When we think of bacteria, we always tend to conjure up the harmful varieties, such as flesh-eating bacteria (Streptococcus Group A), salmonella (Salmonella spp.) or E. coli (Escherichia coli). Few people think about beneficial bacteria and this, in spite of the fact that are actually many of them all around us.
The best-known beneficial bacteria in the gardening world is without any doubt Bt (for Bacillus thuringiensis), a bacteria very abundant in the wild. It is found just about everywhere, almost certainly in your own backyard. We know dozens of strains of this useful microbe, many of which are very specific to certain groups of insect hosts, hence of great interest to gardeners.
All work in much the same way: Bt remains dormant on the plant, in the soil or in water until it is ingested by its host insect. In the insect’s digestive tract, it starts to grow and release toxic crystals, making the insect sick. Soon (within three to four hours of ingestion in the case of caterpillars), the insect stops eating and eventually dies. Since this can take from a few hours to over a week, gardeners have to be patient: those who expect to see the pest die within minutes of the treatment will be sorely disappointed!
Note that Bt has no effect on humans, pets, plants, or wildlife… other than the specific categories of insects Mother Nature designed it to infest.
Several Bt strains are being offered as organic pesticides and probably even more strains will be in the future. In fact, Bt is presently the most widely used bio-insecticide in the world.
Knocking Out Caterpillars
Currently, the most commonly available strain is Btk (Bacillus thuringiensis kurstaki), effective against butterfly and moth larvae: in other words, caterpillars. But be careful! Like any pesticide, even organic ones, it must be used carefully, strictly on infested plants (such as on crucifers to prevent the cabbage butterfly, for example, or on garlic and onions to control leek moth), because it will also kill the caterpillars of any butterfly or moth, even those considered harmless or even endangered. Also, you have to make sure that the pest you use it on truly is a caterpillar (that is, an insect from the Lepidoptera, the butterfly and moth family), not something else. Btk will not be effective, for example, against sawfly larvae, although they look like small caterpillars, because sawflies are actually in the Hymenoptera or wasp family.
Readers living near the coniferous forests of Northern North America may well know about Btk as it is the insecticide most commonly sprayed on forests to control spruce budworm (Choristoneura fumiferana).
Mosquitoes Get Blasted Too
Bti (Bacillus thuringiensis israelensis) is a strain of Bt that is effective against mosquito and blackfly larvae. Many municipalities use it to make life more pleasant for their citizens. And with all the press coverage these days about the Zika virus, a mosquito-transmitted disease, Bti is currently much discussed in the media. However, while mosquitoes and blackflies may annoy people (and yes, sometimes even carry diseases that make them sick), they nonetheless play an essential role in the environment. Many animals depend on mosquitos for their survival and that includes many desirable bird species as well as bats. For that reason, the decision on whether or not to use or Btk, especially on a large scale, will probably always remain controversial.
Down With Potato Bugs
There is even a form of Bt that is used to control the Colorado potato beetle (Leptinotarsa decemlineata), also called potato bug: Btt (B. thuringiensis tenebrionis). This product has not proven as effective as the other strains and is not approved for domestic use in many countries. That’s because, firstly, results on potato beetles are sometimes disappointing, but also, it affects a wide range of beetles, some of which are beneficial. Of particular concern is the fact it is considered toxic (although minimally so) to honeybees.
Although Bt is considered organic (of course, since it is a living bacteria developed by Mother Nature), that doesn’t mean you should abuse it. As with any pesticide, you need to know that you are targeting the right insect and you should only apply it where the problem occurs, not throughout your garden. Admittedly, Btk is less toxic than many other biological insecticides (rotenone, pyrethrum, etc.), but it can still have negative effects on harmless or beneficial insects, so it must always be applied with precaution.
My favorite “pesticides” for treating caterpillars, for example, remain hand picking (for sporadic cases like tomato hornworn) or, when the problem is recurrent, covering the plant susceptible to caterpillars (especially crucifers like cabbages and kale) with floating row cover immediately after planting, thus keeping them off entirely. Still, when you’re hit by an unexpected caterpillar attack, Btk might well be the most environmentally acceptable choice for stopping it.