The addition of wild stock from native
populations of natural enemies may result in
the inadvertent introduction of endemic
pathogens. In some cases, native populations
of natural enemies are known to harbour
pathogens (Geden et al., 1995) but little is
known regarding the prevalence and effects
of endemic pathogens in most cases. It may
be argued that endemic pathogens are of lit-
tle concern in arthropod mass rearings.
However, pathogens in natural field popula-
tions may cause problems in mass rearings if
diseased wild stock is inadvertently added
to the rearing system.
Infected arthropods that are used as food
or hosts in mass rearings may be unsuitable
for the completion of predator or parasitoid
development (Laigo and Tamashiro, 1967;
Beerling et al., 1993; Brooks, 1993). Host
arthropods should be examined routinely to
ensure that they are free of pathogens.
Furthermore, to ensure the success of a bio-
logical control programme, releases should
be made more frequently in areas where
endemic hosts are known to be infected
(Sajap and Lewis, 1988; Schuldet al., 1999).
The release of infected natural enemies
into a previously disease-free environment
may result in the introduction and estab-
lishment of the disease in field populations
where it did not exist previously
(MacFarlane et al., 1995). Therefore, natural
enemies should be screened to ensure that
they are free of pathogens prior to their
release (Geden et al., 1995; Bjørnson and
Keddie, 1999). Once released, there is no
guarantee that pathogen-free natural ene-
mies will remain free of pathogens. It is
possible that they may encounter endemic
pathogens native to the site in which they
are released and become infected with
them. However, pathogen-free natural ene-
mies are more likely to survive and become
established following release than are
infected natural enemies (Dunn and Andres,
1980).
In experimental trials involving life-
history studies, it is important to ensure that
test arthropods are free of pathogens if the
interpretation of data is to be meaningful. In
some cases, the presence of pathogens in
experimental animals may explain some of
the discrepancies in the literature in respect
of life-history traits (Zchori-Fein et al., 1992).
Furthermore, every effort should be made to
test and release pathogen-free arthropods in
preliminary trials when natural enemies are
being evaluated for biological control poten-
tial. Mortality and reduced performance of
diseased individuals may result in the misin-
terpretation of test results (Dunn and
Andres, 1980).158 S. Bjørnson and C. Schütte
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