November, 1911. j 



Possible Relation between Host 

 and Parasite. 



When looking for parasitic fungi, with 

 the possible exception of the black fun- 

 gus (Myriangium Duricei), it must be 

 remembered that the fructifications, by 

 which alone they are made easily visible, 

 cannot be expected to be present in any 

 given locality at all times of the year. 

 They come and go according to the stage 

 in its life-history that the fungus has 

 reached at the time. As an illustration 

 of this, the following observations in 

 Dominica during 1909 may be cited. 

 The fructifications of the red-headed 

 fungus (Sphcerostilbe coccophila) were 

 observed at the Agricultural School in 

 May, but had disappeared ac the begin- 

 ning of June; they did not re-appear until 

 November. These facts should be con- 

 sidered in connection with the following 

 observations made in Montserrat : — 



One field was visited by Mr. H. A. 

 Rallou, the Entomologist to this Depart- 

 ment, and by the author on March 9, 1910. 

 The field had suffered from attack by 

 the purple scale (Mytilaspis citricola) 

 during the preceding twelve months, 

 and had been put under Bengal beans 

 which had been cutlassed a mouth pre- 

 viously. The trees had recovered from 

 the scale insect attack, but practically 

 no red-headed fungus was observable, 

 although it had been seen a month 

 before in large quantities by two reliable 

 observers. On the next field to leeward, 

 however, fructifications of this fungus, 

 both conidial and perithecial, were 

 present in large numbers. The attack of 

 the insects had reached this field some- 

 what later than the previous one, as the 

 insects travelled with the wind, and 

 consequently the fungus was still 

 visible, as it also had started its life- 

 cycle at a somewhat later date than in 

 the first field. The presence of the 

 perithecii, which probably constitutes 

 the last stage in the life-history, would 

 appear to indicate that the fungus was 

 about to disappear from the second field 

 also. Both of the instances just cited 

 tend to show that the fungus has a 

 definite life-peiiod which must be in- 

 fluenced to some extent by the amount 

 of food-supply available. It should also 

 be borne in mind that the scale insects 

 themselves have a definite life-period, 

 though details with regard to this are at 

 present wanting in the West ludies. It 

 is probable that the season of greatest 

 abundance of these insects falls between 

 approximately the same dates each year, 

 and the same would also be true of 

 the season of least abundance. The 

 sequence of events would appear to be 

 somewhat as follows : — The spores of the 



55 



433 Plant Sanitation. 



fungus are blown on to a tree where 

 numerous young scale insects are present ; 

 they germinate if the conditions are not 

 too dry ; the germ tubes penetrate the 

 bodies of some of the young scales by 

 growing in under the scale itself, and 

 once established, the fungus spreads 

 rapidly. After vigorous vegetative 

 growth, during which the only sign of 

 the existence of the fungus is the 

 number of dead insects present, the focd- 

 supply . becomes exhausted, and the 

 fungus produces fructification and is 

 tnen visible. Finally, the fructifications 

 and most of the dead scales are washed 

 off the tree by rain and disappear, leav- 

 ing the tree clean with the exception of 

 spores of the fungus caught in the bark, 

 a small residue of dead scales that have 

 not been removed, and a few healthy 

 scales that have not been infected by 

 the fungus. If the scale insects re-infect 

 the tree, either owing to the increase 

 of the original survivors, or to infection 

 from an outside source the spores 

 germinate again, or re-infection takes 

 place from outside, and as the scales 

 become fairly numerous, the life-story 

 of host and parasite is repeated. There 

 are thus probably two periods when the 

 fungus is not visible : one when it is 

 making much vegetative growth, scale 

 insects being numerous, the other when 

 it has disappeared owing to the absence 

 of a sufficient food-supply ; under the 

 latter circumstance, very little fungus is 

 present at all on the tree. How far the 

 life-period of the fungus is affected by 

 external conditions remains to be seen; 

 probably moisture has an effect on the 

 rate of growth during the period of 

 vegetative activity. The relationship of 

 the fungus life-period to that of the host 

 also requires investigation, but for this, 

 the life-period of the scale insects, that 

 is the time occupied from one brood to 

 the next, must first be carefully 

 determined. In any case, it seems that 

 the food-supply, as afforded by the 

 insects, cannot be expected to be entirely 

 continuous. Green, in his book on the 

 CoccidcB of Ceylon, remarks on the 

 periodicity of the appearance of the 

 green shield scale (Lecanium viride), and 

 says that this is apparently connected 

 with weather conditions, extremes of 

 rain or of drought being alike unfavour- 

 able. Moreover, when the numbers of 

 live scales are decreasing, the shield 

 scale fungus is always found to be 

 present and may kill as many as 90 per 

 cent, of these insects. 



The above suggestions are put for- 

 ward in a purely tentative manner, 

 as the evidence on which they are 

 based is insufficient to permit of any 



