December, '14] McGREGOR: COTTON LEAF-MINER 453 



within speciall}^ constructed cheesecloth shelters. After sufficient 

 exposure to the tender foliage, very careful examinations revealed 

 numerous punctures exactly similar to ovipositional apertures, but 

 no eggs had been deposited. 



Seasonal History 



The leaf-miner becomes noticeable in cotton leaves when the seed- 

 lings are but a few days old. Young larva; were seen the present 

 season (1913) as early as May 10. This was a late season, however, 

 and it is probable that May 1 usually marks the initial appearance 

 in cotton of the miner. From that date the infestation increases 

 rapidly until by the middle of June almost complete infestation, occurs. 

 In the season of 1912 only one computation of infestation was made, 

 namely, upon July 12, at Avhich time 84 per cent of the plants were 

 infested. 



During the present season (1913) more complete data concerning 

 infestation has been secured. From Maj' 26, when an infestation of 

 41.5 per cent obtained, the occurrence rapidly increased until on 

 June 19 an infestation of 98.7 per cent was computed. In fact it 

 can be said that during May and early June the leaf-miner is the most 

 common pest of the cotton plant in the vicinity of Batesburg, S. C. 



It is interesting to record that the activity of the leaf -miner becomes 

 markedly reduced toward the end of June in South Carolina. This 

 is probably brought about through the agency of two factors, namely, 

 parasitism and aestivation. Our observations at Batesburg relative 

 to aestivation are quite in agreement Avith those recorded by Webster 

 and Parks. Subsequent to the 20th of June, the formation of new 

 mines in cotton was reduced to a minimum. During July and August 

 abortive, unfinished tunnels are often seen, but long search is usually 

 necessary before finding well-developed mines during these months. 

 Furthermore, it was noticed that such abortive mines, when removed 

 to the laboratory for breeding, rarely gave issue to adult miners. 

 During these midsummer months, however, it was usually an easy 

 matter to find well-developed mines and larvae in leaves of Lobelia 

 cardinalis growing in shaded woods, or in leaves of the floral nastur- 

 tium {Tropaeolum minus) and of the English aster (Aster sp.) growing 

 in shaded portions of gardens. Since the larvae in these environments 

 must be equally accessible to parasitic attack, there must be some other 

 factor or factors at work in the open fields which enforces this reduced 

 activity. Through analogy with other species where aestivation has 

 been demonstrated as a factor of natural control, it is undoubtedly 

 true that midsummer temperature brings about a cessation of 

 activity. The precise point where activity ceases and aestivation 



