THE COLEOPTERA 



145 



the stubble after the first cutting, or treating such fields by going over them once 

 or twice with a disk or spring-tooth harrow, followed by dragging with a brush 

 drag, to give a dust mulch, will protect the second crop but is probably less valu- 

 able than the earlier spring methods. 



The Potato Stalk Weevil (Trichobaris trinotata Say). This pest of the potato 

 is widely distributed over the United States east of the Rocky Mountains except 

 in the more northerly States. It has also been reported from California. The 

 beetle is gray with a black head and three black spots at the base of the elytra and 

 is about a fifth of an inch long. It winters in the old potato stalks and when the 

 young potato plants are large enough it makes small holes in the stalks and 

 sometimes in the branches, in which the eggs are deposited. The eggs hatch in 

 a week or 10 days and the grubs burrow downward toward the roots and after 

 reaching them turn upward again, enlarging the burrows. This tunnelling 

 weakens the stalks and causes the plant to wilt and die. Pupation takes place in 

 the stalks, usually near the ground, and the adults are produced in from 1 to 2 

 weeks, but generally do not leave the stalks until the 

 following spring. A number of individuals may be present 

 in a single stalk. Other food plants are Jamestown weed, 

 horse nettle, eggplant and other plants of the family 

 Solanacese. 



Control. Where the plants have wilted and dying leaves, 

 and an examination of the stem shows borers to be present, 

 pulling up and burning infested stalks is desirable. Prac- 

 tically the same result may be obtained by collecting and 

 burning all the stalks as soon as the crop has been dug, thus 

 destroying the weevils in them. The destruction of all 

 weeds around, which are liable to be infested by the insects, 

 this work being done after the egg-laying season is over, is 

 also desirable. 



The Sweet Potato Weevil (Cylas formicarius Fab.). 

 This is a tropical insect which was first reported in the 

 United States about 1875. It now occurs in the most 

 southerly States from Georgia to Texas, attacking the 

 sweet potato. The adult (Fig. 141) unlike the other snout 

 beetles here considered, is very slender, about a quarter of an inch long, with 

 a black head, reddish prothorax and legs, and dark blue elytra. The prothorax 

 is strongly narrowed, forming a noticeable "waist" for the insect. 



The eggs of this pest are laid singly in small holes eaten in the stem or any 

 exposed potato. They hatch in a few days and the grubs in the stems burrow 

 through them down to the potato, then tunnel irregularly about, becoming full- 

 grown in 2 or 3 weeks. The grub now forms a cavity and in this it pupates for 

 about a week and then a few days later eats its way out and may leave the potato, 

 or may remain there and lay eggs for another generation in the same potato 

 in which it itself developed, and this process may continue until the entire 

 potato is destroyed. As long as food is available, one generation after another is 

 thus produced, but when no more can be found the adult insects live along for a 

 considerable time without feeding, attacking the plants and laying their eggs in 

 them whenever more appear. Adult beetles feed on the leaves and stems somewhat. 

 10 



FIG. 141. Adult 

 Sweet Potato Weevil 

 (Cylas formicarius 

 Fab.) enlarged over 

 five times. (From 

 U. S. D. A. Farm. 

 Bull. 856.) 



