76 JOURNAL OF ECONOMIC ENTOMOLOGY [Vol. 14 



day the entire tip showed increased injury, and was so badly burned by 

 the twelfth day that the nymphs were removed, there being no food 

 left. Six were still in the fifth instar, the remaining having matured. 

 The plant was photographed a few days later. In addition to 

 demonstrating the local nature of the disorder, the above experiment 

 shows that in severe cases the leaf petiole, stem, and even opening leaves 

 are affected, the nymphs feeding on these places when obliged to. 



Comparative Effect of Different Stages of the Insect 



A set of experiments was conducted to determine the comparative 

 effect of the different stages of the leaf hopper on the potato foliage. 

 In one test first instar nymphs were placed on nonnal leaflets and kept 

 there by means of a wad of absorbent cotton tied around the leaf petiole. 

 Leaves were confined with one, two and three nymphs, respectively. 

 Those confined with one nymph developed the first symptoms in from 

 six to nine days, depending upon the age of the nymphs which were in 

 either the third or fourth instar. Those having two hoppers developed 

 the first symptoms in from four to five days, the insects in this case being 

 in either the second or third instar. Leaves on which three nymphs 

 were placed showed tipburn in from three to five days, the insects being 

 in either the first, second or third instar. When more insects were 

 confined to a leaf, the burning developed still earlier. These tests 

 showed that mali is capable of producing burning in all its nymphal 

 stages, but that when one is confined to a leaf the amount of "toxic 

 substance" introduced is not sufficient to produce the disease and that 

 normally one nymph does not reach an "effective" size until the third 

 or fourth instar. 



In another series one and two fourth instar nymphs, respectively, were 

 confined on potato leaves to determine the effect of this stage in develop- 

 ing tipburn. Five fourth instar nymphs placed on individual leaflets 

 produced burning in from one to se\xn days. In all but one case the 

 insect was in the fifth instar when burning was first noticed. Five leaflets 

 on each of which two fourth instar n^nnphs were placed developed burn- 

 ing in from one to four days, there being little difference in the date of 

 appearance of burning, whether there were one or two insects on the 

 leaf. These and other experiments showed that where mali is in either 

 the fourth or fifth instar, the symptoms are often produced on a normal 

 leaflet within twenty-four hours. 



In cages where equal numbers of adults and nymphs were added, 

 l:)urning developed much earlier and to a much greater extent in those 

 cages having n\Tnphs than in those having adults. Furthermore, 

 adults confined to individual leaflets failed to produce the disease to any 



