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Status of Pollinators in North America
Varroa Mite (Varroa destructor)
The supply of healthy and affordable honey bee colonies for crop pollination clearly has been threatened by the arrival of parasitic mites Varroadestructor and Acarapis woodi. Since 1981—just before the arrival of A.woodi—stocks of honey bee colonies in the United States have declined by 39 percent (Figure 2-1; USDA-NASS, 1995, 1999, 2005, 2006). Parasitism by mites of honey bees is a relatively recent problem in North America. A 1980–1982 survey of samples from 4,400 apiaries in the United States and Canada revealed no evidence of mite infestation (Shimanuki et al., 1983). The varroa mite was first reported in the United States in 1987 (Anonymous, 1987) and within a decade it had become established throughout the United States.
Varroa destructor (Anderson and Trueman, 2000) has caused dramatic declines in honey bee abundance in North America and throughout the world (DeJong, 1990; DeJong et al., 1982a; Sammataro et al., 2000). The varroa mite is an obligate external parasite of A. mellifera and Apis cerana (eastern honey bee) that was first described as V. jacobsoni (Oudemans, 1904) in Java. It exists there in a stable and sustainable association with A. cerana, its native host (Rath, 1999). In eastern honey bee colonies, female varroa mites reproduce almost exclusively on male (drone) larvae or pupae (Koeniger et al., 1983), so they do not affect the population size of the female worker force. The biology of A. cerana, including its relationship with the varroa mite, is discussed by Kevan et al. (1996) and by Oldroyd and Wongsiri (2006).
The association of V. destructor with the western honey bee, A. mellifera, reportedly began in the 1950s (Matheson, 1995) when the mites moved into honey bee colonies brought into the home range of A. cerana. Subsequently, the varroa mite has established a nearly cosmopolitan distribution with respect to its new host, and Australia is now the only mite-free continent (Matheson, 1995). It is not known how this parasite entered the United States.
In A. mellifera, female varroa mites reproduce on both worker and male larvae. Infestation of honey bee colonies of European origin (the source of most A. mellifera introduced to North America) is fatal if untreated, and colony mortality usually occurs 6 months to 2 years after the initial infestation (DeJong, 1990).
Newly emerged adult worker bees parasitized as pupae exhibit a range of symptoms: substantial loss of adult weight (DeJong et al., 1982a,b; Engels and Schatton, 1986), reduced concentrations of serum proteins (Engels and Schatton, 1986), impaired development of (brood food-producing) hypopharyngeal glands (Schneider and Drescher, 1987), severe deformations of