April, '14] MELANDER: SPRAY RESISTANCE 171 



The resistance of the San Jose scale, however, is of a different kind. 

 Spraying affects everj^ tenth generation or so, and it is quite unHkely 

 that an acquired immunity should make its effects felt over so long 

 a period. Sulphur-lime spraying in the orchard districts is compulsory 

 by law and so practically every scale is subjected to its action. We 

 have often noted an individual scale, or a group of scales, probably 

 brothers and sisters, still alive in the midst of a mass of dead insects. 

 Such vigorous individuals have been just as thoroughh^ wet with 

 the sulphur-lime as their neighbors, yet have escaped its action. 

 The sporadic occurrence of naturally immune individual scales finds 

 a parallel in recent work on heredity of protozoa and bacteria. Mu- 

 tants less or not susceptible to certain toxins have been repeatedly 

 found in cultures and from them have been produced immune strains. 

 Similarly, disease-resistant wheat has been produced through Mende- 

 lian segregation, the new strain acting as a pure recessive. 



The data at present at hand do not permit us to determine whether 

 resistance and susceptibility are differences of degree and not of kind, 

 or whether they behave as allelomorphs. That the mortality curve 

 of the Clarkston scales is less steeply inclined from the beginning 

 might indicate a partial immunity already possessed by nearly all 

 ^the scales, but which is pronounced enough to be an absolute immunity 

 •only in a relative few. Such a view is especially interesting when 

 we recall the reported rapid action of sulphur-lime a few years ago. 



What is the economic importance of the appearance in a locality 

 ■of a resistant strain of the San Jose scale? An alarmist might say 

 that a few such scales would soon result in a totally immune insect, 

 brought about by annual spraying. But viewed from a Mendelian 

 standpoint, the consequences are less direful. If only the resistant 

 individuals survived to reproduce then a pure line might result after 

 repeated sprayings. But always there are some scales missed by the 

 spraying, and these, during the ten generations between sprayings, 

 will produce a population in part, at least, non-resistant. If resistance 

 were a dominant characteristic there would already be a larger pro- 

 portion of immune individuals than the data show. If it is recessive 

 the crossing with scales missed by the spray would, by the end of 

 each year, produce a majority of susceptible individuals. Thus we 

 may make the strange assertion that the more faulty the spraying 

 this year the easier it will be to control the scale the next year. 



Practically, a change from sulphur-lime to an oil spray is all that 

 is necessary for effectual control. However, even in the case of the 

 oils we have noted a very few individuals that have manifested a 

 remarkable tenacity of life. Should these result in a resistant strain 

 sometime in the future it would be necessary to use both insecticides, 



