INSECTS INJURIOUS TO AGRICULTURE. 191 



potato, before its introduction lived on other plants. The 

 insects of the apple and other fruit trees before those trees 

 were introduced into America lived on certain forest-trees, 

 such as the oak, elm, ash, mountain -ash, wild cherry, pop- 

 lar, willow, etc. 



Our forest-trees are also peculiarly liable to depredations 

 from insects, certain species of which attack the roots, 

 others the bark, others the wood, many the leaves, and a 

 few the fruit or nuts. Thus the oak harbors between five 

 or six hundred species, the hickory affords maintenance to 

 one hundred and forty recorded species, the birch to over 

 one hundred species, the maple to eighty-five, the poplar 

 to seventy-two, while the pine yields food to over a hun- 

 dred different kinds. 



We will now very briefly notice the most common and 

 formidable pests of some of our cultivated plants, referring 

 the reader for further information to the list of works and 

 reports on economic entomology at the end of this book. 



INSECTS INJURIOUS TO FIELD AND GARDEN CROPS. 

 Injuring Wheat. 



The Joint-worm (Isoso-ma hordei Harris). A minute, 

 footless, yellowish-white maggot often forms blister-like 

 swellings between the second and third joints of the stalk, 

 immediately above the lower joint in the sheathing base of 

 the leaf; remaining through the winter in the stubble, 

 straw, or harvested grain, and changing into a small, 

 slender, black, four-winged insect, which deposits its eggs 

 in the stalks of young wheat late in May and in June. 



This is one of the Chalcididee, and, unlike the majority 

 of the family, lives on plants. When wheat or barley is 

 from eight to ten inches high its growth becomes suddenly 

 checked; the lower leaves turn yellow, and the stalks be- 

 come bent. If the butts of the straw are now examined, 

 they will be found to be irregularly swollen and discolored 

 between the second and third joints, and, instead of being 



