122 



THE AGRICULTURAL NEWS. 



April 19, 1919. 



INSECT NOTES. 



CACAO THRIPS IN GRENADA. 



The subject of cacao thrips in the West Indies has 

 freciuently been referred to in the A^ncutural News. The 

 present article has reference to a report by the Entomologist 

 on the staff of the Imperial Department on a visit to Grenada 

 for the purpose of studying cacao thrips in that colony 

 in .lanuary last. 



It appears that cacao thrips has now been recorded in 

 every parish in Grenada, and that at one time or another 

 these attacks ha^ e been serious in each parish. There appears 

 to be no immunity to thrips in any locality in the island. The 

 occurrence of thrips in epidemic form has consistently been 

 held by this Department to be associated with some condi- 

 tions unfavourable to the cacao trees, and it has been held 

 that the control of thrips depended on the improvement of 

 these conditions rather than on direct attacks on the thrips 

 themselves. Present conditions in Grenada seem to bear 

 this out. Planters have been able to reduce the attacks of 

 thrips, and even to control them satisfactorily by the appli- 

 cation' of such agricultural methods as draining, mulching, 

 and manuring. 



As a result of the visit of Mr. Urich to Grenada in 1917, 

 several planters are spraying for the control of thrips. The 

 ioseciicide largely used is that form of Nicotine sulphate 

 knov^n.as 'Black Leaf 40', applied in conjunction with Bordeaux 

 mixture, and, in some instances, with soap. Black Leaf 40 

 used by itself does not spread well. The Bordeaux mixture 

 combines the functions of a fungicide and spreader, while the 

 soap, in those cases where it is used, is a spreader only. 



Mr. L rich's visit to Grenada to advise in the matter of 

 cacao thrips control was the outcome of representations made 

 in 1916 to His Excellency the Governor to the effect that the 

 cacao thrips was more abundant than usual, and constituted 

 k threat to the success of the industry. How far this belief 

 was justified may be judged from the fact that the crop of 

 1916 was the largest crop which has ever been produced in 

 Grenada being some 27 per cent, larger than that of the 

 preceding year. The crop of 1917 was even larger than its 

 predecessor. 



The occurrence of thrips in epidemic attacks is fre- 

 quently associated with the lack of drainage, but it is also to 

 be noticed that any unfavourable conditions such as shallow 

 soil, with a hard and impenetrable subsoil underlying it, the 

 occurrence of root disease and canker, and the exposure of the 

 soil to the direct rays of the sun are all liable to be accom- 

 panied or followed by thrips attack. As the remit of these 

 conditions or diseases, it is believed that .some change takes 

 place in the cacao plant which makes it less resistant to the 

 ^cao thrips, or pofbibly this may better be stated as follows: 

 the upset in the physiological condition of the cacao plant 

 which follows as a direct result of root a.«phyxiation resulting 

 from bad drainage or from root interference, and such diseased 

 conditions as already mentioned, produce.s changes which 

 make the plant tissues more attractive to thrips and more 

 favourable to their breeding and feeding. This is often to be 

 seen in adjoining trees, one of which being perfectly healthy 

 carries a limited number of thrips without being 'attacked', 

 while another, being unhealthy, is .seriously attacked. 



Shade or the lack of shade appears to Lave a bearing on the 

 thrips situation. C>n wet soils, deep rooted shade trees assist 

 in drainage, and in this way tend to improve the .ioil condi- 



tions. There does not seem however to bejnecessity for shade 

 as a protection to the foliage of the cacao tree, but shade as 

 a soil cover appears to be most important. Exposure of the 

 soil to the sun, and particularly to the afternoon sun, seem.^ 

 to be very deirimenial to the cacao tree and conducive to 

 thrips attack, l^^'.xposure to winds .seems to be associated 

 with thrips attack. 



The beneficial effects of improvement of cacao fields by 

 drainage and mulching continue to be evident. The 

 following suggestion, which is put forward in the report as 

 ofifering a subject for trial, is of interest. In many places 

 where there is an excess of moisture, and a particularly 

 difficult subsoil to be dealt with, it is to be noted that even 

 with an abundance of drains something still needs to be done. 

 The increased number of drains cuts the land up into small 

 patches carrying a few trees each, and every time the drains 

 are opened additional quantities of subsoil, quite useless for 

 plant roots, are brought to the top, The question is raised 

 whether a certain proportion of these drains might not be 

 filled in and made to serve a double purpose. If a 

 drain were taken and thoroughly cleaned, iis bottom being 

 reduced to a smooth gradient capable of carrying water easilj", 

 and then filled with leaves, pods, bush, and any organic 

 matter, would it not continue to function as a drain, and 

 at the same time enormously increase the available feeding 

 space and food material for the cacao roots! 



Reference is made to the use of manure and lime, but 

 particular stress is laid on the value of drainage, and the 

 use of large quantities of organic matter as mulch. An 

 adaptation of the filling in of certain drains appears to be 

 worthy of extended consideration. Thf nidinary pits dug 

 for 'bedding in' pods and leaves ate "ithout drainage On 

 certain estates these are dug on sloping land in the form of 

 a trench, the bottom of which, properly graded, runs out to the 

 surface, or communicates with a drain. The I renches which are 

 filled in, although they would continue to function as drains, 

 would best be considered as trenches, probably 'feeding 

 trenches', in order to emphasize the fact that a very impor- 

 tant function of such trenches is to supply large quantities 

 of food material and additional root space. 



THE POISONING OF THE BOLL 

 WEEVIL. 



In a recent number of the .l^ricul/ural jVews reference 

 was made to the success wLich attended some experiments in 

 the control of Mexican' boll weevil by the application of 

 poison. Information has lately besn received by the 

 Imperial Commissioner with regard to more recent trials on a 

 large scale in the use of poisons. 



'During the season of 191t> the attempt was made to. 

 control the weevil successfully on some 3.j,000 acres of cotton. 

 On the whole, the results were very satisfactory indeed, and 

 demonstrated quite definitely the practicability of utilizing 

 this method of control on a plantation basis. However, there 

 is still much to be worked out before final and definite instruct- 

 tions for general use can be given, and it will be noted that 

 the instructions in the following pages are rather conditional 

 in many ways, but these are made as definite as is possible in 

 the present status of our information.' 



The preceding paragraph is quoted from a circular letter 

 from the United States Department of Agriculture Bureau 

 of ]intomology, issued by the Delta Laboratory for the 

 study of Southern Field Crop Insect Investigations. 



