The Potato-Worm, (Sphinx 5-maculata, Haworth.)—This well-known insect, the larva of which (Fig. 3,) is usually called the potato-worm, is more common on the closely allied tomato, the leaves of which it often clears off very completely in particular spots in a single night. When full-fed, which is usually about the last of August, the potato-worm burrows under the ground, and shortly afterward transforms into the pupa state, (Fig. 5.) The pupa is often dug up in the spring from the ground where tomatoes or potatoes were grown in the preceding season, and most persons that meet with it suppose that the singular jug-handled appendage at one end of it is its tail. In reality, however, it is the tongue-case, and contains the long, pliable tongue which the future moth will employ in lapping the nectar of flowers. The moth itself (Fig. 4) was formerly confounded with the tobacco-worm moth, (Sphinx Carolina, Linnæus,) which it very closely resembles, having the same series of orange-colored spots on each side of the abdomen.
The gray and black markings, however, of the wings differ perceptibly in the two species; and in the tobacco-worm moth there is always a more or less faint white spat, or a dot, near the centre of the front wing, which is never met with in the other species. The potato-worm often feeds on the leaves of the tobacco plant in the Northern States. In the Southern States, in Mexico and the West-Indies, the true potato-worm is unknown, and it is the tobacco-worm that the tobacco-grower has to fight. The potato-worm, however, is never known to injure the potato crop to any serious extent.
The Striped Blister-Beetle, (Lytta vittata, Fabr.) This insect (Fig. 6) is almost exclusively a southern species, occurring in some years very abundantly on the potato-vines in Southern Illinois, and also in Missouri, and according to Dr. Harris, it is occasionally found even in New-England. In some specimens the broad outer black stripe on the wing-cases is divided lengthwise by a slender yellow line, so that, instead of two, there are three black stripes on each wing-case; and often in the same field may be noticed all the intermediate grades; thus proving that the four-striped individuals do not form a distinct species, as was supposed by the European entomologist Fabricius, but are mere varieties of the same species to which the sixth-striped individual appertains.
The striped blister-beetle lives under ground and feeds upon various roots during the larva state, and emerges to attack the foliage of the potato only when it has passed into the perfect or beetle state.
This insect, in common with our other blister-beetles, has the same properties as the imported Spanish fly, and any of them will raise just as good a blister as that does, and are equally poisonous when taken internally in large doses. Where the striped blister-beetle is numerous, it is a great pest and very destructive to the potato crop. It eats the leaves so full of holes that the plant finally dies from loss of sap and the want of sufficient leaves to elaborate its juices. In some places they are driven off the plants (with bushes) on a pile of hay or straw, and burned. Some have been successful in ridding their fields of them by placing straw or hay between the rows of potatoes, and then setting it on fire. The insects, it is said, by this means are nearly all destroyed, and the straw burning very quickly, does not injure the vines.
The Ash-Gray Blister-Beetle, (Lytta cinera, Fabr.)—This species (Fig. 7, male) is the one commonly found in the more northerly parts of the Northern States, where it usually takes the place of the striped blister-beetle before mentioned. It is of a uniform ash-gray color. It attacks not only the potato-vines but also the honey locusts, and especially the Windsor bean. In particular years it has been known, in conjunction with the rose-bug, (Macrodactylus subspinosus, Linn.,) to swarm upon every apple-tree in some orchards in Illinois, not only eating the foliage, but gnawing into the young apples.
This beetle does considerable damage to the potato crop, especially in the North-Western States. Like the other members of the (Lytta) family, it lives under ground while in the larva state, and is troublesome only when in the perfect or winged state.
The Black-Rat Blister-Beetle, (Lytta murina, Le Conte.)—This species (Fig. 8,) is entirely black. There is a very similar species, the black blister-beetle, (Lytta atrata, Fabr.,) from which the black-rat blister-beetle is distinguishable only by having four raised lines placed lengthwise upon each wing-case, and by the two first joints of the antennæ being greatly dilated and lengthened in the males, of the lath species. It is asserted by some authors that the black blister-beetle is injurious to the potato; but I can not see how it could do much damage to that crop, as the perfect insect does not appear until late in August, when the potato crop is nearly out of its reach. Not so, however, with the black-rat blister-beetle, which is on hand ready for business early in the season. This insect does considerable damage to the potato in Iowa, and neighboring States; it is also found, though in not so great numbers, throughout the whole of the Northern States.
The Margined Blister-Beetle, (Lytta marginata, Fabr.)—This species (Fig. 9) maybe at once recognized by its general black color, and the ash-gray edging to its wing-cases. It usually feeds on certain wild plants, but does not object to a diet of potato-leaves. Though found over a large extent of country, it seldom appears in numbers large enough to damage the potato crop materially. Like other blister-beetles, it goes under ground to pass into the pupa state, and attacks the potato only when it is in the perfect or winged state.
The Three-Lined Leaf-Beetle, (Lema trilineata, Olivier.) The larva of the three-lined leaf-beetle may be distinguished from all other insects which prey upon the potato by its habit of covering itself with its own excrement. In Figure 10, a, this larva is shown in profile, both full and half grown, covered with the soft, greenish excrementitious matter which from time to time it discharges. Figure 10, c, gives a somewhat magnified view of the pupa, and Figure 10, b, shows the last few joints of the abdomen of the larva, magnified and viewed from above. The vent of the larva, as will be seen from this last figure, is situated on the upper surface of the last joint, so that its excrement naturally falls upon its back, and by successive discharges is crowded forward toward its head, till the whole upper surface is covered with it. There are several other larva, feeding upon other plants, which wear cloaks of this strange material.