Vol. XIV. No. 350. 



THE AGRICULTURAL NEWS. 





pondence and interviews with plai ei md others; il hasbeen 



pin forward in departmental publication's and at Agricultural 



3 from time to time during the pasl few years, and 



ception to the general idea has ■ i been taken. In the 

 circular on Cacao Thrips bj I". W. I 'rich, published by the 

 Trinidad Board of Agriculture, the saini belief is expn ed 

 As a result of a recen visit t< Grenada made bj the 

 Entomologist and Mycologist of this I )epartment, the opinion 

 is formed i hat in Grenada, esp those estates which 



, the lower lands around the periphery of the island, 

 the cacao thrips is most abundant during the last three 

 months of the year, October, November and December, and 

 that in the dry months a considerable improvement in the 



condition of the trees takes place. 



With regard to the nature and the amount of injury 

 caused by thrips, it was found that general!) speaking, 

 planters notice only the injury in terms of pods picked too 

 green ami of labour, and time lost in scratching the pod to 

 see if it is ripe enough to be pick d 



It is true that iii some instances thrips is credited 

 with causing the leaves to fall and with preventing the 

 development of pods, when these are attacked while very 

 young, and it appears that in a few instances thrips is 

 believed to have killed trees out-right. 



Thrips live and the leaves ind pods of the 



cacao. When leaves are attacked they may be so seriously 



injured as to cause them to drop, but unless the entire crop of 



were destroyed and the destruction repeated in a short 



time, it doe- U0t seem to me as if[the tree would lie seriously 



injured, and it certainly would not be killed from that cause 

 alone. The attack of thrips on the growing pods would not have 

 : ,i,v effect i n the health and vigour of the tree, even if all the 

 pods were killed. The effect would be entirely an effect on 

 the crop. In most instances planters have spoken of 

 a severe thrips attack as being a time when thrips were 

 abundant on the pods, as a result of which pods were badly 

 discoloured. Planters have been asked to state how much 

 injury to crops have been caused and how much permanent 

 n done to In nearly every case the 



has been that very little shortage of the crop is 

 traceable to thrips attack and that the few leaves which fall 

 are quickly replaced by new growth, but the loss occurs in 

 the pods picked before thej an i d in the loss of time 



by the pickers, who often have to examine nearly all the 

 large pods on a tree in order to know which are ready for 

 picking. 



A remarkable unanimity of opinion appeared to prevail 

 among Grenada cacao planters in regard to the occui 



f thrips. In every in thrips attacks or the 



effects of thrips were seen, it was stated that 'this 



something besides thrips 



could be shown to be wrong with the tree-. Before 

 considering what these things are which are wrong with the 

 four point- may be discussed with regard to the cacao 

 thrips in Grenada as follows: — 



, Thrips tire present throughout the island, and at 

 certain times n is doubtful if on those where out 



thrips occur, there ue any trees which are entirely 

 free from this insect. 



(b) Thrips in the adult stage are capable oi flight and 

 the young are active enough so that they might be 



from tree to tree by birds and 1 u s 



(c) Thrips attacks occur on the same limited ana- yeai 

 after year. 



(d) Thrips is a dry W6>ther and a dry locality insect a- 

 a general thing, and yet in Grenada it generally 1" 



troublesome toward the end oi the wet season. 



-^ . 



In the case of an insect (a) which is at all times well 



distributed throughout the island, (b) which can migrati I 



can be easily transported, it is remarkable (c) that 



attacks should always occur on limited areas and on the 



areas year after year and (d) 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 a pted that thrips maj 



present year after year in ca aoand never increase in numbers 



sufficiently to attract attention or to cause any injury. 



for several years the convict ion has I n growing that 



the so-called attacks of thrips indicate that something is 



wrong with the plant, or with thei ditions under which it is 



growing. It is now suggested that thrips in Grenadaat least 

 ought to be considered a useful insect, because it is a. certain 

 indicator that the treesaiv suffering from untoward condition. 

 Thrips might well be called a danger signal, a watch dog, or 

 a trouble indicator, but should not be called a pest. To 

 explain this statement, it may be stated that when thrips 

 aiea-were examined, it was found that root disease was 

 present, or that the drainage was insufficient, or the -oil was 

 shallow with terrace or heavy ela\ beneath, or there was 



evidently a lack of humus in the soil. 



Root di-ea-e is the result of the action of a specific 

 organism which produces a well known and recognizable effect 

 in cacao, ftsattacks seemalwaya to be accompanied by thrips, 



which probably indicate the area of infection by root di 

 better than it can be determined in any other way. The worst 

 affected trees, probably in the centre of the infected p 

 will be dying wholly or in part, and of course theeffect ol root 

 disease is easily to be seen in such cases, but on the edge of the 

 root disease patch, where its effects might not be easily ^ffii, 

 the thrips will almost certain!} be found to decrease in 

 numbers. In dealing with root disease accompanied by 

 thrips, the exjtent of the area to be isolated will probably be 

 accurately indicated by the extent of the thrips attack-. R il 

 disease probably produces a physiological upset in the 

 tree long before it causes the death of the t rees or even of many 

 of the roots. This condition i- favourable to thrips, and 

 result- in their increase in numbers. Cacao trees growing in 

 soils not properly drained are also subject to physiological 

 upset and consequently to thrips attack-. 1 was shown 

 several instances where thrips areas had been rendered free 

 from thrips attacks by improved drainage. In this connexion, 

 the conviction is expressed that on some estates iii Grenada, 

 at least there is room for much improvement in drainage. 

 Drains should allow rain and soil water to thoroughly 

 saturate the soil and at the same time they should provide 

 ample means for the quick run-off of all surplus water. For 

 this purpose contour drains are to be preferred to drains 

 running straight down the slopes. The depth of drains 

 varies according to local conditions, and it may happen that 

 on lands which seem to 1 Bciently well drained, thrips 



occur year after year. 



In such i ■■■- it i- possible that merely deepening 



drain- will restore the trees to health and reduce the Si 



of thrips attacks. 



Lack of proper drainag isidered to be i ol the 



mostfrequent causes ot unhealthy trees and attacks bj thrips. 



Etool disease iscaused by a specific organism and requires 

 definite treatment. As already stated thrips attacks 

 sometime-, perhaps often, indicate not only the presence of 

 r00 t di I also thi urea attacked by it. 



Directions for the treatment of root di-ea-e are given in 



the Aaricultuw.1 Few for Jury 31, last. 



II. A. B. 



