68 DISEASES OF CROP-PLANTS 



Susceptibility and Resistance to Insect Infestations. 



It has been frequently remarked that there is a reverse 

 relation as well as the direct one between the development of 

 infestations of certain insects and the state of health of the plant 

 infested. Plants growing under wholly favourable conditions 

 may exhibit a resistance — or in unfavourable conditions a 

 susceptibility — which determines the extent to which the insect 

 concerned can establish itself upon them. Such a relation 

 exists in several of the stigmonoses. A. F. Woods, who originated 

 the term, remarks that aphides are specially fond of plants which 

 have made a poor starved growth, and increase rapidly upon 

 them. The cacao thrips, with a whole country-side of cacao 

 apparently open for it to feed upon, infests certain patches year 

 after year, only occasionally and temporarily extending to the 

 surrounding fields. Usually it can be shown that the infested 

 areas are on poorer or less well-drained soil, are insufficiently 

 sheltered, or perhaps have been depressed by some more tem- 

 porary disability. 



It is the general experience with the froghopper blight of sugar- 

 cane in Trinidad that it is in fields in poor tilth, in ratoons on 

 red clay soils, which quickly lose the effects of cultivation, and 

 in local patches subject to some similar defect, that the insect 

 finds the most suitable conditions for rapid multiplication. 



Infestations of the bug Helopeltis which causes the mosquito 

 blight of tea are reported to depend similarly on cultural con- 

 ditions, being especially liable to follow upon water-logging of 

 the soil. Bugs of this species in captivity are said to have 

 proved unable to maintain themselves on twigs supplied from 

 resistant bushes, but to recover and thrive on material from 

 plants in a susceptible condition. Scale insects frequently show 

 greatly increased powers of infestation on plants in an unthrifty 

 condition or growing in situations in some respect unfavourable. 

 Thus young lime trees grown with a shelter crop, the system under 

 which they thrive best, have remained practically free from scale 

 when plants of similar age in the open have had to be frequently 

 sprayed to keep them alive. 



