Bacillus thurengiensis

Bacillus thurengiensis is a wide spread bacterium of the family Bacillaceae. Different varieties (B. thurengiensis kurstki, B. thurengiensis aizawai, B. thurengiensis tenebrionis) and strains of this bacterium are effectively used in agricultural practices for pest control. The insecticidal effect of biopesticides made of B. thurengiensis culture is mainly spread on scale-winged insects (Lepidoptera); the number of insect species that are susceptible to B. thurengiensis exceeds 150. But the insects of the orders hymopteran (Hymoptera), two-winged (Diptera) and beetles (Coleoptera) are not vulnerable to this bacterium. The most harmful insects that are susceptible to the biopesticides of B. thurengiensis are: Yponomeuta malineus (the apple moth), Yponomeuta evonymellus (the full-spotted moth), Yponomeuta padellus (the common hawthorn moth), Yponomeuta cogwatellus (the allied moth), Yponomeuta mahalebellus, Caloptilia roscipennella, Agryresthia conjugella (the apple fruit moth), Caloptilia syrengella (the black leaf hopper), Plutella maculipennis (the cabbage moth), Pieris rapae (the small white butterfly), Pieris crataegi (the thorn butterfly), Stilpnotia salicis (the willow moth), Dendrolimus pini (the pine moth), Malacosoma neustria (the common lackey), Lymantria dispar (the gypsy moth), Euproctis chrysorrhoea (the brown-tail moth), Torticidae spp. (the fruit-tree leaf-rollers or totrix moths), Evergestis forficalis (the garden pebble), Ephestia kuehniella (the flour moth), Operophthera bramata (the winter moth), Itame wauaria (the v-looper), Hyphantria cunea (the white tiger moth), Laspeyresia pomonella (the codling moth), Caliroa cerasi (the pear slug sawfly), Ostrinia nubialis (the corn borer), Pectinophora malvella, Heliothis armigera (the cotton bollworm) and some other species.

Invnatural conditions B. thurengiensis is mainly a soil-living organism. However it is able to inhabit the foliage of trees and shrubs, where the bacterium exists owing to the waste products excreted by leaves. The life cycle of the bacterium consists of two phases: a division phase and a sporulation phase. During the second phase the bacterium produces specific protein crystals. These crystals in joint action with spores are able to kill caterpillars after which the bacterium continues to  feed and multiply inside a dead insect. Besides, the bacterium also occurs as a saprophyte consuming dead organic matter. The spores give the bacterium possibility to survive unfavorable conditions (for example, the deficiency of food) and can be found in different places (host-plants, soil, water, foliage). Usually the bacteria are spread by wind. However it can be spread with the aid of carrying organisms, which are insensitive to the bacteria of this species. The reproduction and the spreading of B. thurengiensis in nature don't constitute danger because they don't reach epidemical threshold.

The definitive feature of the bacteria B. thurengiensis is the ability to produce specific crystalloid toxins possessing a high insecticide activity. Crystals have a rhombic shape. They are found inside cells in the process of sporulation by means of the usual microscopy of alive bacterium preparations and they are often called as parasporal inclusions or endotoxins. After the completion of spore formation process the toxins are excreted in environment and discovered in free form. The size of the crystals varies within 1–3 μm (micrometers) according to the kind of bacteria culture. The insecticidal crystals are the matters of protein nature and they consist of 18 aminoacids. The effectiveness of biopesticides made on the base of B. thurengiensis depends greatly on several factors such as the behavioural features of caterpillars, their biotype, climatic conditions, the methods of application and doses.

The effective action of B. thurengiensis is exhibited when it penetrates in the alimentary tract of pest insects. The young stages of pest development are better controlled than the elder ones and their activity of feeding is less intensive. The bacterium is not effective against eggs and adult moths therefore it is necessary to know the life cycle of pests for opportune response. The determination of treatment periods is especially important for species leading the hidden mode of life.

The bacterium produces the spores and the protein crystals, which cause the death of caterpillars. Their affection occurs owing to the action of enzymes under alkaline medium in the intestine of caterpillars. Small ferment molecules destroy the walls of the intestine making holes in it through which the spores of the bacterium penetrate in the body cavity of the caterpillar where they multiply. The metabolism of the caterpillar is broken and the paralysis of jaws occurs. As a result of it feeding ceases during several hours. Paralyzed and dead individuals dangle from leaves, decompose slowly, and in course of time fall on the ground.

The incontestable advantage of biopesticides containing B. thurengiensis is the absence of toxic influence on entomophages and beneficial entomofauna. For example, they are harmless for melliferous bees. Biopesticides of crystalloid toxins also harmless for homeothermal (hot-blooded) animals, fish and plants.

The plant harmlessness of biopesticides containing B. thurengiensis allows them to be applied in any phase of plant vegetation. The rate of biopesticide application depends on insect species, meteorological conditions, the size of treatable plants and the kind of sprinkle. It is perspective the joint application of the bacterium pesticides together with various chemicals. Herewith it is observed the synergism of their insecticidal action. The range of the insecticidal effect of these substances widens and their rate of application decreases.