378 



THE AGRICULTURAL NEWS. 



December 1, 1906. 



INSECT NOTES. 



Thrips on Cacao. 



For some years past a small insect called thrips 

 {Physopus riihrocincta) has been known to occur on 

 cacao trees in different parts of the West Indies. 



These are small insects with slender bodies, the adults 

 being dark-brown or blackish in colour, while the young are 

 greenish or yellowish. Many of the young have a bright-red 

 band across the abdomen. The accompanying illustrations 

 (figs. 25 and 26) show the young and adult thrips, respectively. 



The adults have four narrow wings which are fringed 

 with hairs. They are able to leap and fiy actively. The 

 young resemble the adults in the shape of the body, but differ 

 from them in having no wings, in being much lighter in 

 colour, and in having the red band across the abdomen, 

 which the adults do not have. Thrips are peculiar in their 

 feeding habits, being rather intermediate between the biting 

 insects and the sucking insects. 



time to time on the leaves and pod and when it dries, a thin, 

 dark-coloured flake remains on the surface of the leaf or pod. 



There are two effects of a severe attack of thrips on 

 cacao. One is a loss of the seriously attacked leaves and the 

 con.sequent dying back of the tips of the branches, which is 

 supposed to be due to repeated defoliation of the trees. The 

 other effect is the discoloration of the pod due to the attacks 

 of thrips, which renders it difficult for pickers to distingui.sh 

 between ripe pods and the unri|ie ones that are discoloured. 

 A considerable loss has been experienced from this condition. 



The remedies to be employed against thrips on cacao 

 are cultural methods, to put the trees in the most vigorous 

 growing condition, and spraying with a contact insecticide. 

 In Grenada, rosin wash is reported to have given good results. 



A detailed account of this pest of cacao trees will be 

 found in the West Indian IJidi/'tin, Vol. II, pp. 175-90. 



Fig. 26. Adult Thrips. 



The young thrips are generally to be found on the under 

 side of the leaves and on the pods. The leaves that have 

 been attacked by thrips have a characteristic appearance, as 

 if th« lower eiiiderniis and a jwrtion of the interior of the 

 leaf bad been eaten away, leaving the upper epidermis and 

 a portion of the interior. When this dries, the attacked 

 areas are plainly to be seen. As the thrips generally occur 

 in small groups or colonies on the leaf, there are sometimes 

 to be seen only one or two small attacked areas, while in 

 cases of severe infestation the whole under surface of the 

 leaves shows the scarified or skeletonized appearance. On 

 the pods the effect is perhaps even more noticeable than on 

 the leaves. Pods that have been seriously attacked turn 

 a deep russet or mahogany-brown colour over the whole 

 surface, while in cases of lesser infestation these colours 

 appear in spots or blotches on the surface of the pod. The 

 young thrills are often to be seen with the abdomen elevated 

 in a threatening attitude, and at the tip of the abdomen 

 a drop of brownish liquid. This liquid they deposit from 



Food. Plants of Cotton Stainers. 



In a recent number of the Ayritidtural Nmvs (Vol. V, 

 p. 314), information was asked for as to food plants of cotton 

 .stainers. Mr. A. W. Bartlett, B.A.,B.Sc., Government Botanist 

 for British Guiana, sends the following note in response to 

 that request, and adds information in regai'd to the insect 

 mentioned as a cotton stainer by Mr. Beckett in his Hints 

 on Agriculture ; — 



The only species of Di/sderciis I have found in this 

 colony is D. anmdiger, Uhler,* of which I am sending a pair 

 per sample post by this mail to verify my identification. 



It is everywhere plentiful on cotton, and other plants on 

 which I have found it are Eriodendron anfractuosum, Hibiscus 

 tiliaceus, ochro, sorrel (Hibiscus i^abJariffa), Hibiscus 

 Abelmoscluis, Malarhra ccvpitatn, and Sida sp. It appears 

 to have a wonderful instinct in discerning plants belonging 

 to the natural order Medvacuie, and I do not remember 

 finding it feeding or breeding on plants belonging to other 

 orders. I notice that in the article on ' West Indian Cotton 

 Stainers' in the West Indian Bulletin (Vol. VII, p. 8-1) 

 is mentioned that ' in British Guiana, Beckett records 

 a cotton stainer (Dysdeixus sp.) which can readily be 

 distinguished by a dirty whitish cross with two dark patches 

 on the back.' On my recent visit to England I identified 

 the insect which Mr. Beckett refers to at the South Kensing- 

 ton Natural History Museum as Oncopeltus fasciatus, Drury 

 (Family Lyijo.eidae). Its real food plant is Asclejnas curas- 

 savica, on which it is found abundantly in its various stages, 

 of which the immature ones are red in colour and much 

 resemble young cotton .stainers, and it is possible that 

 occasional examples of the mature insect may be found on 

 the cotton plant. 



Leaf-mould. Well-rotted powdery leaf-mould is 

 needed in all cases where plants are grown in pots. It can 

 be prepared in a month or two, in tropical countries, by 

 making a pile of weeds, dead leaves, pruning.s, etc., in some 

 out-of-the-way corner. In dry weather a little water may be 

 thrown on it occasionally. Good leaf-mould can always be 

 obtained by a spade from the bottom of this pile. The chief 

 use of leaf-mould is to hold water. It also provides a small 

 .supply of nitrates and the other salts needed by all plants. 



* Exaniiiiatidu by the Entomologist on the staff of the 

 Inii)erial Department of Agriculture of the specimens forwarded 

 by Mr. Bartlott shows that this insect is not Dysdercus (annuliger) 

 dehi)nif:ili. It has not yet been identified, but is more nearly 

 related to the Trinidad sjiecies {Di/sdercns liowardi). 



