Over 90 species of naturally occurring, insect-specific bacteria have been isolated from insects, plants, and the soil, but only a few have been studied intensively. The two studied the most have been Bacillus thuringiensis and Bacillus popilliae.
Background on Bacillus thuringiensis
Bacillus thuringiensis (Bt) occurs naturally in the soil and on plants. Bt produces a crystal protein that is toxic to specific groups of insects. Bt has been available in North America as a microbial insecticide since the 1960's. The products have a excellent safety record and can be used on crops until close to the day of harvest. The bacteria must be eaten, so good spray coverage of the crops is essential.
Bacillus thuringiensis is found on, and may be used on many crops. Including, vegetables, cotton, tobacco, tree crops, and landscaping.
The primary insects controlled by Bacillus thuringiensis are numerous moth and butterfly caterpillars, as well as, certain beetle and fly larvae. Different varieties of Bacillus thuringiensis have been formulated to control certain insects. For example, the Bacillus thuringiensis variety kurstaki controls many caterpillar pest such as horn worms, bag worms, and cabbage worms to name a few. The variety israelensis is used to control mosquito, black fly and certain gnat larvae. When purchased commercially, many of the varieties are combined to attack a broader spectrum of pest.
How it works
The commercial product is applied to the crops by spraying the crops with a solution containing the Bacillus thuringiensis . A susceptible insect ingest the bacteria when the insect ingest the treated crop. The crystal protein toxin damages the gut lining, which leads to gut paralysis. This affectively stops the insect from feeding. The combined effects of gut lining tissue damage and starvation causes death. There are disadvantages in using this control. The insect not only must be in a susceptible stage of development, but the insect must also ingest enough of the bacterium to cause sufficient damage. Genetic engineers have also developed several plant species to express the Bacillus thuringiensis toxin as part of the plant's normal development. This leads to genetically engineered "insect-resistant" crops.
Successful use of Bacillus thuringiensis formulations requires application to the correct target species of pest at a susceptible stage of development, in the right concentration, at the correct temperature to ensure feeding, and before the pest enter the plant or fruit where they would be protected. As with any microbial insecticide, determining when the pest is most susceptible is the key to getting the best results.
Background on Bacillus popilliae
Bacillus popilliae. is a naturally occurring bacteria that have been mass-produced for the control of Japanese beetle larvae in turf since the 1940s. Bacillus popilliae. cause "milky disease" in the beetle larvae and establish a resident population capable of causing mortality over several seasons if soil conditions are appropriate. Bacillus popilliae was the first insect pathogen to be registered in the U.S. as a microbial control agent.
Bacillus popilliae prefer nutrient rich media containing a yeast extract, an amino acid source, and sugars. When used as a pesticide, it is typically spread onto the turf so it can soak into the underlying soils.
The Japanese beetle, Popillia japonica, is the specific insect that Bacillus popilliae controls. Other varieties of Bacillus popilliae have been found to work on other beetles in the family Scarabeaidae, which include the Japanese beetle, the chafers (a pasture pest) and the beneficial dung beetles.
A commercial product, such as "milky spore", is applied to the turf using a tube applicator as seen below. After application, the larval grubs ingest the spores of Bacillus popilliae when they ingest the roots of the grass. The bacteria begins to multiply inside the gut of the grub killing it in about 14 days. Once the grubs begins to decompose, billions of new bacteria are released into the soil. The treatment begins working upon application wherever grubs are feeding. Warm climates can achieve complete control in 2-3 years. Colder climates may require 3-5 years. Once the treatment is applied and established in your lawn it can be effective for 15 to 20 years. The cause of death is most likely physiological starvation caused by the growth of bacterial cells in the hemolymph.
The treatment is most effective when applied on a region or state wide basis to reduce the overall levels of beetle infestation. It is less effective when used by small landowners, who may control larvae in their own turf only to have their plants eaten by beetles from neighboring properties.