Vol. 23, No. 4
from the World Health Organization
Bacillus thuringiensis, Environmental Health Criteria No. 217
1999, xv + 105 pages (English, with summaries in French and Spanish),
ISBN 92-4-157217-5, CHF 27.-/USD 24.30; In developing countries: CHF
11.20, Order No. 1160217.
This book evaluates the risks to human health and the environment posed
by the use of Bacillus thuringiensis (Bt) as a microbial agent for pest
control. Products containing various Bt subspecies are increasingly
used worldwide to control the larvae of several insect pests that threaten
major agricultural crops and forests. Bt products are also being used
to control the insect vectors of malaria, onchocerciasis, and other
diseases of major public health importance. The bacterium is also a
key source of genes for transgenic expression to provide pest resistance
in plants and microorganisms.
The report opens with an overview of the biological properties of Bt
and commercial Bt products. Particular attention is given to the mechanisms
by which sporulation produces inclusion bodies, containing insecticidal
crystalline proteins, which are selectively toxic for insect species
in the orders Coleoptera, Diptera, and Lepidoptera. Tables show the
current classification of 67 Bt subspecies and the large number of genes
coding for the insecticidal crystalline proteins. A review of Bt metabolites
found in commercial products concludes that they pose no hazards to
humans or the environment. Chapter 2 reviews data elucidating the mechanisms
by which Bt exerts its toxic action on susceptible insect larvae. Data
on insect populations that are resistant to Bt are also briefly considered.
Chapter 3, which focuses on the survival and activity of Bt in the environment,
compares habitats where Bt subspecies occur naturally with treated habitats.
Particular attention is given to the ability of Bt to form endospores
that are resistant to inactivation by heat and desiccation and that
persist in the environment under adverse conditions. A chapter on commercial
production describes methods of production and general patterns of use
in agriculture and forestry, and in large-scale programs to control
the vectors of malaria and onchocerciasis.
The most extensive chapter evaluates the large number of studies conducted
to assess the toxicity of various preparations containing insecticidal
crystalline proteins, spores, and vegetative cells. Laboratory studies
in a range of species have failed to demonstrate toxic or pathogenic
effects. Field studies have likewise failed to demonstrate adverse effects
on birds, earthworms, fish, other aquatic vertebrates, and nontarget
aquatic invertebrates. An evaluation of effects on humans draws on studies
in volunteers, case reports from occupationally exposed workers, and
extensive data from countries where Bt products are added to drinking
water for mosquito control or used to treat rivers for blackfly control.
On the basis of this review, the report concludes that Bt products
are unlikely to pose any hazard to humans or other vertebrates or to
the great majority of nontarget invertebrates, provided that the commercial
product is free from non-Bt microorganisms and biologically active products
other than the insecticidal crystalline proteins. The report further
concludes that Bt products can be safely used for the control of insect
pests of agricultural and horticultural crops and forests. These products
are likewise judged safe for use in aquatic environments, including
drinking water reservoirs, for the control of mosquito, blackfly, and
nuisance insect larvae.
The report stresses, however, that vegetative Bt has the potential
to produce Bacillus cereus-like toxins whose significance as a possible
cause of human gastrointestinal disease remains unknown.