The bee’s susceptibility to pathogens (Gupta, 1986), although the

The effect of insecticides on the
immune systems of pollinators, whilst not heavily studied, is predominantly
negative (for review see James & Xu, 2012). An in-depth study
into the effect of three different neonicotinoids on the immunocompetence of
honey bees (Brandt et al., 2016) found that exposure to
field-realistic concentrations of all three neonicotinoids resulted in significantly
reduced counts of hemocytes (invertebrate phagocytes) within 24 hours.
Hemocytes are an important component of the insect cellular immune response as
they carry out phagocytosis, meaning that reduced hemocyte densities could increase
bee’s susceptibility to pathogens (Gupta, 1986), although the
consequences of reduced hemocyte density on the disease susceptibility in honey
bees are not fully understood (Brandt et al., 2016). Interestingly, the
findings of Brandt et al (2016) on total hemocyte count directly contrast those
of a similar study which reported no change in total hemocyte count (Alaux et al., 2010). A key difference between
the two papers was that all the bees in the Alaux et al (2010) study were
infected with Nosema apis, a
unicellular parasite fungus that is widespread in honey bees. This may suggest
that neonicotinoids interact with the immune system of infected bees
differently compared to healthy ones, although more research is needed on the effect
of pesticides as a whole on infected bees (Collison et al., 2016). Neonicotinoids have
been reported to have other impacts on the immunocompetence of honey bees,
including a significantly reduced encapsulation response by hemocytes of
foreign pathogens and the subsequent melanisation reaction to destroy them  when exposed to all the major neonicotinoid types
at field-realistic concentrations (Evans et al., 2006). Encapsulation of
foreign pathogens is positively correlated to viral infection and parasite
resistance (Washburn et al., 1996), suggesting that
neonicotinoid exposure could increase the risk of infection in honey bees from
these sources. 

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