412 PLANT GROWTH SUBSTANCES 



Although no evidence exists showing that insects can produce a 

 natural auxin, it is significant to note that in 1940 Link reported 

 curvature responses in Avena tests from extracts of certain aphid'. (15), 

 and in addition, from extracts of plant tissue upon which insects had 

 fed (27). More recently, Nysterakis (18,19) ^^^^ reported that certain 

 ones do secrete auxin which affects plant growth. 



A review of investigations relating to this subject shows that there 

 are certain similarities in growth changes in plants caused by hormones 

 and the deformities in plants caused by insect feeding. In both cases 

 enzyme activity appeals to be involved. An interesting case suggesting 

 a relationship between plant hormones and the effects of insect feeding 

 is the fact that bean plants treated with hormones have failed to show the 

 expression of certain insect damage. 



Similarity in Growth Changes. — As has been previously mentioned 

 perhaps the most significant evidence that disturbances in plants caused 

 by insects and by hormones are similar is the change produced by the 

 feeding of insect and mite gall-formers. It is the opinion of most workers 

 interested in such insects that gall formation is caused by the introduction 

 of substances into the plant tissue. In 1936 Felt (5) stated that stimula- 

 tion by such substances was the fundamental principle in gall formation; 

 and Martin (16) concluded in 1930 that since stem galls in sugar cane can 

 be produced by injection of macerated leaflioppers, possibly auxins 

 were involved. Brown and Gardner (2) state that the formation of galls 

 caused by insects is a tissue response paralleling the response to growth- 

 promoting substances of some plant tissues. 



Further similarities in changes in plants caused by insects and by 

 plant hormones may be seen in the development of adventitious buds 

 and reduction of length and width of internodes which may follow the 

 application of hormones (9,25). Ripley (21) has reported that some 

 mirids cause malformation in young trees and a witches'-broom rather 

 than a single stem is produced from the terminal leaf bud. A disease 

 associated with the feeding of a cercopid on sugar cane has been described 

 as reducing the length and width of plant internodes with a development 

 of adventitious buds (10). A rosetting and shortening of internodes of 

 alfalfa and clover by the feeding of spittle insects has recently been 

 recorded (6). Smolak (24) describes a witches'-broom of lilac caused by 

 Eriophyes lowi (Nel). Psyllid yellows, which is associated with the feeding 

 oi Paratrioza cockerelli (Sulc), possesses symptoms that include rosetting 



