Farm Forestry New Zealand

PESTS AND DISEASES OF FORESTRY IN NEW ZEALAND

Bacillus thuringiensis

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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 No. 26, November 1993.

Products based on Bacillus thuringiensis (B.t.) are among some of the most successful microbial insecticides. The use of  B.t. in the recent campaigns in North America against the Asian gypsy moth and the possibility that it may have to be used for the same purpose in New Zealand has prompted the production of this short note.

Bacteriology
Bacillus thuringiensis was first isolated from diseased silkworms in Japan in 1901. It was described and given its present name by Berliner in 1915. The specific epithet means 'of Thuringia', a province in Germany. It grows well on normal bacteriological media. Twenty-two distinct varieties of B. thuringiensis , based on serotyping and esterase patterns, are recognised. These vary in production of toxins and in their host spectrum. The original strain described by Berliner is now known as B. thuringiensis var. thuringiensis ( B.t.t.) and its toxicity is used as the standard against which the toxicity of the other strains is measured. Commercially, the most important varieties are B. thuringiensis var. israelensis (B.t.i.) and B. thuringiensis var. kurstaki (B.t.k.) . The former is used to control mosquito and sandfly larvae and the latter for the control of lepidopterous larvae. Var. kurstaki, which was first isolated in France by Kurstak in 1962, is approximately 15 times more active against lepidopterous larvae than var. thuringiensis.

Toxins
The various varieties produce several different toxins. The two main types are the δ-endotoxin and the β-exotoxin. Not all varieties produce the β-exotoxin. The δ-endotoxin is a crystalline proteinaceous protoxin which yields toxic peptides when acted upon by digestive enzymes in a strongly alkaline (pH 9-10.5) environment. Thus, susceptibility to this toxin is confined to those insects, mainly lepidoptera, which have a highly alkaline midgut. In these insects, the toxin causes paralysis of the gut and death results through starvation. Caterpillars stop feeding within minutes of ingesting the toxic crystal. Human and vertebrate guts are acid or mildly alkaline and the crystal passes through unchanged. The β-exotoxin is not a protein, has a low molecular weight and acts by interference with ATP metabolism. This exotoxin may be toxic to mammals and its inclusion in a B.t. formulation is not permitted. B. thuringiensis var . kurstaki does not produce the β-exotoxin.

Standardisation
As B.t. is mass-produced in batches in aerobic fermentation vats, the activity of the individual batches varies. The activity of each batch is compared with the activity of a standard B.t. formulation (equivalent to 1000 international units), using cabbage white butterfly caterpillars as the test insect. Activity is expressed in international units per milligram (I.U./mg) for dry formulations and in billions of international units per litre (B.I.U./l ) when formulated as a spray. In Vancouver, 50 B.I.U. were applied per hectare per spray application during their Asian gypsy moth eradication programme.

Toxicity to non-target organisms

a. Mammals
As B.t.k. contains no β-exotoxin it constitutes no hazard to man or other mammals. In trials with B.t.t., a variety which does contain β-exotoxin, eighteen people who ingested 1 gram (10¹⁰ spores) on 5 successive days showed no ill effects. Similarly people who inhaled 100mg of spores daily for 5 days were not affected. Workers in a plant manufacturing the product for up to 7 months exhibited no adverse reactions.

More than 500,000 kg of B.t. are applied annually in the US alone. No adverse effects, attributable to B.t. have emerged in any of the many areas sprayed in the US. Exhaustive clinical studies in Vancouver during the Asian gypsy moth eradication programme failed to show significant differences in the health of the populations living in the sprayed and unsprayed area.

b. Other Animals
There is nothing in the available literature to suggest that B.t.k. poses any threat to animals other than caterpillars. At very high concentrations, at least 1000 times the concentrations likely to occur during spray operations, fish death has been recorded but this has been attributed to the additives in the B.t.k. formulation rather than the bacterial products.

c. Phytotoxicity
There are no reports of damage to plants sprayed with B.t.

John Ray

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