Plant Sanitation, 



54 



[January, 1909. 



per acre, but both be finally efficient. 

 To a member of this class of checking 

 factors, Mr. Elwood Cooper, the former 

 Horticultural Commissioner of Cali- 

 fornia, would apply the term " the true 

 parasite," and those alone he would 

 consider worthy of importation. 



To determine at any time the status 

 of an insect we should have to know the 

 percentage of efficiency of each factor 

 under the existing numerical promi- 

 nence of the host, and in order to prog- 

 nosticate the future we should need to 

 know the ratio of increased or decreased 

 efficiency of each under the changed 

 numbers of the host. 



None of these factors can ever be 

 determined with any greac degree of 

 accuracy because they are each involved 

 in as complicated a system of inter-rela- 

 tions and in many cases the efficiency 

 of a check against any one insect is 

 profoundly influenced by the ups and 

 downs of numerous other insects that 

 serve as alternate hosts. 



The complication of the subject indeed 

 is so great that accuracy even of obser- 

 vation will be impossible, but the failure 

 to reckon with all the factors of the 

 problem will make conclusions of little 

 significance. 



The inter-relation of factors may be of 

 the most complicated nature ; for in- 

 stance, a parasite which of itself might 

 be wholly inefficient due to its slow 

 rate of reproduction as compared with 

 that of its host, might be rendered very 

 efficient by the co-operation of a con- 

 tributing factor which could only delay 

 the rate of increase. 



It will thus be readily seen that the 

 efficiency of all these factors working 

 together is neither the sum nor the 

 average of the potential efficiency of 

 each, though much nearer the latter 

 than the former. Many writers have 

 assumed that by adding a new parasite, 

 its efficiency was simply added to that 

 of others previously existing. This 

 supposition is certainly far from the 

 theoretical conception of the inter-rela- 

 tions of species as presented above, and 

 has not been borne out in actual ex- 

 perience. 



Relation of Life Cycle of Host. 



Thus far the insect whose control is 

 sought is conceived of as existing in but 

 one condition. The growth and trans- 

 formation of insects add still further 

 complications to the subject. The 

 checks are not simultaneous in their 

 action, but at each stage in the progress 

 of its development the insect lives in a 

 different environment. The parasites, 



for instance, that affect the egg will find 

 the next generation of eggs perhaps 

 more profoundly influenced by the 

 checks that have operated during the 

 remainder of the life of the insect than 

 anything they have accomplished, and 

 so perhaps with the checks operating at 

 any stage. A serious attack of one 

 parasite during early larval life might 

 result in protecting the insect from still 

 more efficient destroyers in the late 

 larval stage and really cause more to 

 come to maturity. 



Suggestions for Laboratory Study. 



We can eliminate most causes of death 

 under artificial breeding conditions and 

 often produce one hundred per cent, of 

 survival. When this can be done we 

 are in a position to begin the experiment 

 of testing first one at a time each cause 

 of death, theu to study their inter-rela- 

 tions or the simultaneous or alternating 

 effects of two of these factors, in the 

 case of parasites studying as thoroughly 

 in detail also their environment. Until 

 considerable work of this kind is done 

 the basis for our theories will not have 

 been well enough established to deserve 

 a place as science. 



Economic Relations. 



The power of an insect to do damage 

 is due as a rule to the number present 

 during their chief feeding period, and 

 may be quite independent of the num- 

 bers that finally come to maturity, and 

 is absolutely independent of the ratio 

 between birth and death rates. A 

 temporary disturbance of this rate pro- 

 duces increase or decrease and may 

 place an insect suddenly in the destruc- 

 tive class or remove it, but while an 

 insect maintains itself in injurious 

 numbers the ratio is as low as though 

 the insect were rare. 



In the case of most of our injurious 

 insects the natural increase is more than 

 a hundred fold, so that less than one 

 per cent, is in these cases the established 

 average rate of survival. This is true 

 even of such recently introduced pests 

 as the gypsy and brown tail moths, and 

 the boll weevil, everywhere, except 

 when the conditions are temporarily 

 disturbed by efforts at control and along 

 the border of the infested area where 

 the insects are invading new territory.* 



*This invasion oi new territory probably 

 involves but a narrow strip. Tn the case of the 

 boll weevil the extreme annual migration is 

 about the width of two counties. The total 

 extension of this insect into new territory only 

 requires an average survival of about two per 

 cent, in the outer two tiers of counties, 



