204 



On the Blight of the Pear Tree. 



Vol. XII. 



The effects of insect-blight begin to appear 

 in June, and continue through July ; but the 

 foundation of the disease may be traced to 

 the operations of the parent insect during 

 the previous summer. If all the limbs that 

 were killed by innsect-blight in the summer 

 of 1847, could have been examined during 

 the latter part of the summer of 1846, there 

 would have been found insects in them, in 

 the larva or grub state, eating their way from 

 the buds, where the eggs were deposited, in- 

 wards, through the bark and sap wood. These 

 insects continued the work of destruction till 

 the following spring, when they were trans- 

 formed to beetles, and finally came forth 

 from their burrows. The insects, therefore, 

 preceded the appearance of the disease near- 

 ly a twelvemonth; from which follows the 

 inevitable conclusion, that the blight of the 

 limbs was the natural consequence of their 

 previous attacks. 



To suppose that the female Scolytus Pyri, 

 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 decay. 

 As the young Scolytus is found in limbs that 

 were apparently in a thrifty state during the 

 previous summer, the parent insect, though 

 not indued with the power to foresee frozen 

 sap-blight during the following 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 thrifly 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 heal- 

 thy. 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 Scolytus 

 Pyri requires a year, or thereabouts, to com- 

 plete its transformations. In those limbs that 

 perished by insect-blight in the summer of 

 J847, the seeds of the fatal disease were 



sown by the parent Scolytus, in the summer 

 of 1846. Those limbs that perished by fire- 

 blight during the last summer, according to 

 the theory of Mr. Cox and Mr. Ernst, must 

 have been sun-struck during the same sum- 

 mer. Those limbs that perished by frozen 

 sap-blight, according to the theory of Mr. 

 Beecher, had their death blow dealt to them 

 during the previous 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 

 limbs destroyed by fire-blight, or frozen sap- 

 blight, would be an exception to the natural 

 order of things. But, even on the nnproven 

 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 probability 

 that diseases, having some symptoms 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, 

 1843, a piece of the limb of an apple tree, 

 affected by the Scolytus, was brought to me 

 for examination. It was twenty-eight inches 

 in length, and three quarters of an inch in 

 diameter at the lower end. Its surface bore 

 the marks of twenty buds — thirteen of which 

 were perforated by the insects ; and, from 

 the burrows within, I took twelve of the 

 blight-beetles in a living state; the thir- 

 teenth having previously been cut out. In 

 July, 1844, I took one of these beetles on a 

 plum tree; and, in August following, I 

 found a large number of them in some pieces 

 of a blighted branch of the apricot. 



To check the increase of this kind of in- 

 sect-blight, it is not enough to cut off the 

 infected limbs. The insects contained in 

 these limbs must be destroyed; for, if they 

 are suffered to make their escape, they will 

 certainly continue to propagate the disease. 

 To make sure of the destruction of the in- 

 sects, every limb affected by them should be 



