4 Department Circular 287, U. S. Dept. of Agriculture. 



its present parasitic habit as an invader of colonial honeybees, or 

 whether, as Bouvier (i^) suggests, it was first a parasite of solitary 

 bees. It has not so far been found in solitary bees. The only prac- 

 tical bearing which this question might have is that if it were actu- 

 ally found that the mite exists in solitary bees at this time, the con- 

 trol of the Isle of Wight disease might be much more difficult (60) . 

 Eennie reports (50) that he and his associates have examined a con- 

 siderable number of other insect species but have found the tracheae 

 always clear of mites. The parasite may also have been a plant 

 feeder, as some have suggested, but there seems to be no evidence of it 

 (SO) , except that other tarsonemid mites have this habit. Migratory 

 nymph stages of related species of tarsonemid mites have been found 

 in tracheae of other insects, but it has usually been assumed that this 

 is a mere accidental and fatal situation for these mites, and that they 

 have entered the spiracles after attaching themselves to the insects, 

 as migratory nymphs of many mite species do. A modification of 

 some such behavior may account for the acquisition of a parasitic 

 and pathogenic habit for Acarapis woodi. The course of evolution 

 of this mite is a question which appears to have little practical sig- 

 nificance. 



METHOD OF ATTACK. 



Acarapis woodi apparently enters the body of the honeybee only 

 through the first thoracic spiracles on either side of the thorax, 

 these being larger openings than any other spiracles along the side 

 of the body. Queens, workers, and drones seem equally susceptible 

 to attack, and the mites may enter either one or both sides of the 

 thorax. The structure of the parts of the bee concerned in this in- 

 vasion has been studied by Snodgrass {56) , so that it is useless to go 

 into detail here. All developmental stages of the mite have now 

 been found by Eennie {50)_ and Ewing (^^), indicating that it is 

 able to pass its entire life in this situation. No migratory nymph 

 stage has been described for this species, such as is described for 

 some related species. This species has not so far been found except 

 within the thoracic trachete or as migrants from them, or on the out- 

 side of the bee, as described by Morgenthaler (^i, 4^). It appears, 

 therefore, that it is a highly specialized animal, both in structure 

 and in habit, adapted to a parasitic life, and that the disease is 

 strictly a contagious one. The word " infestation " should be used 

 for this disease, not " infection." 



The means by which these imprisoned mites feed is not whoUy 

 clear, but it is assumed, without doubt correctly, that they draw 

 their nourishment from the blood of their host. They spot the 

 tracheal trunks with feces and thus color the normally white walls 

 brown or black in a characteristic fashion, so that their presence is 

 easily detected on dissection with slight magnification. Efforts to 

 make artificially controlled inoculations of these mites have not 

 been especially successful {30) , and there is some uncertainty as to 

 the manner in which they pass from one bee to another and succeed 

 m entermg the thorax, but there can be little doubt that they do 

 this merely by crawling out of one bee and attaching themselves to 

 another, later to enter the spiracles. The adult and nymph stages 

 of both sexes are capable of locomotion, but, according to Eennie 

 {S2), only the adult fertilized female mite migrates effectively. 



