January, 1909.] 



r,5 



Plant Sanitation 



This being the case it will be evident 

 that the effective portion of the work of 

 any introduced parasite lies within the 

 fraction of one per cent, that would 

 otherwise survive. It therefore follows 

 that, should an insect be introduced that 

 would destroy fifty per cent, of the pest, 

 more than forty-nine per cent, of this 

 fifty per cent, is simply the destruction 

 of individuals that would have died 

 from other causes. The real question to 

 be settled, therefore, becomes whether 

 the new insect replaces a more or a less 

 efficient cause of death. The apparent 

 per cent, of efficiency is really no criter- 

 ion whatever of the value of the intro- 

 duction. That which we are desiring to 

 secure is the reduction of the numbers 

 especially during the period of injury, 

 and therefore the only significant datum 

 is the determination of the relative 

 abundance maintained by the injurious 

 species The number of any particular 

 parasite is not even a safe index of its 

 role in the maintenance of this status, 

 unless one were able to accurately weigh 

 its efficiency as contrasted with that 

 which it replaced. 



All entomologists appreciate that 

 natural enemies are largely, if not the 

 only controlling, factors that maintain 

 the present status of insect abundance, 

 but do not so uniformly appreciate that 

 the change of status though related is 

 nevertheless essentially a different 

 problem.— c. w. woodworth, University 

 of California, in Science, Friday August, 

 21, 1908, New Series, Vol. XXVIII., 

 No. 712. 



RUST ON MANGOES, 



Lovers of the succulent mango will be 

 concerned to hear that it is liable to be 

 a victim to the notorious " red rust, " 

 which is one of the worst blights of the 

 tea crop. What is worse, although this 

 discovery was made as long ago as 

 August, 1905, when many valuable 

 mango orchards at Malda were attacked, 

 we learn from the Biennial Report of 

 the Imperial Department of Agriculture 

 that nothing has been done yet to dis- 

 cover a suitable treatment for the 

 disease, as prolonged investigation on 

 the spot would be necessary before auy 

 suggestion for treatment could be 

 made", and " this is not possible at 

 present, " though why we are not told. 

 One would think, however, that the 

 mango crop is of sufficient importance 

 to justify such an impossibility being 

 speedily overcome. In the meanwhile 

 it is slightly consoling to know that the 

 conditions determining virulence are 

 local. The features of the disease, in 

 fact, correspond closely to those ob- 



served on tea, and, like the latter, 

 serious loss is confined to certain 

 districts, though the alga, its cause, is 

 widely distributed over India. — Indian 

 Agriculturist, Vol. XXXII., No. 9. 



CACAO THRIPS. 



By H. A. Ballou, M, Sc. 

 Entomologist on the Staff of the Im- 

 perial Department of Agriculture, 



The cacao thrips is probably familiar 

 to every grower of cacao in the Islands 

 where this insect occurs. Since it first 

 appeared as a pest in Grenada it has 

 been more or less prevalent from time 

 to time in different places. 



It has been found that thrips becomes 

 numerous enough to attract attention 

 only when, for some reason, the cacao 

 trees are not in the most vigorous con- 

 dition of growth. 



Some of the influences which tend to 

 reduce the vitality of the cacao trees 

 and thus render them more susceptible 

 to the attacks of thrips may be men- 

 tioned—seasons of drought, lack of 

 drainage, lack of tillage, lack of ferti- 

 lity and of humus in the soil ; and, in 

 fact, anything which causes a check in 

 the growth or produces an unhealthy 

 condition of the trees is liable to pro- 

 mote the development of thrips. 



On the other hand, anything that 

 tends to improve in a general way the 

 vigour and healthfulness of the cacao 

 trees would be expected to assist in 

 reducing and keeping down the number 

 of thrips. In all cases where it had 

 been tried it has been found that any 

 improvement of the conditions of the 

 trees that are suffering from thrips 

 results in diminished numbers of, and 

 lessened injury by, these insects. This 

 improvement may be brought about by 

 attending to drainage, by better cul- 

 tural methods, such as tilling the soil, 

 pruning out dead wood, improving the 

 soil by the addition of- manures and 

 humus to give greater fertility and 

 better aeration, and to increase the 

 moisture-retaining properties in times 

 of drought. 



Experiments in Grenada, St. Lucia, 

 and Dominica have all given results 

 bearing out these statements, and from 

 observations on estates where, although 

 no experiments have been carried out, 

 improved cultural measures have been 

 adopted, it has been found that attacks 

 of thrips have been less frequent and 

 less severe, and in certain instances 



