Parasites are considered to be efficient when they approach a commensalistic existence and do not seriously harm their host. A parasite's host is likewise considered an efficient host if it does not react strongly to the parasite. After living together under natural conditions for a long time, the parasite and host populations tend to adjust to one another. The symbiotic relationship becomes more commensalistic and the two organisms have less difficulty coexisting. Varroa mites on worker bee pupa.
Heavy infestations result in under nourished and deformed bees.
Until the 1950s, varroa was a parasite on Apis cerana , the Indian honey bee. Varroa is commensal on the Indian honey bee and causes little harm to its host. When European honey bees, Apis mellifera , were moved into the Indian honey bee's range, varroa found a new host upon which it could proliferate without the checks and balances present with the Indian honey bee. The severe parasitism that occurs when varroa infest European honey bees is not unusual for a parasite that moves to a new host. In nature, it often takes thousands of years of selection for a more commensalistic relationship based on tolerance to develop.