Mar. i, 1921 Biology of the Smartweed Borer 841 



and second instars burrows are often found containing a number of larvae. 

 Their work very quickly results in the wilting of the tender tips above the 

 point of attack, and these drooping tips soon become to the observer an 

 almost certain indication of the presence of the young larvae. As soon as 

 the food supply here is exhausted the larvae desert this portion of the stem 

 and scatter, each reentering at another point to make a burrow of its 

 own, and thereafter only one larva is found in a burrow, although it often 

 happens in a thickly infested stem that these burrows are practically 

 continuous. The stems of Polygonum pennsylvanicum are thick-walled 

 and succulent, with only a very small central cavity. The larvae cut 

 into this cavity, almost invariably entering at the swollen node just 

 below the base of the ocrea, and consume the succulent tissue, leaving 

 only the very thin, fibrous, outer bark. They do not hesitate to abandon 

 a burrow and seek another location whenever the food supply fails. The 

 larger stems are attacked first, but later the branches are utilized, often 

 those so small that the larvae can scarcely crowd into them. The bur- 

 rows are kept clean, all excrement being disposed of through the entrance, 

 which is left open, although with the growth of the plant it often partially 

 heals. 



Larvae of the first generation make no effort to leave the smartweed 

 stems but pupate in them as soon as fully fed. Those of the second 

 generation attack the plants in the same way and feed as did their pro- 

 genitors until they are fully grown. This stage is reached about the last 

 of August, and thereupon many of the larvae abandon their host plant 

 and seek shelter elsewhere. Those entering cornstalks have been par- 

 ticularly noted. Neither thoroughly dry nor green stalks suit them as 

 well as those of intermediate condition. They enter preferably under a 

 leaf sheath or behind an ear. Their presence is indicated by the fluffy 

 white pith showered from the entrance hole upon the leaves below. The 

 entrance hole is perfectly round and clean-cut, and the burrow within is 

 of equal diameter, 3 to 3.5 mm., and is kept clean and free of all cuttings 

 and excrement. It turns downward from the entrance and is from 1 

 to 4 inches long. Early in October the larva closes the entrance with a 

 drum-tight sheet of silk, quite effectively camouflaged by the incorpora- 

 tion of a few brownish particles of the chewed bark. 



As far as determined the larvae are not torpid during the period of 

 hibernation. Repeated collections of larvae in the field during the winter 

 show them always quick to respond when disturbed. There is no evi- 

 dence that they consume any food before pupation after leaving their 

 food plants in the fall. In making their winter burrows in the shelter 

 plants they do not swallow the tissue but discharge it from the mouth 

 in sawdust- like particles. 



No very definite cocoon is constructed by either generation. In 

 smartweed the burrow is lightly plugged above and below the pupa 

 29666°— 21 5 



