302 BOARD OF AGRICULTURE. [Pub. Doc. 



woods in the spring and fall migrations. Those who have 

 watched the flights of wood warblers during the present season 

 cannot but marvel at their vast and constantly changing pro- 

 cession. On May 11 of the present year, at Amesbury, 

 Mass., Blackburn ian warblers were seen all through the 

 woods at daybreak. Having come in the previous night, 

 they were not singing, but were busily feeding until seven 

 o'clock. At eight o'clock not one was to be seen. They 

 had passed on, and other species had taken their place. 



The great hosts of migratory warblers feed largely on 

 young caterpillars and })lant lice, — two of the worst ene- 

 mies of trees. These birds come at a time when the first 

 broods of these insects appear, and so do yeoman service in 

 preventing their enormous increase. One needs only to 

 know the possibilities in the way of reproduction among the 

 plant lice to appreciate the services of birds in destroying 

 these early broods. Lintner says of one species, — the hop- 

 vine aphis, — that, according to Riley, it has thirteen gen- 

 erations a year, and that, giving the average numl)er of young 

 produced by each female as 100, if every individual should 

 attain maturity and produce its full complement of young, 

 the twelfth brood alone would amount to ten sextillions. 

 "If this brood," says Lintner, "were marshalled in line, 

 ten to the inch, touching one another, the procession would 

 extend to the sun (a space travelled by light in eight 

 minutes), and beyond that to the nearest fixed star (a dis- 

 tance travelled by light in six years), and onward into space 

 beyond the most distant star that the strongest telescope 

 may bring to our view, to a point so inconceivably remote 

 that light would only reach us from it in twenty-five hundred 

 years." It need hardly be said that no such multiplication 

 as this can ever occur in nature ; still, the calculation shows 

 the possibilities of great danger to vegetation should any of 

 the forces be withdrawn which hold these insects in check. 

 Dr. Fitch, by a careful enumeration and computation, esti- 

 mated that several young cherry trees about ten feet in height 

 were each infested by at least 12,000,000 aphides.* 



The increase of these creatures is largely controlled by 

 birds, but in greenhouses, where birds cannot go, plant lice 



* American Journal Agricultural Science, 1846, p. 282. 



