Everybody is talking about Btk because of the gypsy moth outbreak this year. This microbial pesticide kills caterpillars but does not harm any other insects. Unfortunately, its popularity has generated a number of misconceptions about the product.
I see this all the time in the gardening world. A little bit of misunderstood true science morphs into solutions that just don’t work. Let’s have a close look at Btk and separate scientific fact from fiction.
What is Btk (Bacillus thuringiensis var. kustaki)?
Bt is the short form for Bacillus thuringiensis, a bacteria that is naturally found in soil. Researchers found that it can negatively affect insects and after some testing they determined that a specific strain of Bt, called kustaki was particularly good at killing caterpillars. This special bacteria is now called Btk.
Btk is effective at killing crop pests such as European corn borer, cabbage looper, tomato hornworm, alfalfa caterpillar, and leafrollers, as well as pests on ornamental plants; tent caterpillar, fall webworm.
There are another 20 or so Bt strains that are either in use or being researched for other pest control applications.
Bti is Bacillus thuringiensis subsp. israelensis and this is very effective at controlling mosquito and black fly larvae. It’s the active ingredient in Mosquito Dunks, a commercial product for controlling these larvae in water.
Bacillus thuringiensis aizawai is used for the wax moth larvae in honeycombs and Bacillus thuringiensis san diego is used for the Colorado potato beetle. BeetleGONE, a commercial product containing Bacillus thuringiensis galleriae (Btg) claims to be effective against both the larvae and adult Japanese beetle.
I see a number of comments on social media about the use of Bt for pest control, but such statements are misleading, unless the specific strain is specified. Bt and Btk are not the same thing. If you don’t buy the right strain it won’t work.
How Does Btk Work?
Bt produces crystal proteins during sporulation, called insecticidal delta-endotoxins. Different proteins work against different insects by attaching to specific binding sites in the insect’s gut. They only become active once dissolved in the alkaline gut environment. Once attached they form holes in the gut, which eventually leads to death. This process can take days but the insect stops eating soon after ingestion.
This pesticide is not as quick acting as many chemicals which causes some gardeners to conclude it doesn’t work.
This type of gut binding site is not found in mammals so the bacterium just moves through our gut without any effect.
Btk is normally found in soil and on plant leaves, but the natural amount on plants is not enough to kill the larvae. It does not take a lot of extra Btk to be fatal, but it is important that it gets ingested by the insect – it is a stomach poison.
Do Btk products Contain Live Bacteria?
If you read most of the available information about Btk, it leaves you with a sense that these products contain bacteria, or bacterial spores, and when these are eaten by larvae, they cause death of the larvae, but this is not true.
The following is from the manufacturers of Safer’s Btk (personal communication), “Our Btk product does not contain living bacteria/spores/solids and these will not replicate on the plant surface. To complicate matters, there are other pesticide products that are live and replicate after application, Btk just happens to not be one of them.”
Canada health also confirms this. When you use Btk you are not spraying live bacteria.
What Insects are Killed by Btk?
Btk is only effective on insects in the family Lepidoptera; the moths and butterflies. Even though it kills all of these, it has to be ingested by the insect to cause harm. So it won’t harm eggs and it is not effective on adults since they don’t spend much time eating leaves coated in the bacteria.
In many cases it is only effective when the caterpillars are small. For example, it works well on the gypsy moth when the caterpillar is less than 1/2″ in size, but it won’t kill larger ones.
Any type of Bt has limited use for boring larvae that spend most of their time eating inside the plant.
Btk does not harm insects that are not in the Lepidoptera family. Suggestions for using it to control various beetles like cucumber beetle and potato beetle are just wrong.
Does Btk Kill Sawflies?
The term caterpillar should only be used to refer to the larvae of moths and butterflies, but gardeners tend to use the term for any similar-looking larvae that is eating their plant leaves. One such group is the sawfly. They have larvae that look very much like a caterpillar, but they are not real caterpillars. Most sawfly adults look like wasps.
This is one reason why it is so important to ID the pest you are trying to control. Just because it looks like a caterpillar, or because you found leaf damage that looks like it was made by a caterpillar, is no reason to start spraying Btk.
Is Btk Safe?
One of the attractions to Btk is that it is extremely safe. ” In over 30 years of safe use, no significant environmental problems have been observed.” It does not harm beneficial insects, humans or other wildlife. It has a short life span once sprayed on plants and is gone in less than a week. It is a natural bacteria found in soil, so there is little concern that it will harm soil microbes and it becomes inactive in water within 48 hours.
Commercial products also include inert ingredients along with the Btk that allow the spray to stick to vegetation. “These ingredients include food products that also provide nutrition for bacteria, such as potato starch, glucose or sucrose, proteins from corn or soy, and water. Additional ingredients might be sodium hydroxide (a chemical also used to adjust pH levels in chocolates, ice cream, and margarine), potassium phosphate, and a thickening agent found in cream cheese and ice cream. Other inert ingredients might be used but always in much smaller quantities than those mentioned above.” Note that the bacterial food is left over from growing the bacteria during manufacturing – it is not there to feed bacteria on the leaves.
Development of Pest Resistance
A claimed benefit of microbial insecticides is that since they are not chemicals, pests won’t develop a resistance to them. This is not true.
The delivery mechanism in this case is a bacterium, but the active insecticide is a chemical, the crystal protein. There are now a couple of cases of resistance to Btk, including the diamondback moth, and the corn borer.
Efficacy Depends on the Environment
How effective is Btk? That depends on a number of factors.
- High light degrades the product
- High temperatures and low temperatures deactivate it
- Rain and irrigation wash it off leaves
Newer formulations try to solve these problems, but it is important to follow directions. Avoid high or low temperatures to prolong it shelf-life.
Efficacy Depends on Timing
The timing of application is critical. In most cases the treatment needs to be done when young larvae are feeding. Eggs, pupae and adults (in most cases) are not affected.
Some pests will have a second or even third wave during the year. Timing the spray for the start of each wave is most effective.
Should Gardeners Use Btk?
Yes. This is an effective product that is very safe to use, has almost no environmental drawbacks and is easy for the home owner to apply, except on large trees.
The problem is that many gardeners won’t take the time to use it properly. Use it only of you are going to follow these steps.
- Identify the pest. Be sure you know what you are trying to kill.
- Decide if the destruction by the pest warrants a spray program – small infestations are easy to pick off by hand.
- Research the pest so that you know when to spray. Spraying at the wrong time is a waste of time and resources. By the time you see the damage, it may be too late to spray.
- Follow all label instructions.