PESTS AND DISEASES OF FORESTRY IN NEW ZEALAND
BTK (Bacillus thuringiensis var. kurstaki)
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Formerly known as the Forest Research Institute, Scion has been a leader in research relating to forest health for over 50 years. The Rotorua-based Crown Research Institute continues to provide science that will protect all forests from damage caused by insect pests, pathogens and weeds. The information presented below arises from these research activities.
From Forest Health News 116, March 2002.
The aerial spraying campaign to eradicate painted apple moth from Auckland using Btk is now underway, so it is timely to recall the nature and mode of action of this widely applied organic insecticide (FHNews 26:2). Bacillus thuringiensis var. kurstaki (Btk) is a bacterium found naturally in soil, which produces toxins injurious to lepidopterous insect larvae (caterpillars of moths and butterflies). Bacillus thuringiensis was first isolated from diseased silkworms in Japan in 1901, and later, in 1911, from Mediterranean flour moths. Btk is only one of many varieties of B. thuringiensis . Bacillus thuringiensis var. israelensis (Bti), for instance, is very active against mosquito and sandfly larvae, but has only a fifteenth of the effectiveness of Btk against lepidopterous larvae. Btk grows well on standard bacterial media, and is mass-produced industrially in aerobic fermentation vats. The commercial product is prepared as either a wettable powder or an aqueous suspension, containing a mixture of bacterial endospores, crystals of a proteinaceous protoxin (toxin precursor) known as delta-endotoxin, and various additives. This mix is deposited on leaves and ingested by the larvae when they eat foliage that has been sprayed. Delta-endotoxin is broken down enzymatically into toxic peptides in a high pH environment. This is why susceptibility to Btk is confined to insects, mainly lepidoptera, which have a strongly alkaline midgut. In these insects the breakdown products damage the gut lining, and the larvae are unable to feed. The endospores then germinate, and the larvae become filled with bacteria and die. The gut of humans and other vertebrates is acidic or only mildly alkaline, and the harmless protoxin passes through unchanged. All Bt insecticides are highly specific to their target insects, and compared to many synthetic insecticides are environmentally benign. Although there have been reports of adverse effects, risks to humans are negligible, and normal formulations and spray concentrations are generally non-toxic to mammals, birds, fish and plants.
Ian Hood, Editor
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