20 MASS. EXPERIMENT STATION BULLETIN 189. 



are attacked by the insect. It grows plentifully as a weed in cultivated 

 areas, and also in waste places, generally preferring rather moist soil. 



The newly hatched borer feeds first upon the tender seed head, or upon 

 the epidermis of the tender leaves. As the larva develops it tunnels 

 through the leaf petiole, and when about half grown enters the main 

 stalk. It then usually tunnels downward, feeding through nodes and 

 internodes, and consuming in its progress nearly all the interior of the 

 stalk. This causes a weakening of the plant which soon breaks over at 

 the point where the larva entered. The broken-over portion soon dies 

 and turns brown in color, thus rendering it a very conspicuous object 

 among plants not infested. A mass of conspicuous yellowish-white frass, 

 extruded by the larva within, generally adheres to the point in the stalk 

 where the larva entered. This serves to distinguish plants infested by 

 P. nuhilalis, even in instances where the plants do not break over. 



The number of dock plants per acre is generally rather limited, so that 

 all plants of this species in a given area are commonly infested, depending, 

 of course, upon the degree of infestation. 



Economically, dock is important in that it serves as an early season 

 host plant for the European corn borer in areas where corn is absent. 

 The second generation adults emerging from dock deposit their eggs 

 upon late corn and other commercial crops. 



Barnyard Grass. 



Barnyard grass is the most important and the most commonly infested 

 weed among the uncultivated hosts of the European corn borer. All 

 parts of the plant, except the root, are fed upon by the larva, including 

 the seed head, the leaves and the stalk. Barnyard grass grows luxu- 

 riantly in almost any waste area of gromid, or in the spaces between 

 economic plants in cultivated fields. It seems to prefer well-fertilized 

 soil, and under favorable conditions may reach a height of 5 or 6 feet, 

 with a diameter at the base of nearly half an inch. It is very abundant 

 in all parts of the area infested by the European corn borer, and serves 

 as a food plant for both generations of larvae. 



The newly hatched larvae feed for a short time upon the green buds 

 of the seed head, and also upon the upper or lower epidermis of the 

 leaves. They soon enter the main stalk of the plant, however, and tunnel 

 upward or downward according to their individual preference. A doz«n 

 or more are sometimes found in each stalk, and as the stalks grow very 

 thickly clustered together in clumps, a foot or more in diameter, the 

 aggregate number of larvae infesting each clump of barnyard grass often 

 equals the number normally found in a hill of badly infested corn. Many 

 areas of vacant land, large or small in extent, throughout the infested 

 region, are thickly covered by barnyard grass clumps of this description, 

 which contain untold numbers of the depredating larvae. 



Owing to the small diameter of most barnyard grass stalks, the tun- 

 neling of the larva leads to an early collapse of infested stalks, which 



