532 AN INTRODUCTION TO ENTOMOLOGY 



color, except the wing-covers, which are orange, with three dark-blue 

 spots on each (Fig. 650). There is considerable variation 

 in the size and shape of these spots; frequently the two 

 near the base of the wing-covers are joined so as to make 

 a continuous band extending across both wing-covers. 

 The larva feeds on milkweed (Asdepias) . 



Fig. 650. The diabroticas. — Several very important pests belong 



to the genus Diabrotica. In the East they are known as 

 cucumber-beetles; but on the Pacific Coast, where they are more 

 feared on account of their injuries to fruit and fruit-trees, they are 

 commonly called the diabroticas. They are chiefly greenish yellow 

 beetles, marked with black stripes or spots. The striped diabrotica, 

 D .vittataihsiS two black stripes on each wing-cover. The adult feeds 

 on the leaves of cucumber, squash, and melon; and the larva, which 

 is a slender, worm-like creature, bores in the stems and roots of the 

 same plants. The twelve-spotted diabrotica, D. duodecimpunctdta, 

 and Diabrotica sdror, agree in having six black spots on each wing- 

 cover. The former is very common in the East; the latter occurs on 

 the Pacific Coast, and is the most destructive of all of the diabroticas. 

 Diabrotica longicornis is a green species, which feeds on the pollen 

 and silk of com and on the pollen of other plants. Its larva is known 

 as the com root -worm ; it is very destructive to com in the Mississippi 

 Valley. Its injuries are greatest where com is grown on the same 

 land year after year; hence a rotation of crops should be practised 

 where this pest is troublesome. The other species of Diabrotica 

 mentioned above are difficult to combat, as the leaves of cucumber, 

 melon, and squash are very apt to be injured by the use of arsenical 

 poisons. The most practicable way of protecting these vines is to 

 cover them while young with frames covered with netting. Where 

 they infest fruit-trees they can be fought with Paris green ; but this 

 poison must be used with great care on such trees as prune and 

 apricot. Squashes should not be grown in orchards, as is sometimes 

 done in California. 



The flea-beetles. — There is a group of leaf-beetles, of which we 

 have many species, in which the hind legs are fitted for leaping, the 

 thighs being very large. These are commonly called the 

 flea-beetles. 



The striped flea-beetle, Phyllotreta vittdta, is exceed- 

 ingly common on cabbage, turnip, radish, mustard, and 

 allied plants. It is a small, black, shining beetle, with a 

 broad, wavy, pale, dull yellow stripe upon each wing- 

 cover (Fig. 651); it measures about 2.5 mm. in length. 

 These beetles eat numerous little pits in the thicker ^^' 

 leaves that they infest, and minute perforations in the 

 thinner-leaved plants. The larva is a slender, white worm, about 8 

 mm. in length ; it feeds on the roots of the plants infested by the adult. 

 The adult beetles can be destroyed with kerosene emulsion. 



