454 JOURNAL OF ECONOMIC ENTOMOLOGY [Vol. 7 



begins has not been determined with accurac}'. However, from a 

 coordination of the most rapid development of the species with the 

 prevailing temperatures, it becomes evident that the most favorable 

 conditions exist between 65° and 75° Fahrenheit, also that the 

 reduction of the species begins with a mean temperature of 77° 

 Fahrenheit. (See fig. 29 for seasonal curve and zone of aestivation.) 

 It has not been definitely e.stablished exactly how many generations 

 develop in the season at Batesburg, but it is quite certain that 

 three broods occur on cotton prior to the beginning of sestivation. 

 Following the period of sestivation, development on cotton is dis- 

 couraged greatly by the toughening of the leaves. 



From the general economic standpoint the serpentine leaf-miner 

 as Webster and Parks have stated, is primarily an enemy of forage 

 crops. The loss to these forage crops is occasioned through the fact 

 that the useful part of the plant is the foliage. However, it is doubtful 

 whether any crop plants — including the clovers — exceed the cotton 

 plant in degree of infestation. As above recorded, we observed cotton 

 fields wherein 98.7 per cent of the plants were infested. 



Since the fruit, rather than the foliage, is the portion of the cotton 

 plant which is utilized, it is difficult to estimate the injury which is 

 occasioned through the ravages of the leaf-miner. It is probable 

 that the greatest damage occurs while the plant is very young, since 

 a few mines at that time might greatly weaken the struggling seedling. 

 In fact, the eotjdedons and the earliest foliar leaves are often shed 

 prematurely through the work of the pest. 



Parasitism 



In the course of the observations on the leaf-miner at Batesburg, 

 it became evident as the season advanced that several species of 

 parasites were at work. Only one computation of the degree of para- 

 sitism was made, which was as follows: total number of leaf-miner 

 pupse, 74; pupse parasitized, 21; percentage of parasitism, 28.4. Parks 

 found that at Salt Lake City, Utah, in September, 1911, 89.7 per cent 

 of all Agromyza individuals were parasitized. It would seem, then, 

 that natural enemies are much more of a factor of control in the alfalfa 

 districts of the West than in cotton fields in the South. 



Although no special attention was given to the rearing of leaf- miner 

 parasites it develops that 7 Chalcidids, 2 Braconids, and 1 Dijoteron^ 

 a total of 10 parasitic species — were bred during the investigations 

 of 1913. As determined by Mr. J. C. Cra^^ord, of the Bureau of 

 Entomology, these are as follows: Chalcidids — Zagr ammo soma multi- 

 lineata Ashm., Derostenus diastatce How., Derostenus 2 spp., Pleurotropis 

 sp.; Closterocerus sp., Chrysocharis sp.; Braconids — Opius (2 spp.); 

 Diptera — one species. 



