1922 



ENTOMOLOGICAL SOCIETY. 



39 



Parthenogenesis. In order to determine whether this species is parthenogene- 

 tic, six female deutonymphs were placed in separate cages and were kept isolated 

 during their entire life. Four days after reaching maturity, all the- females 

 commenced to lay eggs, and each 'deposited from 10 to 41 eggs, the average being 

 25 eggs. All the progeny proved to be males. 



Proportion of Males to Females : At no time, during the past season, did our 

 observations in various orchards show a preponderance of males over females. 

 Counts made in the orchard at different times gave an average ratio of about 1 

 male to 10 females. In the insectary, the ratio was 1 to 4. This would 

 indicate that, under normal conditions, most of the females axe fertilized. 



Number op Generations. 



The activities of the mite extended this past year from May 2nd when the over- 

 wintering eggs commenced to hatch, to mid-October, when the last egga were laid. 

 During this period — 5y 2 months — a maximum of six ("first hatched series") and a 

 minimum of three generations ("last hatched series") developed in the insectary. 

 This, of course, means that under our conditions, there are three full generations 

 and three additional partial generations. 



Overlapping op Broods. 



Except for two weeks in the spring, namely, from May 11th to 25th, when 

 only immature forms were found, all stages from eggs to adults were present in 

 the orchard throughout the season. The overlapping of the different generations 

 is shown in table No. 6. 



TABLE No. 6— Showing Number, Duration and Overlapping of Generations. 



Winter 

 1920-21 



May 



June 



July 



Aug. 



Sept. 



Oct. 



Winter 

 1921-22 



1st 



2 . 



. . ..23 





. .3 





..14. . 





Gen. 



2nd. 



25.. 







Gen. 



3rd 

 Gen. 



19. . 











4th 

 Gen 



11. . 







.14.. 







5th 30 

 Gen. 







..14.. 







6th 24. . 





..14.. 







Gen 









Period of Maximum Infestation. 



From the first appearance of the mite in early May until the middle of June, 

 the degree of infestation was very light. However, during the month of July 

 the mites increased marvellously, at this time as many as 800 to 1,200 eggs being 

 found on a single plum leaf. The mites spread throughout the trees and attacked 

 every leaf. From the third week in August to. the end of the season their numbers 

 fell even more rapidly than they had increased. The explanation of this is -noted 

 under the heading of Natural Control. 



Comparative Susceptibility of Varieties of Plums. 



Notes made on the comparative susceptibility of over fifty varieties of Euro- 

 pean plums are presented herewith in tabular form. 



