Crithidia are members of the trypanosome protozoa. They are parasites that exclusively parasitise arthropods, mainly insects. They pass from host to host as cysts in infective faeces and typically, the parasites develop in the digestive tracts of insects and interact with the intestinal epithelium using their flagellum. They display very low host-specificity and a single parasite can infect a large range of invertebrate hosts . At different points in its life-cycle, it passes through amastigote, promastigote, and epimastigote phases; the last is particularly characteristic, and similar stages in other trypanosomes are often called crithidial.
Crithidia bombi is perhaps the most well documented species and is a parasite of bumblebees. Other species include C. fasciculata, C. deanei, C. desouzai, C. oncopelti, C. guilhermei and C. luciliae. C. deanei is atypical of the Crithidia genus, and it has been argued not a member of the Crithidia at all. It is also not typical of trypanosomatids because of its unusual shape and it harbours endosymbiotic bacteria .
These parasites may be at least partially responsible for colony collapse in wild bee populations.They cause the bees to loose their ability to distinguish between flowers that contain nectar and those that don't. They make many mistakes by visiting nectar scarce flowers and in so doing, slowly starve to death. Commercially bred bees are used in greenhouses, to pollinate, for example, tomatoes and these bees typically harbor this parasite, while wild bees do not. It is believed that the commercial bees transmitted the parasite to wild populations in some cases. they escape from the greenhouses through vents and a simple mesh could help prevent their escape.