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THE AGRICULTUKAL NEWS. 



June 17, 1916. 



PLANT PESTS AND DISEASES. 



CACAO THRIPS AND DIE-BACK IN 

 ST. VINCENT. 



An article which appeared in the Aqricultiiral Nen'S for 

 September 2.5, 1915 (Vol. XIV, p. .314), entitled What 

 Cacao Thrips Signify in (Grenada, gave an account of the 

 status of the cacao thrips (Heliotkrips riihrocinctus) in the 

 cacao cultivations in Grenada. The article was an abstract 

 of a report by the Entomologist to the Imperial Commissioner 

 of Agriculture on a visit to that island in February-]\Iarch 

 191.5, and embodied the conclu.sions arrived atbj- that officer. 



In .January 1916, the Entomologist and Mycologist 

 visited St. Vincent for the purpose of studying the pests and 

 diseases of cacao, with .special reference to the occurrence of 

 cacao thrip.», and the conditions which lead to its develop- 

 ment in such numbers as to be considered a pest. 



The conclusions arrived at as a result of careful study of 

 the .situation in Grenada were ( 1 ) that the cacao thrips is 

 always present on cacao trees, (2) tFat it increases in numbers 

 when unfavourable conditions produce an unhealthy state in 

 the cacao plant, (.3) that the unfavourable conditions which 

 are known to so affect the cacao as to result in the so-called 

 attack of thrips are (a) insufficient or improper drainage, 

 <b) root disease, (c) poor or unsuitable soil, which may be thin, 

 overlying an impervious subsoil, impoverished by washing or 

 by exposure to sun, (d) e.xposure to wind, and (e) lack of 

 shade. 



It was held by agricultural officers and planters in 

 (St. Vincent that the conclusions arrived at as a result of 

 a study of Grenada conditions would not apply to St. Vincent, 

 where the cacao thrips continued to give trouble as a pest of 

 importance. 



In a report to the Imperial Commissioner of Agriculture, 

 ■signed jointly by the Entomologist and Mycologist, on the 

 observations made in St. Vincent, it is shown that while the 

 circumstances of the cacao cultivations in St. Vincent differ 

 con.siderably from those in Grenada, the same general state- 

 ments apply to the conditions which re.sult in the outbreaks 

 of thrips in the two islands. 



The following is copied from the joint report of the 

 Entomologist and Mycologist already referred to. The five 

 estates visited are referred to by letter instead of by name. 



GENEK.4L COKDITIONS OK CaCAO IN St. ViNCENT. 



Although consideration of the rainfall and topography 

 in St. Vincent might incline one to the opinion that the 

 conditions would be very suitable for the cultivation of 

 cacao, it is significant that cotton rather than cacao is the 

 crop which has made important progress in recent years. 



The available figures show that as much as twenty-three 

 years ago (1893) the cacao industry 'was well established and 

 the development since that time has not been very great. 

 Cacao has never assumed a place of great importance in 

 the agriculture of the island; the output in normal years has 

 remained practically stationary since 1897. 



Cotton, on the other hand, which requires for its suc- 

 cessful cultivation climatic conditions which in many respects 

 are the opposiie of those most suitable for cacao, has, since 

 its introduction in 1903 become a much more valuable crop 

 than cacao, and has approached the position held by arrow- 

 root, which is the principal crop of tbe island. 



If tbe conditions existing in Grenada, which is emi- 

 nently a cacao-producing colony and not at all a cotton 

 locality are contrasted with those of St. Vincent, it is 

 readily seen that in matters of climate and topography, the 

 two islat.ds are very si]nilar. Tlie one outstanding point 

 of difference is to be found in the character of the soil. 



The Grenada soil is moderately heavy, retentive of 

 moisture, and is underlaid at varying depths by a relatively 

 impervious subsoil. 



The St. Vincent soil is remarkably light, offering very 

 little resistance to the passage of water. 



We are of opinion, and in this we are in accord with the 

 Agricultural Superintendent, that as a consequence of this 

 condition of soil, the' atmospheric humidity is much below 

 what would be e.xpected from a consideration of the rainfall 

 statistics of the island. In other words, in spite of an abun- 

 dant rainfall, .St. Vincent presents the conditions of a relatively 

 dry island. 



We hold that these conditions e.xplain the position with 

 respect to cacao and cotton above referred to, and that it is 

 indicated that in St. Vincent cacao is being grown near to 

 the margin of permissible dryness. 



In this we refer more especially to the absence of that 

 degree of atmospheric humidity which is recognized as 

 essential for the successful development of a cacao cultivation; 

 but in addition to this the soil condition.* are such that the 

 roots are liable to suffer from a relatively short period of dry 

 weather. Under these circumstances, the cultivation calls 

 for very careful attention to such measures as tend to 

 conserve the atmospheric humidity and the soil moisture. 



This indicates the necessity of providing a fairly dense 

 top shade, of covering the soil in all open places by quick- 

 growing plants, and the use of wind-belts and marginal 

 screens to prevent as far as possible air currents in cacao 

 cultivations, and also the free and regular use of organic 

 manures and mulches in order to make the soil more 

 retentive. 



These light soils not only lose their moisture rapidly, 

 but they possess little ability to retain plant food for any 

 length of time, and it is necessary to maintain the soil 

 fertility by .systematic manurial treatment, by using such 

 materials as those indicated above. 



CoN&lTioN.s AT Estate A. 



At estate A, thrips, though pre.sent in all parts of the 

 cacao cultivation and perhaps on every tree, were at the 

 time of our visit nowhere particularly abundant. We are 

 told that this insect has two periods of abundance on this 

 estate; one in September and October, in the midst of the wet 

 season, and the other in April and May. in the dry season. 

 During the past two years they have been very abundant • 

 during these periods. 



The Entomologist visited this cacao in October 1912. 

 The trees then were in good condition, having greatly 

 improved after a severe attack of thrips which had occurred 

 a few months previously. In the report on this visit, pub- 

 lished in the Annual Report of the Botanic Station, 1912-13, 

 it was stated that this improved condition was believed to be 

 due, in part at least, to manuring, forking, and general 

 agricultural conditions. This improvement appears to have 

 continued through 1913 and 1914, the severe attacks of 

 thrips which led to the present conditions having begun 

 towards the end of 19l'4 or early in 191.5. 



The conditions which we found that were associated 

 with, or were supposed to be the results of, e.vtremely severe 

 thrips attack, were (1) .several patches in which trees had. 

 died where root disease due to two species of liosellina was 

 found by the Mycologist: (2) groups of trees and individual 

 trees which were dead or dying without any manifestation of 

 disease; their failure took the form of a progressive die- 

 back. The trees so affected had been subject to repeated severe 

 attacks of thrips, and the dead branches were infested with 

 the die-back fungus, Diplodia. 



