Dec. i S , 1920 Opius fletcheri, Parasite of the Melon Fly in Hawaii 435 



from all cucumbers collected during 191 8 was 18.1 per cent. Parasitism 

 from larvae developing in cucumbers collected in the first eight months of 

 1 91 9 amounted to 7.3 per cent. These records were secured from only 

 those larvae that emerged from the cucumbers the first two to four days 

 after collection. Larvae emerging after this time would not give a true 

 representation of parasitism under field conditions, because at the 

 time they were collected they were comparatively small and had been 

 subject to parasitism only a short time. These cucumbers were specially 

 selected by the collector as being the most heavily infested ones in the 

 fields. Considering the fleshy nature of cucumbers and the fact that 

 those from which these data were obtained were from 4 to 10 inches 

 long, it is remarkable that Opius fletcheri is able to destroy such a high 

 percentage of the melon-fly larvae developing in them. 



Considerable effort has been made to establish a series of records a 

 comparison of which would show the amount of infestation by the melon 

 fly from time to time and which would determine the extent of control 

 exerted by Opius fletcheri. Infestation records of the Mediterranean 

 fruit fly have been secured by recording the average number of larvae 

 per fruit, this average being obtained from a large number of fruits of 

 the same species. The great variation in size of cucumbers made this 

 method impracticable, and the following method was used: All cucum- 

 bers that were collected for records of parasitism were weighed and then 

 held until all the melon-fly larvae had emerged. Accurate records of 

 these larvae were kept, and at the end of December, 191 8, and of August, 

 191 9, the average number of larvae per pound of host fruit was obtained. 

 From July to December, 191 8, inclusive, 200 pounds of cucumbers were 

 collected, which contained 47,888 melon-fly larvae, or an average of 239.4 

 per pound. From 337 pounds of cucumbers, collected during the first 

 eight months of 1919, 57,921 melon-fly larvae were secured, giving an 

 average of 172 larvae per pound. These averages indicate that the 

 melon-fly infestation of cucumbers in and about Honolulu was approxi- 

 mately 28 per cent less during the period from January 1 to August 

 31, 1919, than it was between July 1 and December 31, 1918. 



It appears from observations of melon-fly infestation in Hawaii made 

 during the past several years that this decrease in the numbers of the 

 melon fly is due to a great extent to the activities of Opius fletcheri. 

 Before this parasite was introduced into Hawaii in 19 16 it was almost 

 impossible to find a cucumber in the Honolulu markets that did not 

 show more or less evidence of attack by the melon fly. From observa- 

 tions made by them in 1915 and 1916, Back and Pemberton state ' 

 that one rarely sees cucumbers offered for sale in the Honolulu markets 

 that do not show some evidence of attack, even when carefully selected, 

 and that during midwinter 150 out of 152 cucumbers ready for market 



1 Back, E. A., and Pemberton, C E. the melon ply in Hawaii, U. S. Dept. Agr. Bui. 491, 64 p., 

 24 pi., 10 fig. 1917. Bibliography, p. 57-64. 



16917°— 20 3 



