658 STATE BOARD OP AGRICULTURE. 



inade, the incubation period proved to be 12 and 13 days respectively. 

 Ackerman^- found tliat the incubation period of Empoasca mail in 

 Pennsylvania was from 7 to 9 days when the temperature averaged 

 70.89. The longer incuJtation ])eriod found under Micliigan conditions 

 is probably due to the cooler weather which prevailed, (62° F) and per- 

 haps somewhat to the shading caused by the cages. The average nymphal 

 period of six different broods was found to be 16 to 17 days. Under 

 Michigan conditions, approximately a month elapses from the time a 

 potato leafhopper egg is laid until the njanph develops to maturity and 

 is ready to lay eggs for the next generation. 



Assuming that the over-wintering adult leafhoppers migrate to po- 

 tato fields about the middle of June and allowing a' month for the de- 

 velopment of each generation, it would indicate that there are two 

 complete generations and a partial third generation on potatoes each 

 season in Michigan. Nymphs of the third generation are no doubt 

 killed by early fall frosts before reaching maturity. 



EXTENT OF INJURY 



The injury caused by the overwintering adults and nymphs of the 

 first generation is. slight, owing to the small number of individuals 

 present as compared to the leafhopper population of the second genera- 

 tion. The injun- is also masked by the new leaves which appear on the 

 rapidly growing vines. 



The greatest damage occurs during the latter part of July and dur- 

 ing August, because the leafhopper population by that time has in- 

 creased greatly. For each over-wintering female which flies to the 

 potato field in June there is a progeny of at least 450 individuals in the 

 second generation, from which it can be realized that the leafhopper 

 population during this period runs up into the millions per acre.' When 

 it is known that a single leafhopper nymph can cause the destruction 

 of an entire potato leaf the damage caused by the millions of individuals 

 on each acre of potatoes can well be imagined. 



EFFECT OF TIME OF PLANTING 



There appears to be a definite period in the spring during which 

 the leafhoppers migrate from other hosts to potato fields. This migra- 

 tion occurs about the middle of June, and as a result eggs are deposited 

 on the early-planted potatoes. That there is such a spring migration 

 of leafhoppers was seen in a very striking manner during surveys of 

 potato fields made in 1919 and 1921. Those fields in the Upper Penin- 

 sula which w^ere planted during May and early June were very severely 

 damaged by hopperburn, while fields in the same neighborhood which 

 were planted after June 15th, remained green throughout the season. 

 These observations were confirmed by evidence obtained on September 2, 

 1921, from the time of planting series at the Upper Peninsula Experi- 

 ment Station which is tubulated on following page. 



