THE CANADIAN ENTOMOLOGIST 235 



NOTES ON ALLOGRAPTA FRACTA O. S. (DIPTERA: SYRPHIDAE). 



*BY W. M. DAVIDSON, U. S. BUREAU OF ENTOMOLOGY, SACRAMENTO, CALIFORNIA. 



During the spring and early summer of 1918 the writer was stationed in 

 the Imperial Valley of southern California, and was afforded good opportunity 

 to observe the habits of the predaceous fly, Allograpta frada O.S. 



Previous to the first settlement and cultivation, some twenty years ago, the 

 Imperial Valley was a flat, almost treeless, dry plain and, therefore, was not a 

 habitat congenial to Syrphidie, a family most of whose members prefer moist 

 forested localities. In 1918 with several hundred thousand acres under cultiva- 

 tion to grains, corn, alfalfa, cotton and grapes and with canals everywhere a 

 fairly rich syrphid fauna might have been expected. This was not the case, 

 and with three exceptions the writer failed to observe during five months' time 

 other than aphidophagous types and some of these, the species of Melanostoma, 

 abundant elsewhere in California were conspicuous by their absence. The 

 three exceptions above mentioned consisted of the species Mesograpta geminata 

 Say, M. marginata Say, and Ceria sp., the last-named breeding in wounds in 

 the trunks and limbs of cottonwood (Fopulus fremontii). From February to 

 July AUograpta fracta w'as without doubt the most abundant species present, 

 and the larvae were very beneficial, acting as an undoubted check upon the 

 barley and corn aphis (Aphis maidis Fitch). So mild was the winter that 

 numbers of adults were observed January 3rd, on the occasion of a visit to the 

 valley. After the middle of February, when the writer took up his duties, 

 until the end of June, when he left the valley, the adult flies were seen nearly 

 every day, often in abundance, about barley and corn fields infested with aphids. 

 The first larva was observed February 19th, and thereafter larvae and pupae 

 were to be found at any time first on barley and later on corn. The larvae were 

 especially beneficial to barley from March 15th to April 30th, at which date most 

 of the grain had ripened, and to corn during May and June. In some fields 

 they were more abundant than in others, and in those in which they especially 

 abounded it was found that abcut 25% of the infested heads had larvae working 

 on the aphids. Experiments on the number of aphids a larva could destroy 

 indicated that one could in its life-tim.e eat all the aphids on from three to four 

 heads of grain of average infestation. It therefore appeared that if at any 

 one time larvae were found to be worliing in a quarter of the infested heads in a 

 field they might be expected to wipe out in due course between 75% and 100% 

 of the infestation of aphids. In one field of 20 acres examined on a number of 

 occasions, it appeared that A. fracta was responsible for an almost total de- 

 struction of barley aphids. 



Larvae of Eupodes volucris O.S., Syrphiis america^ius Wied., AUograpta 

 obliqiia Say, and Catabomba pyrastri L. were present in the barley and corn fields, 

 but in much smaller numbers than those of AUograpta fracta. 



In the Los Angeles district of southern California AUograpta obliqua is very 

 abundant and A. fracta comparatively scarce, whereas in the Imperial Valley the 

 reverse evidently holds true. Fracta is com.mon in the San Diego mountains. 



In 1918 fracta was not bred from any other host than Aphis maidis, except 



that a single larva was taken attacking Aphis pseiidobrassiccE Davis. Aphis 



*Published with the permission of the Secretary of Agriculture 

 October, 1919 



