within a few days.
Bt is considered safe, and can be used up to day of harvest. Available as either a power or spray, thorough coverage is important because the caterpillars must ingest the bacteria to become infected. It also is susceptible to degradation by sunlight, and may not remain effective for more than a few days.
Okay, so much for the technical discussion of Bt. Now let me share with you my take on its use in the garden.
First, let me say that I’ve used Bt for nearly three decades, and with great results. I know of no better product for the control of cabbage loopers, imported cabbage worms, tomato hornworms (although handpicking is arguably just as effective), corn borers, leafrollers, tent caterpillars, and fall webworms.
But – and this is a very big but – Bt is indiscriminate, meaning that it will kill all species of caterpillars. And there are certain caterpillars I don’t want to destroy, no matter how much damage they do to my plants.
Case in point – the parsley worm. This cute little thing is the larval form of the black swallowtail butterfly. In its adult form, it’s as beautiful as the caterpillar is destructive, especially if you grow parsley. The two-inch long, colorful caterpillars have a voracious appetite, and can strip an entire parsley plant down to a few stubs in no time.
But is my need for parsley, however great, more important than the life of a single black swallowtail butterfly? And the answer is no. I grow up to 20 parsley plants a year, and they all succumb to damage, but they often generate new growth once the caterpillars pupate. Besides, I can always buy parsley at the market. And the caterpillars have entertainment value. Brush their backs and they send up two menacing orange horns, as if to say “Don’t mess with me!”
Of course, you can always grow trap crops of dill or even Queen Anne’s lace (both parsley relatives) and hope that the caterpillars will favor them over your parsley. You can even pick them off your parsley and move them over to the trap crops. But I prefer to let them eat all the parsley they want. That way I know my garden will be full of black swallowtails each spring, which is a sight more beautiful than all the parsley in the world.
(By the way, there are others strains of Bt on the market, including Bt israelensis, which is used to control mosquito, black fly, and fungus gnat larvae; and Bt San diego, which is used to control Colorado potato beetles and elm leaf beetles.)
