Bacillus thuringiensis or BT is a naturally occurring bacterium that is used as a pesticide for various pests that attack gardens. It's often used for caterpillars and mosquitos, which are arguably two of the most annoying pests in the garden. You can find BT in many forms including granules, sprays, and dunks. BT spray is one of the more common varieties available for home gardeners or commercial use.
There are a few different types of BT that can target specific pests, so it is advisable to have a few different types on hand. BT is believed to be safe around people and pets, and it may not cause harm to some species of insects. There are few concerns about its safety, but most of the concerns can be circumvented or avoided when using it in your garden.
We're going to go into the details of BT so you can learn how to effectively use this natural wonder in your garden.
What is bt
BT spray, powder or granules are common organic pesticides. Source: OSU master gardener
Bacillus thuringiensis is a bacterium found in the soil. There are several different species that conveniently target different insects but remain harmless to others. Although found in the soil, it is made commercially using media such as fish meal, soybean meal, and other ingredients to boost the production of the spores. It is found as an active ingredient in a number of organic pesticides.
Bacillus thuringiensis (BT) spray is the most popular form because it is easy to use. Hand bottles are available for gardeners or can be sprayed over a large area by air application. Other forms are available, such as powder, granules, or cakes, which you can put in water. Sprays are good for leaves, powders and granules for soil, and cakes for water. Always read the directions on the label to make sure you are using your particular brand correctly.
How does BT kill insects?
BT kills insects by destroying their intestines once ingested. When a caterpillar eats BT, the bacteria produce a protein toxin that breaks down the intestines and eventually leads to infection. It can also cause insects to starve to death by damaging or paralyzing their intestines, as is the case with caterpillars, the Bacillus thuringiensis var. Kurstaki, or BTk. Bacillus thuringiensis var. Israelensis, or BTi, kills mosquito larvae in the soil and mosquito larvae in the water.
Each BT toxin is effective against certain insects because of the highly specific proteins contained in these strains. It will not harm birds or other mammals, nor harm the human stomach. Most BT will not harm the intended insect if it just lands on their bodies because they will have to eat the BT toxin in order to be killed by it. A notable exception is spraying larvae with liquid forms, as they can ingest some of it through their skin if they are thoroughly covered. Still, this control method is much more effective when it comes to consumption.
Types of BT
A view of a BT strain (in this case 4A4) magnified 1000 times through a microscope. Source: BlueShift 12
Bacillus thuringiensis var. Israelensis (BTi) and Bacillus thuringiensis var. Kurstaki (BTk) are the most common forms of BT control methods available, but they are not the only ones. Let's take a look at some of the different types of BT and what they can be used for.
Bacillus thuringiensis var.Israelensis (BTi)
BTi can be used to kill black flies, midges, and midges. It can be used on plants, soil, or standing water to kill the insect larvae before they reach their adult stages. Since it is non-toxic to humans and animals, you can safely apply it to pools, rain barrels and other areas that harbor annoying mosquitoes. You can also use it on indoor plants to keep the flies away.
BTi comes in many forms including liquid, granules, dunks, tablets, pellets, and briquettes. It leaves no residue, so it is safe to use on plants. Once they swim on the surface of the water, mosquitoes will not set foot down to lay their eggs, so this is a very effective control for these populations. For flies or mosquitoes, a granular shape can also be used to cover the surface of your indoor plant potting mix to reduce the incidence of infestation.
Bacillus thuringiensis var.kurstaki (BTk)
BT kurstaki (BTk) is effective in removing caterpillars. It treats a wide range of species including (but not limited to) gypsy moths, tomato hornworms, cabbage worms, cabbage claws, groundworms, and leaf rollers. It kills young caterpillars in the feeding phase and does not kill anything that is in the egg or adult stage. If you have a butterfly garden, this area should not be used during butterfly season as it kills caterpillars just as easily as the caterpillars of pest moths.
BTk is most commonly found in liquids and granules. You can apply it directly to growing medium, trees and plants where you can find the caterpillars.
Bacillus thuringiensis var. San Diego
Bacillus thuringiensis var. San Diego is used to kill the Colorado potato beetle, poplar leaf beetle, elm leaf beetle, boll weevil, black weevil, and leaf-eating beetle. It's incredibly effective against these specific beetles, but it may not work for every species of beetle.
Bacillus thuringiensis var. Tenebrionis
Bacillus thuringiensis var. Tenebrionis kills the same beetles as the San Diego variant. Just like other types of BT, it is safe to use around people and animals so you can use it in your yard when needed.
Bacillus thuringiensis var.aizawai (BTa)
BTa is used to treat the grape berry moth and the wax moth, a moth that nests in abandoned beehives. The moths can breed quickly, so using BTa is a wise choice.
BTa can potentially harm honey bees so be careful how you use it. A study published in 2010 found that BTa mixed with sugar water was harmful to bees after ingestion, but did not appear to cause any harm when bees came into contact with it through pollen.
There are other forms of BT that are still being explored. For example, a strain currently known as Bacillus thuringiensis 4A4 is currently being investigated because it appears to be extremely toxic to nematodes such as the root-knot nematode. We do not yet know about every strain of these soil bacteria, and more are being discovered, but they must be studied carefully before safe products are developed and commercialized. As scientists learn more about BT and its many variations, we will see more potential uses over time.
Benefits of Using BT Spray
Hornworms (like this tobacco hornworm) can be treated with BTk. Source: garden solutions
The obvious benefit of using a Bacillus thuringiensis spray is that you can keep mosquitos, caterpillars, and beetles from harming your garden by killing the larvae before they become adults. BT is natural so you don't have to worry about exposure to chemical pesticides in your garden. It is better for the environment and your health if you can stick to natural methods.
While BT is not toxic to humans, you can be sure that you will not be exposed to it for long as it spreads when exposed to sunlight. This way, no residues will remain on your plants in the long term and it will not stay in the water. The data suggest it may remain present and as an active control on plants for up to 22 days. Acid soil also accelerates its spread so it doesn't linger in your soil. BT does not pollute groundwater or soil so it is a safe control and environmental treatment that you are comfortable with.
BT is highly toxic only to the larvae affected by the toxins it produces, so using Bacillus thuringiensis products will not put you, children, pets, and most beneficial insects at risk. Just wash all products before use and any BT that is present will be washed away.
Disadvantages of using BT spray
One of the few problems with using BT spray is that some forms such as Bacillus thuringiensis var. Aizawl (BTa) can be harmful to bees and possibly other beneficial insects such as wasps. It is only harmful when consumed or sprayed directly on the insect. This can easily be avoided by spraying when the bees are not active, for example in the late afternoon when they have returned to their hives. Once dry, this is not harmful to your beneficial insects.
Many caterpillars are harmed by Bacillus thuringiensis var. kurstaki (BTk), which will be a problem if you have a butterfly garden that is supposed to attract butterflies. Avoid spraying the garden if you don't want to cause harm to these butterflies. Keep in mind that BT only kills young caterpillars, so the beneficial butterflies won't run so much of a risk if you can spray the garden once all of the beneficial larvae have matured into adult butterflies.
That brings us to the problem that BT is heavily dependent on timing. Unlike chemical insecticides, BT needs to be applied at the right time. This method of control does not kill eggs or adults, so you need to use it when the insect pests such as caterpillars or other larvae are actively feeding. If you're new to gardening or haven't carefully studied the habits of the insects around you, it may take some practice to get it right.
It should also be noted that BT does not kill immediately, as it does with chemical insecticides. It can take a few days for the larvae to starve, so some patience is required in order to use Bacillus thuringiensis products.
How to use Bacillus Thuringiensis
Another close-up of Bacillus thuringiensis. Source: TheRubinLab
Using Bacillus thuringiensis products in your garden is easy. Liquids are available as ready-to-use formulas or as concentrates. For the ready-to-use options, all you have to do is spray it on your plants or soil. Make sure to follow the directions on the package to make sure you apply it in the right places.
Concentrates must be added to the water and are usually intended for larger areas. For this type of BT spray control, you need to put the liquid in plain water and mix it before applying. Read the label to find out the liquid to water ratio. It can vary between brands, but a little goes a long way, so you probably only need a few teaspoons per gallon.
Liquids must be applied regularly as BT spreads quickly in sunlight. Once a week to every two weeks should be enough for as long as you notice pests. If it rains you will need to reapply it after it dries as it will wash off the leaves. Apply BT in the early morning or late evening to avoid direct, strong sunlight during application. Treatment in the afternoon with this control is generally easier as the risk to pollinators is lower.
When applying BT, be sure to coat the entire area where you will find caterpillars, mosquitos, or beetles. Cover the top and bottom of the leaves, stems, and the soil around the plant. The pests have to eat the BT for the toxicity to take effect. So, spray any part of the garden where you find an insect.
Powdered BT pesticides work in a similar way, but only in a different form. Squeeze out some powder directly on the affected area or use a duster to lightly spray it on the foliage. You may find that this type of pesticide works best on the cob or on the soil surface, but you can still use it anywhere it's needed. Granules are best used in the soil as they are small BT pebbles that will not stick to plants. For powder and granules, you will need to reapply it when it's gone or after it has rained.
Some BT products come in the form of cakes best known as mosquito dunks. Mosquito dunks are designed to swim in stagnant water and are safe to use in ponds with fish or in cattle troughs. Mosquito larvae living in the water eat it and die before they become adults. Add another dunk once it's gone. They should last about a month before needing to be replaced.
frequently asked Questions
Q: is BT spray safe?
A: BT is an insecticide that occurs naturally and is perfectly safe to use on all plants. BT is a bacterium that is only poisonous to certain insects. They do no harm to humans, mammals or fish, so you can use them anywhere in your garden.
Q: is BT toxic to humans?
A: BT is non-toxic to humans and is not known to cause serious health problems or long-term illness. It's perfectly safe to use, but if you are concerned about using it, avoid exposing the area you applied it to for up to thirty minutes to allow the BT to set and dry. Once dry, its toxicity is limited solely to the specific species of insect that consumes the organic pesticide. It is popular in agriculture as one of the safest biological control methods available to the general public.
The National Pesticide Information Center found that some people with hay fever had some symptoms such as nose and throat irritation after airborne BT in agricultural areas. However, it is undetermined whether the reported symptoms are related to seasonal pollen or BT exposure. If you are sensitive to pollen, wear a mask and gloves to avoid exposure during use.
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