Bacillus Thuringiensis in Horticulture
One of the best natural pesticides a grower can use against pest insects is a naturally occurring bacteria called bacillus thuringiensis (Bt). Eric Hopper explains why Bt is so effective.
Pest insects can wreak havoc on an otherwise healthy vegetable garden. Unlike gardeners who specialize in ornamental crops, those who grow vegetables or other plants for human consumption must be careful with the products and methods used to treat pest insects. Many chemical pesticides and herbicides can leave behind residuals that can be unhealthy to humans or other mammals when consumed.
Although they are not always 100 per cent safe, natural- and organic-based pesticides tend to be safer options for vegetable crops. Some of the most commonly used and effective types of insecticides for protecting vegetable crops are derived from a naturally occurring bacteria called bacillus thuringiensis (often abbreviated to Bt). There are many recognized subspecies of Bt; however, bacillus thuringiensis kurstaki (Btk) and bacillus thuringiensis israelensis (Bti) are the two particular strains of Bt that are most commonly used in horticulture. Each of these Bt subspecies offers protection against particular types of pest insects.
History of Bacillus Thuringiensis (Bt)
Bacillus thuringiensis (Bt) was first discovered by Japanese biologist Shigetan Ishiwatar, when he was trying to find the cause of the sudden collapse disease that was killing large populations of silkworms. He successfully isolated the bacterium Bt and identified it as the cause of the disease in 1901.
In 1911, a German microbiologist, Ernst Berliner, rediscovered Bt as the culprit responsible for killing a Mediterranean flour moth. The bacterium was named bacillus thuringiensis after the German town, Thuringia, where the moth was found. Through his research, Berliner reported the existence of a crystal toxin within Bt. However, the specifics of how this crystal worked was not realized until much later. Due to Berliner’s work, farmers started to use Bt as an effective pesticide in 1920.
There was even a commercialized spore-based formulation sold under the name Sporine during the 1930s. At the time, Sporine was primarily sold and used to kill flour moths.
In 1956, a group of researchers found that the main insecticidal activity found in Bt could be attributed to its parasporal crystal body.
A parasporal body is a crystalline protein that forms around a spore in some bacteria and can act as a toxin precursor when ingested by particular insects. The discovery of the parasporal crystal body inspired more research and interest in the crystal structure, biochemistry, and mode of action of Bt.
In other words, many scientists jumped on board to learn more about Bt. This led to Bt being used commercially in the US in 1958 and, in 1961, Bt was officially registered as a pesticide by the Environmental Protection Agency (EPA). Up until the late 1970s, all the discovered subspecies of Bt were used to treat lepidopteran (the order of insects that includes butterflies and moths) larvae. In 1977, a subspecies of Bt was discovered that was toxic to dipteran (the order of insects that includes flies, mosquitoes, and fungus gnats) species. In 1983, another subspecies was discovered that was toxic to coleopteran (beetles).
The popularity of Bt as an insecticide expanded significantly throughout the 1980s because pest insects were becoming increasingly resistant to synthetic insecticides. It was also during this time that scientists and environmentalists became aware of the negative effects chemical insecticides have on the environment. Agriculturists and gardeners who were concerned with these negative effects turned to Bt as a safer and effective alternative.
Bacillus Thuringiensis Kurstaki
Bacillus thuringiensis kurstaki is one of the commonly used Bt subspecies to treat lepidopterans on consumable crops. Some of the most common pest insects that can be treated with Btk are cabbage loopers, gypsy moths, tent caterpillars, and tomato hornworms.
During its sporulation, Btk forms crystal proteins that are deadly to lepidopteran larvae. For Btk to be effective, it must be ingested by the pest insect. Put another way, Btk must be applied directly to the foliage where the pest insect feeds. Once ingested, Btk breaks down the pest insect’s gut from the inside, thus killing it.
Bacillus thuringiensis products are favored by many horticulturists because of their ability to target specific pests, while having little or no effect on other insects that do not feed on the foliage, so Bt products won’t harm beneficial insects, unlike many chemical pesticides. This allows horticulturists to employ beneficial predatory insects, in conjunction with Bt, as a treatment against pest insects.
Bacillus Thuringiensis Ilsraelensis
Bacillus thuringiensis israelensis is a Bt subspecies commonly used in horticulture to treat fungus gnat infestations. Bacillus thuringiensis israelensis is also effective at killing mosquitoes and black flies.
The biggest advantage of Bti, like other Bt subspecies, is its ability to effectively kill targeted species, while having almost no effect on other organisms. Bacillus thuringiensis israelensis contains Cry and Cyt proteins which are pore-forming toxins that break down the cells of the midgut (part of the insect’s digestive tract) by inserting into the cell membrane and forming pores. In layman’s terms, Bti creates holes in the digestive system of the targeted pest insect and kills them from the inside out.
Application Instructions for Bt
Although horticulturists should follow the specific application instructions provided by the manufacturer of a Bt insecticide, there are a couple of general rules of thumb that will make Bt applications more effective against pest insects.
First, Bt products are only effective when ingested by the pest insect. Spray irrigation or rain will wash the Bt from the foliage, making it ineffective against the pest insects. Therefore, horticulturists should avoid applying Bt products right before watering or scheduled rain.
Second, depending on the particular pest insect being targeted, horticulturists should try to apply Bt products when the pests are most actively feeding. For example, cabbage loopers are most active on hot, dry days. Applying Bt on the days when the atmospheric conditions make pests most active will make the treatment more effective.
Genetically Modified Crops Using Bt Genes
Due to advancements in molecular biology, scientists are able to move the Bt gene that encodes the toxic crystals into a plant. The first Bt-based genetically engineered plant (Bt corn) was registered with the EPA in 1995. Commonly referred to as Bt crops, today there are multiple crops that have been modified with genes from Bt, including cotton and potatoes. When pest insects feed on these genetically modified crops, the toxic crystals are ingested by the pest insect and break down the insect’s digestive tract, resulting in death.
Concerns Regarding Bt
Although Bt pesticides are one of the safest pest insect treatment options, overuse of Bt products could pose problems. Some studies have shown that, although Bt is non-toxic to non-targeted organisms, the accumulation of Bt toxins in the soil might lead to Bt resistant insects. Put another way, it is possible targeted pest insects could build up a resistance to Bt if overused. The same holds true for genetically modified Bt crops as well. Overuse of Bt crops could lead to pest insects with a heightened resistance to the effects of Bt.
Regardless of whether a horticulturist chooses chemical or organic pesticides to treat a pest insect problem, he or she should always follow the manufacturer’s suggested handling and application instructions. Also, a grower is responsible for researching the possible negative effects of any pest insect treatment before applying it in the garden.
For vegetables or other crops that will eventually be consumed, organic pesticides tend to be a safer option. Generally speaking, certified organic pesticides break down more quickly than their chemical counterparts and will not leave behind harmful residuals. Most Bt-based pesticides are allowed in organic farming practices because Bt is a naturally occurring, non-pathogenic bacterium found in soil. Bacillus thuringiensis specifically affects the digestive system of pest insects that feed on treated foliage and does not persist in the digestive systems of mammals. In fact, the EPA has not found any human health hazards related to Bt pesticides.
Overall, Bt is one of the safest insecticidal options for horticulturists due to its ability to specifically target pest insects, without affecting innocent bystanders. Gardeners who are especially concerned about protecting their crops, while also protecting themselves, can responsibly use Bt-based insecticides for both safe and effective results.