13 



the moths begin to emerge in the jelly-glasses the orchardist should 

 expect the larvae of the second generation to put in an appearance, 

 and he should be prompt to spray accordingly. The same method 

 may be followed for the third generation also. 



This program may seem a little complicated, and does undoubtedly 

 require careful and constant attention, but since it will enable the 

 fruit grower to get for himself the information which he most needs 

 in regard to the hatching of the larva? in his orchard long enough 

 beforehand to enable him to adjust his schedule of spraying opera- 

 tions to the actual facts of the season, it seems to us well worth the 

 effort which it will require. There is this difficulty, of course — that 

 the weather of the season may be so extraordinary that even this close 

 watching will not give wholly dependable results, and for this, as we 

 have already said, there is no remedy except expert observation in 

 the general interest. It must be, however, continuous work at three 

 or more fixed stations, and not casual field observation by a traveling 

 agent. 



There is one point of which a good deal was made in the pre- 

 liminary circular of last spring, but of which we have made no men- 

 tion in this paper ; that is, the fact that in determining the effect of 

 temperature on the codling-moth, or any other insect, all low tem- 

 peratures must be dropped out of account — all temperatures at and 

 below a certain minimum called the zero of development — since in- 

 sects remain physiologically dormant at these lower temperatures. We 

 have ignored this matter here because this developmental zero has 

 never been accurately determined for the different stages and the 

 different generations of the codling-moth, and we have thought it 

 best to avoid all use of hypothetical guess-work in this purely prac- 

 tical paper. We are happy to say, however, that Ave are now getting 

 an equipment which will enable us to take up this subject for in- 

 vestigation next spring — an equipment in the new vivarium building 

 at the University, by means of which we can place an insect in any 

 of its stages at any degree of temperature and of atmospheric hu- 

 midity and either hold it right there during its whole life or vary 

 these conditions from time to time at our own pleasure. By this 

 means we shall be able to tell by precise experiment at just what de- 

 grees of both conjoined developmental activity and growth begin and 

 continue, and just how they are advanced and stimulated by additional 

 degrees up to the optimum for each insect species and stage. When 

 these facts are known, the practical problem of following and fore- 

 telling the progress of the codling-moth in any season will be greatly 

 simplified and facilitated, and the orchardist will be much better able 

 than now to put his labor and his money in at just the points where 

 and when they are needed and thus to get more certain results than 

 are now possible, and at much less cost. 



