OBSERVATIONS ON INSECT PESTS IN GRENADA. 175 



In the case of an insect which is at all times well distributed throughout the island, 

 and which can migrate and can easily be transported, it is remarkable that their attacks 

 should always occur on limited areas and on the same areas year after year, and that 

 these attacks should occur in the wet season, which would be expected to be the time 

 when the plants would do their best and the thrips would be reduced in numbers. 

 It seems to be generally accepted that this species may be present year after year in 

 cacao and never increase in numbers sufficiently to attract attention or to cause any 

 injury. 



For several years I have been convinced that the so-called attacks of thrips indicate 

 that something is wrong with the plant, or with the conditions under which it is 

 growing. I now suggest that thrips, in Grenada at least, ought to be considered a 

 useful insect, because it is a certain indicator that the trees are suffering from some 

 untoward condition. It might well be called a danger signal, but should not be called 

 a pest. To explain this statement, I may say that whenever I was shown thrips 

 attacks, I found root disease, or insufficient drainage, or very shallow soil with terrace 

 or heavy clay beneath, or a lack of humus. 



Root disease is the result of the action of a specific organism which produces a well 

 known and recognisable effect on cacao. Its attacks seem always to be accompanied 

 by thrips, which probably indicate the area of infection by root disease better than it 

 can be determined in any other way. The worst affected trees, probably in the centre 

 of the infected patch, will be dying wholly or in part, and of course the effect of root 

 disease is easily to be seen in such cases, but on the edge of the patch where its effects- 

 might not be readily detected the thrips will almost certainly be found to decrease 

 in numbers. Root disease probably produces a physiological disturbance in the cacao 

 tree long before it causes the death of the tree or even of many of the roots. This 

 condition is favourable to thrips and results in their increase in numbers. Cacao 

 trees growing in soils not properly drained are also subject to physiological derange- 

 ment and consequently to thrips attacks . I was shown several instances where trees 

 had been freed from the insects by drainage . 



In this connexion, I wish to record my impression that in Grenada there are many 

 instances of insufficient drainage due to the wrong construction of the drains. Too 

 many drains run nearly straight down the slope of the gradient instead of across it. 

 In other words, there are too many direct run- off drains, and not enough contour 

 drains. The proper arrangement and construction of drains in cacao is one that 

 allows rain and other water thoroughly to saturate the soil, and at the same time 

 provides for a rapid removal of all surplus water. Proper drainage also reduces 

 surface wash in times of heavy rainfall and rapid run-off, and prevents or reduces land- 

 slipping. Direct down-hill water-courses increase wash and land-slipping. In the 

 matter of depth also many drains are deficient, and this is somewhat due perhaps 

 to a wrong method of calculating depth. I have heard a planter give the depth of 

 his drains as to the top of the bank, not to the original level of the surface soil. 

 Another weak point in drainage in Grenada is that in some instances the soil thrown 

 out of the drains is allowed to lie in ridges in such a manner that pockets or depressions 

 are formed in which there is little or no chance for the surface water to get directly 

 into the drains ; it must stand on the surface until it percolates. In such instances, 

 the tree stands in the lowest part of such a place and the water settles round it. These 



