Objectives of COST Action 850

Background

Approximately half of global food production is lost annually before it reaches the consumer. About 40 % of production is lost before harvest, mainly to pests, diseases and weeds (Oerke et al., 1994). Chemical control, even where it is available, successful and relatively benign, has become less acceptable, because of greater appreciation of its environmental impact and concerns about chemical residues in food. This has increased demand for (and allocation of funds towards) the development of alternatives. The alternative pesticidal agents are natural enemies.

Symbioses: Nematodes (Phasmarhabditis, Steinernema, Heterorhabditis) that kill slugs or insects have specialised bacteria (Xenorhabdus, Photorhabdus) that live with the nematodes and are usually essential for the nematodes to kill their prey. Other bacteria living intimately with those same nematodes or their insect hosts (e.g. Wolbachia sp.) may influence sex ratios or otherwise manipulate nematode and insect life histories. These intimate associations (symbioses) of biocontrol organisms are the subject of the proposed COST Action. The Action will bring bacteriologists, nematologists, entomologists, molluscologists, biochemists and a number of kinds of molecular biologists together with industrialists with the aim of integrating their efforts to understand and exploit the peculiar attributes of symbioses.

Greater understanding of symbiotic complexes has become important because of the need to:

understand and control the host specificity of natural biocidal symbioses,
recognise novel bioactive molecules such as those that mediate symbioses,
manage symbiotic complexes better in industrial production and quality control,
create symbiotic complexes that can occupy new niches for biological control and environmental tolerance and
exploit the broader potential of symbiotic parasitic nematodes as delivery systems for highly targeted pesticides.

Biocontrol nematodes have been produced and marketed in Europe for the past ten years. Currently there are seven European biocontrol companies marketing 21 nematode-based biocontrol products (Blum, 2000). These products are used with success to control vine weevil, a major pest in nursery stock and soft fruit production throughout northern and middle Europe. Another substantial biocontol nematode market is for controlling larvae of mushroom gnats (Sciaridae) in mushroom cultivation (nine products from seven European producers). Other developing biocontrol nematode markets are for controlling beetle grubs (Phylloperta sp.) in amenity turfgrass and the use of Phasmarrhabditis hermaphrotida to control slugs in domestic gardens.

Bacterial symbionts: Bacteria function as internal and external symbionts in a broad spectrum of plant and animal hosts, ranging from protozoa to legumes to vertebrates. The molecular interactions between bacterial symbionts and their hosts are under study by several of the experts consulted. So far, only a small number of economically or biotechnologically important internal symbionts are well understood. Those (e.g. nitrogen-fixing bacteria and the plant transformation vector Agrobacterium tumefaciens) have, however, yielded quite extraordinary biotechnical advances.

Nematode symbionts: The slug and insect killing symbiotic nematode/bacterium symbioses already mentioned are the second most important industrially produced and internationally traded natural enemy, after Bacillus thuringiensis. They are the only industrially produced agents that are capable of actively seeking out target pests in soil. The nematodes deliver their natural bacterial symbionts to the insect's blood system and the insects die of bacterial septicemia. These nematodes could also be used to deliver other biocidal agents to the target pests.

Nematode/external bacterium symbioses: These three genera (Heterorhabditis and Steinernema, and Phasmarhabditis) of biocontrol nematodes are obligatory and lethal parasites of their respective hosts. The bacterial symbiont is carried outside of the nematode cells. It is required to kill the host and to digest the host tissues, thereby providing suitable food for multiplication of the nematodes.
Both bacteria and nematodes play complex parts in suppression and overwhelming the immune responses of the host. All three nematode genera can be mass produced in culture media containing
their symbiotic bacteria.

The symbiotic relationship between species of Heterorhabditis and their unique symbiot ic bacteria is very specific. Heterorhabditis grows best on its own symbiont, and only cells of that symbiont are packaged and carried between insect hosts, in the intestine of the infective migratory stage nematode. In the case of Steinernema the symbiotic relationship is a little less specific, and in Phasmarhabditis, the strength of the nematode bacterial symbiosis and its role in the pathogenic process is less clear. Unlike the other two species, Phasmarhabditis can retain many different species of bacteria in its intestine and it can grow and reproduce on a wide range of bacteria.
However, of nine strains of bacteria isolated from the intestine of Phasmarhabditis, only two were pathogenic when injected alone into the blood system of slug hosts. Further, Moraxella osloensis, the bacterium which produced the most pathogenic nematode/bacterium combination, was not toxic when injected alone into the slug haemocoel (Wilson et al., 1995).

Although the importance of the symbiosis between biocontrol nematodes and their symbionts has long been recognised, it is only in the past 2-3 years that a concentrated effort has been made to isolate and characterise the entomotoxins produced by the symbiotic bacteria (reviewed by ffrench-Constant and Bowen, 1999). These entomotoxins are proteins encoded by several genes.
They are secreted by the bacterium into the insect haemocoel, but some of these toxin complexes also show oral activity against insects. The insecticidal effects of the symbiotic bacteria also involve secreted proteases, lipases, chitinases and lipopolysaccharides. These bacteria also produce several broad spectrum antibacterial and antifungal antibiotics (reviewed by Forst & Nealson, 1996).

Internal Bacterial Symbionts: The a-proteobacterium Wolbachia pipientis is a very common cytoplasmic symbiont of insects, mites and terrestrial isopod crustaceans. W. pipientis has evolved a variety of mechanisms which enhance its transmission. Its most common effect is to cause crossing incompatibility between infected males and uninfected females. This phenomenon has been extensively studied in Drosophila and in the parasitoid wasp Nasonia. Other mechanisms include the induction of parthenogenesis in which infected virgin females produce daughters, feminisation in which infected genetic males reproduce as females, and male killing in which infected male embryos die while female embryos develop into infected females. Recently a virulent strain of Wolbachia has been identified in Drosophila melanogaster. It is quiescent during the fly's development, but starts to multiply rapidly in adult tissue, causing degeneration of a variety of tissues, resulting in premature death. Wolbachia bacteria have been detected in the majority of filarial nematodes tested so far (reviewed by Taylor and Hoerauf, 1999) - filarial nematodes are important pathogens of humans and animals. This finding is directing attention to a new kind of nematode/bacterial symbiosis that may have major possibilities for both insect and nematode pest management, and for novel molecules.

So these symbiotic partnerships produce immunity suppressants, toxins, novel signalling chemicals,
and exploit highly specialised niches. They need to be studied in a wide collaboration, because other biologically active molecules (e.g. Cry of Bacillus thuringiensis) may find application along with nematode toxins in pest control. This COST Action will encompass diverse disciplines and focus on how symbiotic complexes can be manipulated and managed for the effective biocontrol of insect and slug pests.

This new Action is necessary

to gain a better understanding of symbiosis biology;
to extend the usage of biocontrol nematodes and identify new target pests and markets for them;
to gain a better understanding of biotic interactions in the soil, because successful biocontrol relies on and is affected by a range of biotic interactions in the environment where the biocontrol agents are used and
to maintain European competitiveness. At present Europe is a world leader in the research, development and application of biocontrol nematodes. The biocontrol industry is a knowledge-based industry. The European biocontrol companies (which are all SMEs) rely on European researchers to elucidate the biology and genetics of biocontrol nematodes in order to exploit the biology of these agents more fully and efficiently in biocontrol. This research can be best coordinated and disseminated by means of a COST Action.