366 



ON THE BLIGHTS OF THE PEAR TREE. 



rows. The insects, therefore, preceded 

 the appearance of the disease nearly a 

 twelvemonth ; from which follows the in- 

 ■evitable conclusion, that the hlight of the 

 limbs was the natural consequence of their 

 previous attacks. 



To suppose that the female Scolytus Py- 

 ri, by anticipation, deposits her eggs only 

 on those limbs which are to become affected 

 by fire-blight, or by frozen sap-blight, is un- 

 warrantable ; because none of these insects 

 have been found in limbs that have perished 

 by these kinds of blight, and consequently 

 none could have existed in them during the 

 previous summer. Hence, it follows that 

 these blights are not caused by the attacks 

 of Scolytus Pyri. 



Observation sufficiently proves that this 

 insect attacks young and thrifty trees, ex- 

 hibiting no visible signs of disease or de- 

 cay. As the young Scolytus is found in 

 limbs that were apparently in a thrifty state 

 during the previous summer, the parent in- 

 sect, though not indued with the power to 

 foresee frozen sap-blight during the follow- 

 ing winter, or fire-blight the next summer, 

 must have had some instinctive perception 

 of the nature and actual condition of the 

 tree on which she deposited her eggs ; and, 

 in performing this act, she would not have 

 been left to blind chance. As the Scolytus 

 lays her eggs on sound and thrifty limbs, it 

 follows that she does so from choice ; and, 

 hence, it is fair to conclude that she would 

 not voluntarily lay her eggs on unhealthy 

 or diseased limbs, so long as she could find 

 those that were healthy. Therefore, the 

 opinion is untenable that the insects, found 

 within the diseased limbs of the pear tree, 

 are not the cause of the disease, but that 

 they are attracted thither by the previous 

 existence of the disease. 



It should be remembered that the Scoly- 

 tus Pyri requires a year, or thereabouts, 



to complete its transformations. In those 

 limbs that perished by insect-blight in the 

 summer of 1847, the seeds of the fatal dis- 

 ease were sown by the parent Scolytus, in 

 the summer of 1846. Those limbs that 

 perished by fire-blight during the last sum- 

 mer, according to the theory of Mr, Cox 

 and Mr. Ernst, must have been sun-struck 

 during the same summer. Those limbs 

 that perished by frozen sap-blight, accord- 

 ing to the theory of Mr. Beecher, had their 

 death blow dealt to them during the previ- 

 ous winter. If the theories advanced by 

 the above named gentlemen be correct, as 

 to the time of attack, the specific difference 

 in these three cases of disease is obvious. 

 The occurrence of the insects in lirnbs de- 

 stroyed by fire-blight, or frozen sap-blight, 

 would be an exception to the natural order 

 of things. But, even on the unproven 

 and unnatural supposition that insect-blight 

 might, for a time, coexist with other blights, 

 it is evident that the attack of the insects 

 must have preceded the disease. Insect- 

 blight, therefore, differs essentially and spe- 

 cifically from fire-blight and frozen sap- 

 blight, in origin and duration. 



" What can we reason but from what we know." 



Had the history of the Scolytus Pyri been 

 well known and duly considered, the blight 

 produced by the attacks of the insect would 

 never have been mistaken for fire-blight, or 

 frozen sap-blight. The mistake has arisen 

 from taking too much for granted, and 

 from not considering the possibility or pro- 

 bability that diseases, having some symp- 

 toms in common, and alike ending fatally, 

 might be really and essentially distinct 

 from each other. 



It may not be generally known that ap- 

 ple, apricot and plum trees are attacked by 

 the Scolytus Pyri, though less frequently 

 than pear trees. In the latter part of May, 



