Certain parasites may force changes in foraging behavior. Late-instar larvae of conopid flies, which occupy much of the host's abdomen, prevent filling of the honey crop (Schmid-Hempel & Schmid-Hempel 1991); these bees concentrate on pollen foraging. On the other hand, parasitism by the protozoan Crithidia bombi is associated with reduced pollen foraging (Shykoff & Schmid-Hempel 1991). These parasites can be common. Shykoff & Schmid-Hempel (1991) found 20.2% and 35.7% infection rates by conopids and C. bombi, respectively, in bumble bees in the Swiss Alps. Schmid-Hempel & Stauffer (1998) also found that parasites affected floral preferences and switching behavior, but since both parasite load and experience may correlate with age, these changes might also have been driven directly by experience.
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