Bacillus thuringiensis subsp. kurstaki
Bacillus thuringiensis subsp. kurstaki.
Bacillus thuringiensis is common in soil , insect-rich environments mills and warehouses. Strains that are used in crop protection are selected from those isolated in nature on the basis of their potency in test insect species, spectrum of host insects and the ease with which they can be grown in fermenters.
Produced in controlled fermentor in deep tanks of sterilised nutrient liquid medium. The endotoxins and living spores are harvested as water dispersible liquid concentrates for subsequent formulation.
Target Pests and Crops:
Lepidopteran larvae, particularly the American Bollworm (Hellicoverpa armigera),
Pink bollworm (Pectinophera
species), spotted bollworm (Erias
insulana)diamond back moth (Plutela
xylostella (Linnaeus)) and other vegetable pests such as Colorado
potato beetle (Leptinotarsa
decemlineota (Say)) and forest insects.
Mode of action:
Bacillus thuringiensis produces parasporal, proteinaceous, crystal inclusion bodies during sporulation. Upon ingestion, these are insecticidal to larvae of the order Lepidoptera and to both larvae and adults of a few Coleoptera. Once in the insect, the crystal proteins are solubilised and the insect gut proteases convert the original pro-toxin into a combination of up to four smaller toxins. These hydrolysed toxins bind to the insect's midgut cells at high-affinity, specific receptor binding sites where they interfere with the potassium-ion dependent, active amino acid symport mechanism. This disruption causes the formation of large pores that increase the water permeability of the cell membrane. A large uptake of water causes cell swelling and eventual rupture, disintegrating the midgut lining. Different toxins bind to different receptors in different insect species and with varying intensities: this explains species specificities.
The crystal inclusions derived from Btk are generally lepidopteran specific. Because they have to be ingested and then processed within the insect's gut, they are often slow acting (two to forty-eight hours in comparison to conventional chemicals). The toxin results in starvation leading to death; insects not killed by direct action of the toxin may die from bacterial infection over a longer period. Different toxins have different spectra of activity. Different strains and serotypes have been developed by different companies. In addition to producing the endotoxins, many strains of Bt are potent insect pathogens. (Many Bt genes ( Cry IA) have been isolated and used to transform crops, also known as Genetically Modified Crop (GMO) or Transgenic Crop ( Cotton ECH-162, MECH-184, MECH-12) thereby making them resistant.
Effective against lepidopteran species, however, light instability can cause problems if exposed to high light intensities. Rapidly hydrolysed under even mild alkaline conditions.
Sold as a combination of endotoxin crystals and living bacterial spores. Formulated as a suspension concentrate (SC), a granular bait (GB), a ready to use bait (RB), a suspo-emulsion (SE), a granule (GR), an oil miscible flowable concentrate (oil miscible suspension) (OF), a dispersible powder (DP) and a wettable powder (WP).
Dipel BL, Delfin WG, Bactin, Bio-Tek, Bio Bit WP, WOCK Biological (Halt-Bt).
Use at rates of 100-300 g active ingredient (a.i.) per hectare ensuring that the crop is well covered with the spray suspension. Apply while larvae are small and repeat every five to seven days if infestations are high. Bt-based sprays can be applied up to the day of harvest.
All formulations are standardised at a toxin content expressed in terms of international units active against a target pest per mg of product.
Storage conditions and shelf-life:
Do not expose to direct sunlight, and keep in cool conditions. If stored under cool dark conditions, the products remain viable for two years or more.
Do not use in combination with broad spectrum biocides such as chlorothalonil. Compatible with a wide range of acaricides, insecticides, fungicides, stickers, spreaders and wetters. Do not use water with a pH above 8.0.
Environmental impact and non-target toxicity:
Btk has a short persistence owing to its sensitivity to UV. light. No adverse effects have been recorded in approved field use and none are anticipated. Btk should not be used near water-courses, however no adverse effect have been observed on birds, fishes and honeybees.
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