EXPERIMENT STATION BULLETINS. 657 



weekly until May 1, after which the field trips were made more often. 

 During these observations trash in the college orchard and woodlot and 

 along fence rows was carefully looked over. Sweepings were also made 

 with an insect net. . 



On April 3, a species of leafhopper was collected in the woodlot which 

 was of the same size and degree of delicacy as Empoasca mail and this 

 led to the belief of the possibility of the latter also wintering over in the 

 adult stage. Tl)e first specimen resembling the potato leafhopper was 

 collected on May 7. This individual was green and had the charac- 

 teristic white spots on the anterior margin of the prothorax, but did 

 not have the white "H" marking on the mesothorax. Wing venation 

 showed that the insect belonged to the genus Empoasca. Although the 

 markings were not quite complete, it is quite probable that the specimen 

 was of the mali species. The first authentic specimens of Empoasca m^ali 

 were found on May 28. Twelve adults, all with the characteristic mark- 

 ings and wing venation were collected in a clover cover-crop in a young 

 orchard. 



That these leafhoppers did not 'Come from early hatclied eggs was 

 shown by life history studies made on the rose leafhopper, Empoa rosac, 

 which passes the winter in the egg stage. The first nymphs of this leaf- 

 liopper to hatch outdoors from over-wintering eggs were observed on 

 May 14. Three Empoa rosac nymphs were found on an apple tree in 

 the orchard and one nymph was observed on a maple tree on the campus. 

 A careful search for nymphs on other trees in the orchard failed to re- 

 veal further specimens. The average length of the nymphal period of 

 the first generation nymphs of the rose leafhopper according to Acker- 

 man^-, is 33.4 days, from which it is evident that the first nymphs to 

 hatch from over-wintered eggs on May 14 could not have been more 

 than in their second or third instars at the time when the first authentic 

 Empoasca mali adults were collected. 



The experimentation during the winter of 1919-20 ancl the collec- 

 tions, and observations during the spring of 1920 indicate that the po- 

 tato leafhoppers, Empoasca mali, hibernates in Michigan ais an adult 

 and not in the egg stage. 



NUMBER OP GENERATIONS 



Various investigators who have studied this leafhopper as a pest of 

 fruit trees, particularly the apple, have made observations as to the 

 number of generations during the season. Thus Washburn^* iu 

 Minnesota, thought there were two and possibly three generations; 

 Webster^^ recorded four generations for Iowa ; . Lathrop^^, two for New 

 York, and Ackerman^- reported three generations under Pennsylvania 

 conditions. 



In order to ascertain the number of generations of the potato leaf- 

 hopper under Michigan conditions the lengths of the incubation and 

 nymphal periods were determined by means of cages similar to those 

 used in the greenhouse experiments. (Fig. 3). Young adult pairs were 

 confined on caged potato plants and were removed when the first signs 

 of egg blisters were found. The incubation period was determined as 

 the length of time elapsing from the date the first egg scars were ob- 

 served until the appearance of the first nymphs. In the two trials 



