1996). Pathogenic effects of C. bombi appear to
be slight, resulting in reduced ovarian
growth in infected individuals and slower
colony growth at the start of the season
(Shykoff and Schmid-Hempel, 1991b).
Moreover, field-collected bumblebee workers
infected with C. bombiare less likely to forage
for pollen than uninfected workers (Shykoff
and Schmid-Hempel, 1991a). Bumblebee
mortality may increase if the host is subjected
to periods of starvation (Imhoof and Schmid-
Hempel, 1998; Brown et al., 2000).
NEMATODES.The parasitic nematode,
Sphaerularia bombiis found in several Bombus
species from Europe, North America and
New Zealand (cited by MacFarlane et al.,
1995; McCorquodale et al., 1998). Mated
female S. bombi(1–3 mm long) enter bumble-
bee queens during the autumn and undergo
developmental changes during queen hiber-
nation. Each female nematode is capable of
producing thousands of eggs, which develop
into larvae in the host haemocoel
(McCorquodale et al., 1998). Parasitized
queens fly close above the ground and occa-
sionally dig small holes into the soil, where
they expel juvenile nematodes (cited by
Horton and Moore, 1993). Infection sterilizes
the queen by arresting development of the
corpora allata. Infection does not affect queen
longevity; however, S. bombi-infected queens
usually fail to start colonies and do not repro-
duce. Infected queens forage for longer than
uninfected ones, the latter return to their
colonies sooner and forage activity is taken
over by workers (MacFarlane et al., 1995).
Management of pathogens in Bombusshould
involve routine screening of individuals for
pathogens. Preventative measures and sani-
tation are important means of preventing the
spread of pathogens. Soil for overwintering
queens should be free of fungi and dead
workers suspected of being killed prema-
turely by pathogens should be discarded
(MacFarlane et al., 1995). Although many of
the pathogens that infect Bombushave been
described from field-collected specimens,
high concentrations of bumblebees within
rearing facilities may predispose them to dis-
ease.
Interestingly, bumblebees are not suscep-
tible to some of the more common honeybee
diseases, including viral sac brood
(Moratovirus), bacterial foul brood (Bacillus
and Streptococcus) and some fungal
pathogens (Ascosphaera). It is thought that the
evolution and behaviour of bumblebees have
helped them to develop resistance to some of
the pathogenic microorganisms that infect
honeybees. However, adult bumblebees and
honeybees share some viral and protozoan
pathogens (MacFarlane et al., 1995).Summary
Reports of pathogens from both field-col-
lected and mass-reared natural enemies raise
concerns regarding the quality and efficacy
of natural enemies used for biological pest
control (Kluge and Caldwell, 1992; Zchori-
Fein et al., 1992). Some pathogens affect the
performance of natural enemies by lowering
their efficacy (Geden et al., 1995; Bjørnson
and Keddie, 1999), whereas others alter
arthropod reproduction (Majerus and Hurst,
1997; Stouthamer et al., 1999; Zchori-Fein et
al., 2000). Therefore, the release of pathogen-
free natural enemies in biological control
programmes is of the utmost importance.
Sanitation and screening procedures are
important for controlling pathogens in arthro-
pod mass rearings (see Lacey, 1997). Although
the process of eliminating a particular
pathogen from the rearing system may seem
time-consuming and expensive, preventive
sanitation and exclusion methods will help
minimize the time and expense of such pro-
grammes (Dunn and Andres, 1980). It is
important to be familiar with the types of
pathogens to which each natural enemy is sus-
ceptible and any symptoms that may be asso-
ciated with infection. Some pathogens
produce chronic and debilitating effects and
are able to remain undetected in arthropod
populations for extended periods (Dunn and
Andres, 1980; Chapman and Hooker, 1992;
Bjørnson and Keddie, 1999). Therefore, indi-
viduals should be screened for pathogens on a
routine and continual basis. Special attention
should be given to field-collected arthropods
and those exchanged among rearing facilities.Pathogens of Natural Enemies and Pollinators 157