26 



THE AGRICULTURAL NEWS. 



Jam AKY 



1917 



INSEGT NOTES. 



THE CACAO THRIPS 



This insect has over a period of several years been tlie 

 subject of articles by Maxwell-Lefroy, Elot, Ballou, Uriah, 

 Eussell and others, to whom the \vriter is indebted for most 

 of the information contained in this brief popular aiticle. 

 Observations were also made hy the writer during a recent 

 short visit to Grenada. 



ORIGIN ANI> iJlsTEIBUTION. 



The cacao thrips was first given a definite scientific name 

 from material collected in Guadeloupe about sixteen years 

 ago, but it has doubtless always been present on cacao in 

 the' West Indies. It is now known to have a wide distri- 

 bution, having been recordeil from Grenada, Guadeloupe, 

 St. Vincent, St. Lucia, Dominica, Trinidad, Tobago, the 

 Virgin Islands, Surinam, Uganda and Honolulu, and Hawaii 

 (in greenhouses). It also occurs on a strip nf the east coast of 

 Florida, where it is known as the red-banded thrips, and has 

 been reported from Ceylon by Maxwell-Lefroy. 



Fiii'l)-PLANT.S. 



This thrips is best known in the West Indies by reason 

 of its injury to the leaves and pods of cacao, and under 

 conditions favourable to its development it is capable of doing 

 considerable damage to this crop. It is also found on 

 several other plants, including cashew, guava. Indian 

 almond ( Terminalid catappa ), mango, Liberian coffee, roses. 

 kola, and avocado pear. 



DESCKIPTIoX. 



Tlie adult cacao thrips (Hehothrips yu/>rorincHis Giard) 

 is dark brown, or black, the fi-male being abouD ./^-inch long, 

 while the male is much smaller and is rarely seen. The 

 recently emerged adults are light in colour but soon become 

 darker. 



The eggs as dissected out of the female by Urich, are minute, 

 kidney-shaped, and quite transparent. They are inserted by 

 the female into the leaf and pod tissues of the cacao, accord- 

 ing to Urich. It Is recorded by Russell thai the female of 

 this .same thrips deposits the eggs in the young leaves of 

 the mango. He says 'that after the female has dejiosited each 

 egg she seals the openina with a large drop of excrement which 

 dries to a fiat scale .so that the egg pocket is concealed.' The 

 other immature stages consisting of larvae and nymphs are 

 pale yellow to yellowish green, witli a bright red band across 

 the abdomen. 



HABITS ANI> IX.rURY. 



The adults are usually found on the underside of the 

 younger leaves and on the pods, where they go to feed and 

 lay their eggs. They can sometimes be seen crawling about 

 with the abdomen elevated, and when disturbed, crawl rapidly 

 about or jump away quickly. Urich has observed them Hying 

 in the cool of the afternoon in Trinidad. 



The larvae are found mostly on the half-grown and 

 older leaves, and on the pods, and togeiher with the adults 

 may cause such severe injury to the leaves by sucking the 

 juices that they sometimes dry up and fall oft'. In a severe 

 attack trees may suffer two or three defoliations while the 

 thrips are abundant. The younger stages are always seen 

 with the abdomen in the air, bearing at its tip a small drop 

 of excrement, which from time to^time falls to the leaf or pod 

 and dries to a thin brown flake. 



The injury to the pods causes a brown corky scab to 

 form, and in a severe infestation, the pods have a russett 

 appearance, which makes it diffcult for the cacao picker to 

 tell when they are ripe. The pods themselves appear to be 

 very slightly injured by thrips, and the only appreciable loss 

 seems to result from immature discoloured pods being picked 

 for ripe ones by inexperienced pickers. 



SIGNIFICANCE OF THRIl'S. 



In places like Grenada, where the cacao thrips is an 

 important insect, it has been noticed that its outbreaks are 

 generally confined year after year to areas where the trees 

 are in an abnormal condition. Occasionally when conditions 

 are exceptionally favoi'.rable to the insect or unfavourable to 

 the trees, there comes an unusually severe attack of this pest. 

 At such times the regular thrips areas may be temporarily 

 enlarged, or the insect -may attract attention in places where 

 it has hitherto passed unnoticed. 



It has also been observed that where cacao trees are 

 kept in a normal healthy state, where the soil conditions are 

 suitable, where good cultural methods, such as eflicient 

 drainage, periodical maiiuring or mulching, forking or super- 

 ficial hoeing, etc., are systematically and persistently carried 

 on, where root disease is not prevalent — in such places 

 as these thrips is a negligible quantity, and never 

 seems to become a .pest, though sometimes present. 

 Urich has noticed in "frinidad that thrips, although present 

 in shaded cacao, is more abundant in unshaded or lightly- 

 shaded areas of estates. So far as thrips is concerned, shade 

 is beneficial to cacao under certain conditions. 



Thrips is only one of several troubles which attack 

 the cacao tree, but likt- many other enemies, plant or insect, 

 it does not seem to make progress on normal, healthy trees, 

 although it may be present in small numbers. As soon as 

 the trees show the least signs of weakening, these numerous 

 plant and insect enemies gradually get a hold and may even 

 attract some attention in slightly abnormal seasons, but not 

 enough to warrant special measures being taken against them. 

 Then comes a year in which the climatic conditions may be 

 unusually favourable to one or more of the many enemies. 

 There is a sudden Cfnubined onslaught, and we get what is 

 known as a 'thrips year', thrips being the pest most in 

 evidence. AVhen an attack of this kind comes along, the 

 cacao growers may he caught unprepared, and spasmodic 

 methods of an emergency nature are adopted, and these, 

 as a rule, only serve to hasten the departure of the already 

 disappearing thrips. The trees make a rapid partial recovery, 

 and the growers are lulled into a false security through 

 a succession of good years. The thrips and other enemies 

 are, however, still causing a little trouble here and there in 

 the same old places, hut the temporarily improved health of 

 the trees, coupled perhajis with climatic conditions unfavour- 

 able to thrips, serve to keep this insect more or less in 

 abeyance, and the cai an planters are no longer reminded of 

 the other cause.s, which are all the time contributing to the 

 gradually weakening ot the trees. Then comes anotlier thrips 

 year and we get the same story over a^ain. 



I'ONTROL METHODS. 



Now it will be .seiTi from the above remarks, that thrips 

 is one of those insects which are unable to make any progress 

 on strong vigorous incs; but let the same trees get run 

 down or weakened, and it is certain that thrips will find them 

 out sooner or later, w ill .single them out for marked attention, 

 and will continue to, I,, so year after year until they are 

 restored to their normal vigour by ordinary cultural methods, 

 such as draining, forking, manuring, mulching, etc. These 

 method.s, to be effect ivt, must be .systematically and steadily 



