1931.] FEOGHOPPER BLIGHT IN TRINIDAD. &S 



Tliere is no definite evidence as yet that canes damaged by root 

 fungi are more attractive to froghoppers. On the other hand the 

 conditions which lead to root fungus attacks are nearly always those 

 which allow the froghopper to breed more freely. 



Influence of Environment. A. study of Section IX indicates that 

 the most important factor in determining the prevalence of blight is the 

 humidity just above and just below the surface of the ground. 



(1.) Extreme dryness in dry season tends to increase blight. 



(2.) Moderate dryness in dry season tends to reduce blight. 



(3.) Dry w-eather in wet season tends to increase blight. 



(4.) Normal weathar in wet season tends to reduce blight. 



(5.) Very wet weather in wet season tends to increase blight. 



In a Table an attempt is made to explain these facts by the- 

 known effect of moisture on the cane, the root fungi, the froghopper and 

 its enemies. 



It is shown that with excessive dryness both the froghopper and all- 

 its enemies are dormant, so that no reduction in numbers will take 

 place. At a slightly higher grade of moisture the vermilion egg-parasite 

 is able to breed and reduce the number of eggs. A.t the third grade the 

 froghopper can breed and so outdistance the vermilion egg-parasite. At 

 a still higher huniidity both the s^'rpliid fly and the green muscardine 

 fungus come into play and reduce the froghopper, while the heaviest 

 rains do not affect the froghopper, but reduce its enemies and have an 

 injurious effect on the canes. 



Size of Broods and Periodicity of attacks. There is no definite 

 relation between the size of the broods in one year. Sometimes the first 

 is the largest and sometimes the smallest. The second, is as a rule, 

 that by which most damage is caused. Frequently the third brood is 

 very small after a large and injurious second brood. More accurate 

 study of conditions may lead to an explanation of these facts. 



No adequate explanation is available to explain the periodic recurrence- 

 of severe blight at intervals of three or four years. It is possibly 

 connected with the cycle of agricultural operations. 



The froghopper appears to have been more serious within the last 

 fifteen years than formerly. This is the case not only in Trinidad but 

 in other countries and with other insects besides froghoppers. It is not 

 considered that either the introduction of the mongoose or the develop- 

 ment of seedling canes are responsible for this increase. It is more 

 likely part of a wider influence of the spread of civilisation, population, 

 and cultivation in a country which has not yet worked out for itself 

 reliable rotational methods of agriculture to combat the disadvantages 

 inherent in Man's artificial method of planting large areas with the same 

 crop. 



CONTIIOL. 



With tropical crops, and particularly with sugar-cane, methods of 

 prevention are more practicable with most pests than methods of cure. 

 With the froghopper the most important jireventativc consists of getting 

 the soil and the crop into sucli a condition that a less suitable environment 

 for the breeding of the froghoppers is produced. 



