6 CIRCULAR 18, U. S. DEPARTMENT OE AGRICULTURE 



SOME FACTORS THAT HAVE PROBABLY CONTRIBUTED IN 

 CONTROLLING THE HORSEFLIES 



There has been a general decrease in the number of horseflies on 

 the escarpment since 1914. Several factors have entered that might 

 have augmented this decrease: however, it is believed that the 

 general distribution of the parasite, supplemented by egg collections, 

 has been one of the potent factors. Of the several thousand speci- 

 mens reared from collected egg masses. Phanurus emersoni has been 

 the only egg parasite recognized. Several predators of the fly eggs, 

 larvae, pupae, and adults have been observed, but none of these 

 has been recognized as having any considerable value in keeping 

 the tabanids in check. 



Undoubtedly climatological factors have had a considerable effect 

 on the breeding of the flies and their enemies. A very severe drought 

 during the years of 1917 and 1918 decreased the fly -breeding area by 

 drying up the streams except in the heads of the valleys. This con- 

 dition has interfered with obtaining more accurate data as to the 

 benefits that might be expected from the work of the parasite. There 

 was a general outbreak of tabanids during the summer of 1920. about 

 75 per cent as many as in 1914. The summers of 1913 and 1919 were 

 characterized by heavy summer rainfall. The streams in the area 

 have flowed as freely during most years as during 1913 and 1919. but 

 when the summers have been characterized by light rainfall and a 

 high percentage of sunshine the tabanids have decreased. It is esti- 

 mated that the parasite has destroyed 50 per cent or more of the 

 tabanid eggs during the most favorable seasons for its work, and 10 

 per cent or less in the most unfavorable seasons. 



CONCLUSIONS 



Phanurus emersoni has been a potent factor in the control of 

 tabanids along the south escarpment of the Edwards Plateau, north 

 of Uvalde. Tex. 



The parasite does its most effective work during seasons with a 

 high percentage of sunshine. 



The artificial dissemination of the parasite has apparently estab- 

 lished it more generally over the escarpment on which its host breeds. 



Egg-parasite rearing and dissemination, augmented by tabanid egg 

 collecting (under conditions to be determined by the percentage of 

 tabanid eggs parasitized, the number of tabanid eggs present, and the 

 accessibility of the eggs for collecting), is a feasible method of taba- 

 nid control under certain climatic and physical conditions. 



