November, 1911.] 



435 



Plant Sanitation. 



of drought on the fungi, as already indi- 

 cated ; and secondly, the fact that the 

 black fungus is hardier in this respect 

 than the other species. The second of 

 these conclusions, however, must be 

 tested by future observations before it 

 can be definitely accepted, as there are 

 no records of the relative abundance 

 of the fungus in these two islands 

 during the year under consideration. 



The outbreak of scale insects in 

 Dominica in 1903 presents some further 

 features of interest in connexion with 

 the effect of external conditions on the 

 parasites of these insects. In 1902 the 

 season was dry, the yield of fruit from 

 the lime trees, which were those princi- 

 pally attacked, was very heavy, and the 

 island was covered with dust from the 

 eruption of Mont Pelee. During the 

 ensuing dry season at the beginning 

 of 1903 the trees were badly attacked 

 by scale insects, which, however, rapid- 

 ly disappeared again in the subsequent 

 normal years 1904 and 1905. It is of 

 course clear that the trees had been 

 weakened by the heavy crops, combined 

 with the general unfavourable condi- 

 tions in 1902 ; but it is also fairly certain 

 that the spread of the scale insects was 

 largely due either to the absence of the 

 parasites by which they had formerly 

 been controlled, or at any rate to the 

 check that the parasites had received 

 through want of moisture, together 

 with the effects of the volcanic ash, in 

 1002, and the succeeding dry season in 

 1903. 



The idea that the reduction of the 

 parasites was the factor mainly respon- 

 sible for the outbreak of this scale 

 insect attack is supported by the gra- 

 dual return to the normal conditions 

 which occurred during the years 1904 

 and 1905, and by the absence of any 

 serious subsequent attacks. Moreover, 

 the outbreaks cannot be explained as 

 due to the introduction of new species 

 of scale insects, as the scales concerned, 

 mainly the purple and the white scales 

 (Mytilaspis citricola and Chionaspis 

 citri) were both reported by Professor 

 Riley as being present in the island 

 in 1894. It should also be mentioned 

 that on some estates extensive spray- 

 ing operations were carried on chiefly 

 with kerosene emulsion and rosin com- 

 pound. The trees of such estates 

 recovered much more quickly than 

 those which were not treated, and it 

 seems likely that in certain cases, at 

 any rate, the complete loss of any trees 

 was prevented. It should be noted, 

 further, that the solutions used were 

 those least likely to have any harmful 

 effect on parasitic fungi of the scales, 



and that they probably were of actual 

 assistance by checking those insects 

 sufficiently to enable the parasites to 

 regain their former supremacy. 



As the effect of volcanic ash has been 

 mentioned, it may be worthy of record 

 that there seems to be a fairly general 

 belief among planters throughout the 

 islands, to the effect that black blight, 

 and therefore probably scale insects, 

 have been more common since the big 

 eruptions in Martinique and St. Vincent 

 in 1902. This may possibly be due to 

 the effect of the ash on the parasites 

 of the scales. While ib might be so 

 in the case of the delicate insect para- 

 sites, it is not an easy matter to account 

 for the effect on the fungi. In any case 

 this would seem to be an illustration of 

 the great increase in numbers of an 

 insect, owing to the fact that the natural 

 control exerted by its parasites had 

 been removed, or seriously checked. 

 The scale insects may themselves have 

 been greatly reduced in numbers, but 

 the much larger destruction of the 

 more fragile parasites provided an 

 opportunity for their almost unre- 

 strained natural increase. As there is 

 little doubt that the survival of the 

 parasites occurred to some extent, it 

 is fairly certain that they will ultim- 

 ately reach such numbers as to be 

 capable of exerting the same control 

 as existed before the volcanic eruptions. 



The Effect of Scale Insect Para- 

 sites ON THE PRESENCE OF BLACK 



Blight. 



As a result of enquiries made in all 

 the islands, it has been shown fairly 

 definitely, that black blight fungi 

 {Capnodium and Meliola sp.), but especi- 

 ally Capnodium mangiferce, are never 

 found on trees where they are not in 

 some way or other associated with the 

 presence of one or more species of scale 

 insects. The commonest insects in this 

 connection are the green scale (Leca- 

 nium viride), and the mango shield scale 

 (L. Mangiferce). Consequently, the re- 

 duction of the scale insects by means 

 of their parasites should be attended 

 by a lessening of the amount of black 

 blight, and where a tree is successfully 

 cleared of insects, the blight should 

 also disappear. Experiments with a 

 view to controlling black blight in this 

 way have been suggested for trial in 

 Grenada, an island in which there is a 

 very large amount of this disease. On 

 the other hand, the author has seen a 

 species of black blight fungus on a 

 palm at the Botanic Station at St. Lucia, 

 distributed in such a way under the 

 inflorescences as to lead to the suspicion. 



