MORTALITY OF APPLE MAGGOT IN FRUIT IN STORAGE 7 



within slightly more than 30 days and seem to fix the storage period at 

 32 days. All eggs and larvae are killed within 29 days after the tem- 

 perature of the fruit in the center of the package comes within 1° of the 

 air temperature, or 33° in an air temperature of 32°. 



In commercial cold storages 40 days should provide ample time for 

 such equalization of fruit and air temperatures under various condi- 

 tions and allow an adequate safety margin. The temperature main- 

 tained should not exceed 33° F. if 32° is the approximate temperature 

 indicated for a disinf estation schedule. 



No pupae were found in the fruit of this series in examinations made 

 after larval emergence was completed. Similar results were obtained 

 in earlier experiments. 7 



As the temperature of the fruit approaches 32°, larval progress 

 through the flesh, as well as feeding, apparently stops, and the larvae 

 and eggs then enter a condition which may be termed "forced dor- 

 mancy." That larval activity and development cease is indicated by 

 the fact that fruit held 10 days at 32° snowed essentially the same 

 duration and rate of emergence after removal from storage as did the 

 control lot (fig. 4, A). There presumably was no marked difference 

 in susceptibility between insect stages. Disproportionate mortality 

 in any one stage would have modified the proportion of the several 

 stages among the survivors, which in turn would have affected the 

 periods when the maggots matured and left the fruit. When only a 

 few larvae survived (lots held 24, 26, 28, and 30 days, table 1), they 

 emerged after the fruit had been removed from storage, at a time cor- 

 responding to the median or peak emergence period in the controls. 

 If there is a difference in susceptibility, the most resistant forms seem 

 to be the first and second instars. 



Examination of fruit at the time it was removed from storage showed 

 that the larvae were dead, and not merely enfeebled or otherwise 

 rendered incapable of emerging. 



Tests at 36° F. 



The previous statements regarding the results obtained at 32° F. 

 apply to the findings in the 36° series, except that a longer storage 

 period was required to effect complete mortality (table 1). This 

 period was 45 days, or 42 days after the fruit in the center of the pack- 

 age came to 37°. A temperature of 36° is probably near the upper 

 limit of temperatures that will inactivate Rhagoletis pomonella larvae. 



Tests at 40° F. 



The response of the apple maggot to a temperature of 40° F. clearly 

 differed from that noted at 32° and 36°. Larval activity continued 

 slowly at 40°, and a few larvae emerged in the storage chamber. 



From a sample of 300 fruits held 70 days, 12 larvae emerged in stor- 

 age, or about 1 percent of the theoretical larval expectation for this 

 lot. The first larva was recovered on the eleventh day and the last 

 on the fiftieth day. 



Further evidence supporting the statement that larval growth con- 

 tinues at 40° F. is indicated in figure 4,B. Larval emergence follow- 

 ing refrigeration tended to be progressively earlier as the storage period 

 was lengthened. This suggests simply that the population in the 40° 



7 See footnote 2. 



