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



elude that the infestation is at least of several years' duration. The 

 same thing is, of course, true of the records from France, so that 

 we dare not assume (5) that the disease in continental Europe is 

 traceable to recent introduction from Great Britain. 



Since Morgenthaler has found mites on bees which were not recog- 

 nized by their owners or by inspectors as diseased, this has given 

 rise to the idea that the mite is often not a serious pest or that it is 

 quite harmless in most cases. For some reason this interpretation 

 has been put on his work, but not on that of Rennie, who has de- 

 scribed the same things. This does not seem to agree with the 

 present ideas of Morgenthaler in the slightest degree. It is quite 

 true that he has found mites where he himself has not been able to 

 detect the ordinary symptoms of the Isle of Wight disease, but so has 

 Rennie. In a private communication (January 15, 1923) he states 

 that he sent dead bees from the Liebefeld Station apiary on which he 

 found mites to Rennie and that Rennie found that from 3 to 5 per 

 cent of the bees showed mites in various developmental stages in their 

 tracheae. 



I have been unable to confirm this finding through my own investigation, 

 although I have dissected many such bees. I find the tracheae always clean. I 

 am, however, now about to test this question with new and better material. 



Regarding the assumed harmlessness of these mites, Morgen- 

 thaler has the following to say {4-4-) '■ 



The harmlessness of the parasite has been erroneously asserted from these 

 findings. I may here allude to only two points which must be regarded in the 

 examination of this phenomenon. First, the bee colony possesses a series 

 of contrivances for defense against the parasite, so that it does not succumb 

 without a struggle to the fljst attack. The most powerful of these means of 

 defense is found in the constant renewing of the inmates of the colony through 

 the death of the old infested bees and the emergence of young healthy ones. It 

 is therefore made very difficult for the parasite to obtain a foothold, notwith- 

 standing that it can probably be found for a long time in the colony. Only 

 when it invades the colony in overwhelming numbers because of special cir- 

 cumstances does it get the upper hand. 



Secondly, however, it is very likely that the apparently healthy colonies 

 containing parasites after all are not entirely normal. By rigorous investi- 

 gations it would indeed be shown that many remain in colony strength below 

 what one would expect of them. Exact weighings and measurements * * * 

 would show clearly that for many apiaries which outwardly do not give an 

 impression of disease, the loss of flight bees is too great. The question would 

 here also have to be examined whether the lack of swarming is not also due to 

 infection, concerning wjiich many beekeepers have complained these last years. 



Throughout this lecture, Morgenthaler deals both with Nosema 

 apis and Acaravis woodi as parasites which under certain circum- 

 stances, not understood, become harmful, so that these remarks are 

 not to be interpreted as applying solely to the mite. The experience 

 with Nosema ajns in the United States would appear to support his 

 contentions for that parasite. 



It would therefore appear from the findings in Switzerland that 

 whereas Acarapis woodi may produce a chronic and damaging dis- 

 ease, it can also at times become dangerous to as great a degree as 

 has been described for England. It is also clear that the outbreaks 

 of the virulent form of the disease can not be attributed, as some 

 (6) have attempted to claim, to recent importations from England, 

 for Morgenthaler specifically states regarding these severe outbreaks 

 that " importation of bees from England has nowhere occurred." 



