SECTION IX.— THEORIES OF DAMAGE AND GENERAL PROBLEMS. 123 



(e) The periodicity of the blight, which recurs, at least up to 

 September or October, at intervals exactly corresponding with 

 the time between the successive froghopper broods. 



(/) The occurrence of similar blight in sugar-cane and other 

 grasses in other countries in association with various species 

 of froghoppers. 



Adult or Nymph. 



The froghopper damages the canes at two stages of its hfe in two 

 different ways, the nymph sucks only at the roots, the adult only 

 at the leaves. The nymph lives for about six weeks but is at first 

 very small. The adults live two to five weeks and suck large quantities 

 of sap every night. It is important to decide if one of these is more 

 injurious tha,n the other. 



The evidence that the nymph is the cause of damage is at present 

 \'ery indefinite, but experiments are being planned by which it may be 

 possible to test the question more accurately. 



Heribt (1908) produced artificially blighted Para-grass in three weeks 

 by placing nymph froghoppers on it, but it appears that the adults were 

 also kept on the plants when they hatched out, so that the experiment 

 is inconclusive. 



On the other hand in July 1919 I put a large number of nymphs on 

 the roots of a heilthy stool of cane and failed td produce any symptoms 

 of blight. 



Gough (1911 C. p. 10) says "Nymphs are more numerous during 

 blight, adults during recovery", but with this, after longer experience 

 than he was able to get, I cannot agree. The severe blight visible on a 

 field during the nymph stage is that caused by the previous brood of 

 adults. 



The evidence incriminating the adult is much more conclusive. 



(a) The visible browning and streaking of the leaves, which causes 

 the canes to look "as if scorched by fire" is the direct result of the 

 puncture of the adult on the leaves. 



(b) The staining of the growing point and death of the top of the cane 

 is more likely to result from injury to the leaves than to the roots. 



(c) The damage never becomes serious until some time after the 

 adults have appeared. The field is often "held back", which may be 

 due to the nymphs, but the results are never disastrous until the leaf 

 injury has developed. 



{d) The appearance of fresh green healthy shoots and leaves, on 

 plants not too severely damaged, immediately the blight has passed 

 and the conditions become favourable is an indication that the root 

 system is not permanently injured. 



(e) The sprouting of the upper eyes in blighted canes is an indication 

 of vigour which is being impaired by want of leaves. 



(/) The occasional appearance of blight on the edge of a healthy lield 

 which has been infested with adult froghoppers from fields or abandoned 

 land alongside indicates that the preaence of nymphs is not essen tial 

 for the production of blight. 



(g) In Surinam severe injury has been caused by a froghopper which 

 does not touch the roots of the cane in the nymph stage (see p. 46}. 



(//) In July 1919 I produced typical signs of blight in a healthy cane 

 stool, which had never had nymph froghoppers on it, by caging over it 

 a number of adults. On the other hand Urich (1913 C. p. 10) says that 

 he put 100 adults a day on a cane plant and failed to produce signs 

 of blight. 



