Organic Pesticides part II: Using "Bt" to control fungus gnats and more

I use an Organic pesticide called "Bt" (Bacillus thuringiensis ) to control Caterpillars: and not every one has heard of Bt. It also works well to control fungus gnats, which are those large black gnats that breed in the soil and their larvae feeds on the roots. Here is an image of fungus gnats on a trapping device:

Bt can also be used to control some types of beetle grubs. (I've lost a large Salvia tree each year due to white grubs.) Just mix some up into your water and water your plant normally.

Bt is a series of organic bio-controls (Bacteria) that can be used to combat various garden pests, such as fungus gnats and caterpillars, safely.

I encourage you to read the whole Bt Primer, by Carrie Swadener.

Here is a part of what this lengthy resource has to say:

Quote

"Bacillus thuringiensis (Bt)"

Carrie Swadener
Journal of Pesticide Reform
Volume 14, Number 3
Fall 1994

Introduction

Bacillus thuringiensis (B.t.) is a live microorganism that kills certain insects and is used to kill unwanted insects in forests, agriculture, and urban areas.

In a purified form, some of the proteins produced by B.t. are acutely toxic to mammals. However, in their natural form, acute toxicity of commonly-used B.t. varieties is limited to caterpillars, mosquito larvae, and beetle larvae. B.t. is closely related to B. cereus, a bacteria that causes food poisoning and to B. anthracis, the agent of the disease anthrax. Few studies have been conducted on the chronic health effects, carcinogenicity, or mutagenicity of B.t. People exposed to B.t. have complained of respiratory, eye, and skin irritation, and one corneal ulcer has occurred after direct contact with a B.t. formulation. People also suffer from allergies to the "inert" (secret) ingredients. People with compromised immune systems may be particularly susceptible to B.t.

Viable B.t. spores are known to exist for up to one year following application. Insect resistance to B.t. has been well documented. Genetic engineering may greatly expand use of B.t., speeding up the development of more resistance.

Large-scale applications of B.t. can have far-reaching ecological impacts. B.t. can reduce dramatically the number and variety of moth and butterfly species, which in turn impacts birds and mammals that feed on caterpillars. In addition, a number of beneficial insects are adversely impacted by B.t.

B.t. is less toxic to mammals and shows fewer environmental effects than many synthetic insecticides. However, this is no reason to use it indiscriminately. Its environmental and health effects as well as those of all other alternatives must be thoroughly considered before use. B.t. should be used only when necessary, and in the smallest quantities possible. It should always be used as part of a sustainable management program.

As hazards of conventional, broad acting pesticides are documented, researchers look for pesticides that are are toxic only to the target pest, have less impact on other species, and have fewer environmental hazards. Bacillus thuringiensis (B.t.) insecticides result from this research. However, there is evidence suggesting that B.t. is not as benign as the manufacturers would like us to believe, and that care is warranted in its use.

B.t. is a species of bacteria that has insecticidal properties affecting a selective range of insect orders. There are at least 34 subspecies of B.t. (also called serotypes or varieties) and probably over 800 strain isolates. B.t. was first isolated in 1901 in Japan from diseased silkworm larvae. It was later isolated from Mediterranean flour moths and named Bacillus thuringiensis in 1911. It was not until 1958 that B.t. was used commercially in the United States. By 1989, B.t. products had captured 90-95 per cent of the biopesticide market.

Bacillus thuringiensis products available in the United States are comprised of one of five varieties of B.t.: B.t. var. kurstaki and var. morrisoni, which cause disease in moth and butterfly caterpillars; B.t. var. israelensis which causes disease in mosquito and blackfly larvae; B.t. var. aizawai which causes disease in wax moth caterpillars); and B.t. var. tenebrionis, also called var. san diego, which causes disease in beetle larvae. Other strains of B.t. have been discovered that exhibit pesticidal activity against nematodes, mites, flatworms, and protozoa.

B.t. products are used to control moth pests in fruits, vegetables, and beehives; blackfly and mosquito pests in ponds and lakes; and several beetle pests in vegetables and shade trees. Common brand names include Dipel, Foray, Thuricide (all B.t. kurstaki), Vectobac, Mosquito Attack (all B.t. israelensis), and M-Trak (B.t. tenebrionis).

Mode of Action

When conditions for bacterial growth are not optimal B.t., like many bacteria, forms spores. Spores are the dormant stage of the bacterial life cycle, when the organism waits for better growing conditions. Unlike many other bacteria, when B.t. creates spores it also creates a protein crystal. This crystal is the toxic component of B.t..

After the insect ingests B.t., the crystal is dissolved in the insect's alkaline gut. Then the insect's digestive enzymes break down the crystal structure and activate B.t.'s insecticidal component, called the delta-endotoxin. The delta-endotoxin binds to the cells lining the midgut membrane and creates pores in the membrane, upsetting the gut's ion balance. The insect soon stops feeding and starves to death.

If the insect is not susceptible to the direct action of the delta-endotoxin, death occurs after B.t. starts vegetative growth inside the insect's gut. The spore germinates after the gut membrane is broken; it then reproduces and makes more spores. This body-wide infection eventually kills the insect.

Factors Affecting Selectivity

One of B.t.'s most desirable characteristic is its selectivity; only certain insects are susceptible to the delta-endotoxin. Scientists have identified at least 29 different crystals and delta-endotoxins. Each is effective against specific insects. Each variety of B.t. can produce one or more of these toxins. Alkaline (basic; pH greater than 7) solutions activate the delta-endotoxin, and different varieties may require different pHs. Certain enzymes must also be present in the insect's gut to break the crystal into its toxic elements. In addition, certain cell characteristics in the insect gut encourage binding of the endotoxin and subsequent pore formation. The age of the insect is also a factor, the younger larvae being more susceptible than older larvae.

Environmental Fate

Very little is known about the natural ecology of B.t. It occurs naturally in many soils. In one study, B.t. was isolated from 70 per cent of soil samples taken from around the world, and was most abundant in samples taken in Asia. More than half of these isolates were undescribed varieties of B.t. B.t. has also been isolated from insect bodies, tree leaves and aquatic environments. It has even been recovered from paper.

Soil: B.t. generally persists only a short time in soil. The half life of the insecticidal activity (the time in which half of the insecticidal activity is lost) of the crystal is about 9 days. However, small amounts can be quite persistent. In one experiment, B.t. spore numbers declined by one order of magnitude after 2 weeks, but then remained constant for 8 months following application.

B.t. does not appear to move readily in soil. In one study, two varieties of B.t. were applied in adjacent plots, but did not become cross-contaminated, indicating that B.t. does not move laterally in soil. Other studies found that B.t. was not recovered past a depth of 6 centimeters after irrigation, and that movement beyond the application plot was less than 10 yards.

Foliage: B.t. deposited on the upper side of leaves (exposed to the sun) may remain effective for only 1-2 days, but B.t. on the underside of leaves (i.e. protected from the sun) may remain active for 7-10 days. It is possible for it to be significantly more persistent, however. Viable spores of B.t.k. were recovered from white spruce foliage one year after application. In one experiment conducted in Japan, B.t. persisted for two years in a citrus orchard and remained toxic to caterpillars.

Water: B.t.k. has been recovered from rivers and public water distribution systems after an aerial application of Thuricide 16B. Standard water treatment processes are not adequate to destroy B.t.k. spores.

B.t.i. spores and crystals bind readily to sediments in the water column, which reduces their efficacy by making them inaccessible to mosquito and blackfly larvae.

In one test, B.t.i. was applied to water, then allowed to contact mud particles. Over 99 percent of the B.t.i. spores were found in the mud, rather than in the water, after 45 minutes. The B.t.i. retained viability and toxicity for at least 22 days, killing 90 percent of the mosquito larvae when the mud was stirred and reintroduced to the water column.

In another experiment, viable cells were recovered from the water for up to 200 days and in the sediment for up to 270 days after application.

Air: B.t.k. has been found to drift over 3,000 meters downwind during an aerial application. The distance B.t.k. is capable of drifting depends upon the amount and method of application, as well as the climatic conditions. B.t. thuringiensis was measured in air for up to 17 days following an application.

Biotechnology

Examples of genetic manipulation and genetic engineering with B.t. include the following:

* In the agricultural product Foil, the gene for a toxin with activity against beetles was transferred through conjugation (sexual reproduction in bacteria) to a B.t.k. cell that only affected butterflies and moths. The resulting cell showed insecticidal properties against beetles, butterflies, and moths. Since EPA considers the organisms resulting from conjugation to be genetically manipulated rather than genetically engineered, Foil was registered for use in the U.S. in 1990.

* Pseudomonas fluorescens cells can be engineered to produce the B.t. delta-endotoxin without production of a spore. The crystal protein remains inside the P. fluorescens cell wall. In the products MVP and M-Trak, the P. fluorescens cell is killed after it produces the crystal protein. When the product is applied, the delta-endotoxin remains protected within the now dead cell wall. In this way, the B.t. delta- endotoxin retains its effectiveness for two to three times longer than other B.t. formulations. MVP and M-Trak were the first genetically engineered products to be registered by EPA, since the transgenic organism was not alive when released into the environment.

* B.t.i. used to control mosquito and blackfly larvae that live on the water surface begins to sink, away from the target larvae, within 24 hours. Bacteria that naturally live on the water surface (in the same environment as mosquito or blackfly larvae), have been engineered to produce the B.t.i. crystal proteins.

* Over thirty different crops have been engineered to produce the B.t. crystal protein throughout their plant structure. Any pest that feeds on any part of these plants will be exposed to the B.t. delta-endotoxin, and those susceptible to the toxin will be killed.

Clearly, the possibilities for the genetic engineering of B.t. delta-endotoxins seem endless. However, researchers know so little about the ecology and genetic stability of B.t., that the potential ecological effects of these transgenic organisms are impossible to predict with certainty.

B.t.'s Ecological Impacts

Some of the most serious concerns about widespread use of B.t. as a pest control technique come from the effects it can have on animals other than the pest targeted for control. All B.t. products can kill organisms other than their intended targets. In turn, the animals that depend on these organisms for food are also impacted.

Beneficial insects: Many insects are not pests, and any pest management technique needs to be especially concerned about those that are called beneficials, the insects that feed or prey on pest species. B.t. has impacts on a number of beneficial species. For example, studies of a wasp that is a parasite of the meal moth (Plodia interpunctella) found that treatment with B.t. reduced the number of eggs produced by the parasitic wasp, and the percentage of those eggs that hatched. Production and hatchability of eggs of a predatory bug were also decreased. On collards, aphid-eating flies in the family Syrphidae were reduced by Dipel treatment. Both B.t.tenebrionis and Dipel have caused mortality of predatory spider mites. Dipel also has caused mortality of the cinnabar moth, used for the biological control of the weed tansy ragwort. Finally, B.t.i. has caused mortality of a moth (Synclita obliteralis) that helps control aquatic weeds in Florida.

Other insects: Many insects that do not have as directly beneficial importance to agriculture are important in the function and structure of ecosystems. A variety of studies have shown that B.t. applications can disturb insect communities. Research following large-scale B.t. applications to kill gypsy moth larvae in Lane County, Oregon, found that the number of oak-feeding caterpillar species was reduced for three years following spraying, and the number of caterpillars was reduced for two years. Similar results were found in a study of caterpillars feeding on tobacco brush following a B.t.k. application to control spruce budworm in Oregon. In untreated areas, the number of species was about 30 percent higher, and the number of caterpillars 5 times greater, than in B.t.k.-treated areas two weeks after treatment. The number of caterpillars was still reduced in treated areas the following summer. In Washington, B.t. applications in King and Pierce counties to kill gypsy moths reduced spring moth populations by almost 90 percent. In addition, one rare species appeared to have been eradicated from the treatment zone, and moth populations were "heavily impacted in an area more than double that which was actually sprayed" as moths moved into the treatment zone from surrounding areas. In West Virginia, applications of Foray 48B reduced the number of caterpillar species and the number of caterpillars. The year following application, the number of moth species and the number of moths were both reduced. A recent (1994) study in four different Oregon plant communities found that total weight of caterpillars was reduced between 90 and 95 percent by B.t. treatment; the number of caterpillars was reduced by 80 percent; and the number of caterpillar species was reduced by over 60 percent.

Aquatic insects are also affected by B.t. treatments. Canadian studies found that certain stream insects (Simulium vittatum and Taeniopteryx nivalis) were killed by applications of Thuricide and Dipel respectively. Midges (chironomids) have repeatedly been shown to be killed by B.t.i.

Birds: Because many birds feed on the caterpillars and other insects affected by B.t. applications, it is not surprising that impacts of B.t. spraying on birds have been documented. In Lane County, Oregon studies of chickadees following a gypsy moth spray program found that birds nesting in B.t.- treated areas brought fewer caterpillars to their nests than did birds nesting in untreated areas. The birds were able to find other food, so that nesting success was not significantly impacted. In New Hampshire, when B.t.- treatment reduced caterpillar abundance, black-throated blue warblers made fewer nesting attempts and also brought fewer caterpillars to their nestlings. A Canadian study found that numbers of caterpillars, followed by numbers of two species of warblers and a thrush, were reduced by B.t. treatment. In addition, there were fewer spruce grouse chicks in B.t. treated areas, and the chicks in those areas grew more slowly than chicks in untreated areas.

There is also some evidence that B.t. can be directly toxic to birds. A study of the effects of application of Dipel to ringneck pheasant eggs found that hatching was only half as successful as hatching of untreated eggs. Because the Dipel was applied with a spreader-sticker compound (Plyac) the decrease in hatching may be a result of the Plyac and not the B.t. product.

Other animals: Because shrews often feed on caterpillars, impacts from B.t. treatments are likely. A study in northern Ontario (Canada) found that treatment with Dipel changed the structure of the shrew population. Adult males emigrated, so that the proportion of juveniles increased. The juveniles and adult females who did not emigrate shifted from a diet of caterpillars to alternative prey.

Foray 48B at high concentrations (about 3 percent) is acutely toxic to rainbow trout, probably because the product is highly acidic.

B.t.i. treatments can also affect other animals. Low concentrations of B.t.i. endotoxins decrease the weight of tadpoles and delay their metamorphosis. The B.t.i. formulation Vectobac is acutely toxic to fathead minnows, probably because "inerts" in the product deplete the dissolved oxygen in water. The B.t.i. formulation Teknar was acutely toxic to brook trout fry, probably because of xylene used as an "inert" in the product.

Comparison with synthetic insecticides: Where comparative studies have been done, the ecological impacts of a B.t. treatment are almost always less than those of synthetic insecticides. For example, B.t. treatment of collards caused less of an increase in aphid numbers than did treatment with carbaryl, which killed many aphid predators. Vectobac was much less acutely toxic to an estuary fish than other mosquito insecticides including temephos, fenoxycarb, diflubenzuron, and methoprene.

** NOTICE: In accordance with Title 17 U.S.C. Section 107, this material is distributed for research and educational purposes only. **

Last Updated on 6/28/99