28 THE UNIVERSITY SCIENCE BULLETIN. 



far more important than these are the hymenopterous para- 

 sites belonging to the subfam41y Ayiteonmae and to the family 

 Bethylidse. Dr. F. A. Fenton's paper on this group shows 

 how extensive is the parasitization of leaf hoppers by these 

 forms which parasitize the nymphs and adults. Professor 

 Osborn states that sometimes 20 per cent of the individuals of 

 some of our native species are thus parasitized. The members 

 of the genus Gonotopus parasitize the majority of the Jassini, 

 while Aphelopus is the only parasite of the Typhlocybini. 

 Various hymenopterous egg parasites are also at times very 

 efficient. 



The relation of fungous diseases to leaf-hopper control is 

 yet an open question. Only rarely have they been recorded 

 as attacking these insects. Professors Garman, Webster, and 

 Thaxter are seemingly the only ones reporting such cases. 

 The first two give records of Draeculacephala mollipes being 

 attacked by Empusa grijlli. Professor Thaxter, in 1890, ob- 

 served Empusa killing the grape-leaf hopper in Connecticut. 

 It seems very probable, however, that in favorable seasons 

 this or other fungi may play some part in the natural control 

 of the leaf hoppers as they do for instance in the checking of 

 the grasshoppers. 



Climatic conditions undoubtedly play an important part in 

 the control of insects. Thus many a foreign insect, on in- 

 troduction to this country, has not been able to gain a foot- 

 hold because of the different and untoward weather conditions. 

 It is well known also that even some forms which have become 

 more or less acclimated, as well as native forms, are often 

 kept in check by extremes of heat or cold. Thus in Kansas 

 very severe winters or very hot summers are known to pre- 

 vent outbreaks of Toxoptera. Undoubtedly the same is true 

 of large numbers of insects, and among them, of the leaf 

 hoppers. Johnson quotes Trimble as observing in 1865 that 

 when the thermometer reached 100 degrees Fahrenheit, thou- 

 sands of the grape-leaf hoppers were killed. It is easily shown 

 that grape-leaf hopper nymphs are killed by an exposure of a 

 few minutes to the hot sun, so that it is very probable that 

 when it becomes very hot, and host plants wither, that many 

 may not be able to find suflicient protection and therefore 

 succumb to the extreme heat. No one doubts also that untold 

 numbers of individuals are destroyed by the extreme cold. 



