Importance of Parameters in the Selection Process

Above I have described how crucial the selection the MPCA is and what the main considerations should be. But which parameters should weigh the most for which pathogen? I will indicate the importance of the parameters for the different pathogens in Table 2.2 and explain briefly why that is. Three areas are considered, the first concerns efficacy, the second production efficiency and the third safety (Fig. 2.1).

The traits related to efficacy are:

• dose-mortality response; the dose is critical in fungi, EPNs and viruses since these organisms are difficult to produce and therefore expensive. Bacteria are relatively easy to produce and, thus, the dose is less critical;

• host range; in baculoviruses the host range is narrow, so apart from screening isolates on the target pest, investigations to find out what the host range is are not really useful. For the other pathogens, this is relevant and it could be useful to know other potential targets and markets;

• survival and persistence; survival of EPNs in soil is usually limited and therefore can play a lesser role in screening tests. It is laborious and expensive to test. In bacteria, baculoviruses and fungi, and at foliar applications where 100% targeting is usually impossible, persistence of infective propagules adds to the total mortality. A long persistence increases the chance of contact or ingestion by a target insect. Few studies, however, have reported on the importance of this fact and personally I do not consider it a key trait. Persistence usually ranges between a few days and a week, and is also dependent on the formulation. In glasshouses, rain and UV, usually factors that reduce persistence, are not relevant;

• abiotic factors; mainly temperature and relative humidity (RH) can limit the activity of the MPCAs. Bacteria and baculoviruses have to be ingested and only become active inside the insect. In this case, temperature affects the insect host activity more than the MPCA and selection for a certain temperature range is accordingly less relevant. For EPNs, however, it is very important to know their temperature range. If it is too cold, the nematode is not active, does not search or cannot penetrate the host, at high temperature the same happens or the EPNs even die. In fungi, the RH is the main abiotic factor taken into account as fungi need a high RH for germination and this should be studied in the selection assays.

Production-related considerations are:

• for use of a pathogen as a biopesticide it is necessary to have infective propag-ules that can be applied to a crop. These are usually spores for bacteria and fungi (bacterial cells and mycelial parts are also possible), occlusion bodies for bac-uloviruses and infective juveniles or dauer larvae (L3) for EPNs. Spore-producing bacteria are generally easy to produce and formulate, while fungi are more difficult and more expensive to produce. Baculoviruses are produced in vivo and production is both laborious and expensive. EPNs can be produced in vivo and in vitro; production is both complicated and expensive. For all pathogens it is important to investigate how easy or difficult it is to produce them. This is especially true for the production of infective propagules. In addition to ease of production, a long shelf-life is highly desirable. For example, whether a bacterium can make spores or not influences the selection because spores are stable and can be stored for a long time.

Safety-related considerations are:

• toxicological characteristics; the different types of pathogens vary in potential for harmful effects towards humans and the environment. In registration, safety to the applicator and worker in the crop are the main concern. Regarding bacteria this is critical, because many of them make metabolites and many of the ones used for biopesticides are closely related to harmful bacteria such as Bacillus and Serratia species. B.t is closely related to B. cereus, a food poisoning bacterium for humans. For fungi this is generally less important, although many of them make metabolites. Baculoviruses and EPNs, including their symbiotic bacteria, are specific, and safety to humans needs little attention in the selection process; • environmental effects; the potential of harmful effects of the pathogens on nontarget organisms are usually relatively small. Bt's and baculoviruses have a narrow host range and are quite specific. Fungi and EPNs can have a broader host range and that needs to be studied further. Effects on natural enemies need to be known too, not only for the registration dossier, but also for use in IPM programmes. Depending on the organism and its mode of action (metabolites) there may be concern for aquatic organisms, although serious effects are not known with the current approved biopesticides.

In Table 2.2 I have estimated the importance of the described traits in the selection process. A trait that is critical for the success of the pathogen needs to be studied with great care, such as relative humidity requirements (abiotic factors) for a fungus.

Table 2.2 Importance of traits in screening MPCAs for biopesticide development

Trait Mortality Production Toxicology

Host Abiotic Effects on Non-target

MPCA Dose range Survival factors Propagules humans effects

Host Abiotic Effects on Non-target

MPCA Dose range Survival factors Propagules humans effects

Bacteria

++

++

+/-

+/-

++

+++

+

Fungi

+++

+++

+

+++

+++

++

++

Viruses

+++

+

+

+

+++

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