COST862: Bacterial Toxins for Insect Control
Important agriculture pests and vectors of diseases are insects. Chemical control has become less acceptable as concerns about insecticide residues in food and drinking water have lead to the implementation of restrictions on their use in Europe. This, and the development of resistance among target populations, has increased demand for the development of alternatives. Biological control is such an alternative. In 2000 it accounted for 2% of the overall sales with annual growth rates of up to 20%. In Europe more than half of the sales are beneficial insects, also due to the fact that these organisms do not need registration in most countries. Microbials accounted for approximately 19%. Major microbial product is the insecticidal bacterium Bacillus thuringiensis (Bt). The WHO concludes that "owing to their specific mode of action, Bt products are unlikely to pose any hazard to humans or other vertebrates or to the great majority of non-target invertebrates".
Bt produces a proteinaceous inclusion body which is toxic only to specific groups of insects. It binds to the midgut epithelial cells creating pores in the cell membranes and leading to the death of the insect. Subspecies of Bt produce a variety of different crystal toxins (Cry) with activity against lepidopteran, dipteran and coleopteran insects. Today over 100 different cry genes have been discovered. A second type of Bt toxin is the Cyt (cytolytic) toxin, which can augment or synergise the Cry toxins, enhancing the effectiveness of insect control. Other insecticidal toxin producing bacteria are within the genera Bacillus, Paenibacillus, Serratia, Photorhabdus and Xenorhabdus, of which the two latter are symbionts of entomopathogenic nematodes. This variety of toxin genes naturally by Bt and other species provide the basis for companies to create genetically engineered strains with novel toxin combinations. Such an approach is designed to delay the development of resistance in target pests, because resistance would have to develop simultaneously to several different toxins. Despite the immense diversity of these toxins only 4 subspecies of Bt have been developed to products: kurstaki and aizawai to control lepidopteran pests, israelensis to be used against mosquitos and blackflies in communities along large river systems and one strain belonging to the subspecies morrisoni (known as tenebrionis) against the Colorado Potato Beetle. These apparently ideal microbial insecticides are not yet well exploited for the use in European agriculture.
The COST Action 862 will consider constraints of Bt products and support the search for new Bt products and targets. Since 1996, a wide range of crop plants have been genetically engineered to contain a toxin from Bt. "Bt crops" now available commercially in the USA, include corn, potato, cotton and soybean. Such plants have been genetically engineered to express part of the active Cry toxin in their tissues, so they kill insects that feed on the crops. Recently, genes from P. luminescens have also been transferred into plants. This technology ensures that only those insects that attack the crop will be exposed to Bt toxins. There is no risk to other types of insect. It also ensures that the range of uses for Bt is extended to insects that feed on the roots or that bore into the plant tissues. The pest recently introduced in Europe, the corn root worm Diabrotica virgifera virgifera is such a case, which cannot be controlled by Bt suspensions and even chemical insecticides have only a limited potential. However, there is also a "downside", because the target insects are perpetually exposed to toxins and this creates a very strong selection pressure for the development of resistance to the toxins. Such resistance can develop quite rapidly when only one type of toxin is involved. In fact, the Diamondback Moth (Plutella xylostella), a cabbage pest, developed considerable resistance against Cry1Ab toxin in the Philippines and Cry1Aa, Cry1Ab, Cry1Ac and Cry1Fa in Hawaii and Pennsylvania. The development of resistance to Bt toxins is, however, particularly unfortunate. Bt toxins are more pest-specific and environmentally safe than conventional pesticides, yet as effective against problem insects. For these reasons, commercial spray formulations of Bt are available to organic farmers and are one of their most valuable biocontrol tools. If Bt-based products become ineffective due to resistance, organic farmers will have lost this irreplaceable resource.
The COST Action 862 will search for solutions to prevent the development of resistance against Bt toxins, whether applied by spraying or used in transgenic crops. Bt is a valuable natural resource with a 40 year proven track record as an environmentally safe biopesticide. Developments at national and international level in future will determine whether it is deployed in transgenic plants in the EU. COST Action 862 will contribute to the discussions and decision finding process and will contribute in the dissemination of information of bacterial toxin safety among consumers and other stakeholders. Since the use of cry genes in crop plants the exploitation of Bt as conventional spraying product has been neglected. The benefits and safety as a spray product are firmly established. Today, Bt products are produced by two larger companies based in the USA, which are owned by two Japanese consortia. Due to limited competition, prices for Bt products have increased and the access to product supply for smaller distributors in Europe is severely reduced. This situation has motivated several small companies in Italy, Germany, Spain and France to develop Bt-based products. However, they do not yet have the critical size to meet the increasing registration demands for new products focussing on European pest insects. COST Action 862 will organize cooperation between these companies on aspects of production and formulation, use of novel toxins and connect with registration authorities to review the registration requirements. It can also produce the environment for a more intensive collaboration between the private and the scientific sector to gather information on safety and efficacy and communicate the results to the consumers. Btk products had originally been standardized on the basis of spore counts, which did not relate to total insecticidal activity. In the 1960s, potency was expressed in biological units based on an insect bioassay expressed as the number of International Units (IU) per unit product determined against the cabbage looper, Trichoplusia ni. Supplies of this important research tool are exhausted. Therefore, a universally available standard is urgently needed for scientific and commercial purposes and also for standardisation of products based on other Bt subspecies. COST Action 862 will contribute to the increase in product quality by supporting the development of innovative quality control methods and agreements on standards.