Many authors suppose that the object of the larva in all these cases is to protect itself from the heat of the sun. In all probability the real aim of nature in the case of all these larvæ is to defend them from the attacks of birds and of cannibal and parasitic insects.
There are two broods of this insect every year. The first brood of larvæ may be found on the potato-vine toward the latter end of June, and the second in August.
The first brood stays under ground about a fortnight before it emerges in the perfect beetle state, and the second brood stays under ground all winter, and only emerges at the beginning of the following June.
The perfect beetle (Fig. 11) is of a pale yellow color, with three black stripes on its back, and bears a strong resemblance to the cucumber-bug, (Diabrotica vittata, Fabr. Fig. 12.)
From this last species, however, it may be distinguished by its somewhat larger size, and by the remarkable pinching-in of the thorax, so as to make quite a lady-like waist there, or what naturalists call a "constriction." The female, after coupling, lays her yellow eggs (Fig. 10,d) on the under surface of the leaves of the potato plant. The larvæ hatching, when full grown descend into the ground, where they transform to pupæ (Fig. 10, c) within a small oval chamber, from which in time the perfect beetle emerges.
This insect in certain seasons is a great pest in the Eastern and Middle States, but has never yet occurred in the Mississippi Valley in such numbers as to be materially injurious.
The Cucumber Flea Beetle, (Haltica cucumeris, Harris.) This nimble minute beetle (Fig. 13) belongs to the flea-beetles, (Haltica family,) the same sub-group of the leaf-beetles (Phytophaga) to which also appertains the notorious steel-blue flea-beetle (Haltica chalybea, Illiger) that is such a pest to the vineyardist. Like all the rest of the flea-beetles, it has its hind thighs greatly enlarged, which enables it to jump with much agility. It is not peculiar to the potato, but infests a great variety of plants, including the cucumber, from which it derives its name. It eats minute round holes in the leaf of the plant it infests, but does not always penetrate entirely through it.
The larva feeds internally upon the substance of the leaf, and goes under ground to assume the pupa state. It passes through all its stages in about a month, and there are two or three broods of them in the course of the same season. This is emphatically the greatest insect pest that the potato-grower has to contend with in Pennsylvania. It abounds throughout most of the Northern, Middle, and Western States. Large fields of potatoes can any summer be seen in the Middle States much injured by this minute insect, every leaf apparently completely riddled with minute round holes, and the stalks and leaves appearing yellow and seared. Plaster frequently and bountifully applied is sure to prevent the attacks of this insect, or to disperse it after it has commenced operations.
The Colorado Potato-Bug, (Doryphora 10—lineata, Say.)—This insect, which, according to Dr. Walsh, has in the North-West alone damaged the potato crop to the amount of one million seven hundred and fifty thousand dollars, came originally from the Rocky Mountains, where it was found forty-five years ago, feeding on a wild species of potato peculiar to that region, (Solanum rostratum, Dunal.) When civilization marched up the Rocky Mountains, and potatoes began to be grown in that region, this highly improved pest acquired the habit of feeding upon the cultivated potato. It went from potato-patch to potato-patch, moving east-ward at the rate of about sixty miles a year, and is now firmly established over all the country extending from Indiana to its old feeding-grounds in the Rocky Mountains. In about twelve years it will have reached the Atlantic coast.
There is another very closely allied species, known as the Bogus Colorado potato-bug, (coryphora juncta, Germor,) which has existed throughout a great part of the United States from time immemorial. This latter insect, however, feeds almost exclusively on the horse-nettle, (Solanum carolinense, Linn.,) and is never known to injure the potato. Both insects are figured, so that one need not be mistaken for the other.