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Insects Injurious To The Potato







There are ten distinct species of insects preying upon the potato-plant within the limits of the United States. Many of these ten species are confined within certain geographical limits. Their habits and history differ very widely. Some attack the potato both in the larva state and in the perfect or winged state; others in the perfect or winged state alone; and others again in the larva state alone. In the case of seven of these insects, there is but one single brood every year; while of the remaining three there are every year from two to three broods, each of them generated by females belonging to preceding broods. Eight of the ten feed externally on the leaves and tender stems of the potato; while two of them burrow, like a borer, exclusively in the larger stalks. Each of these ten species has its peculiar insect enemies; and a mode of attack which will prove very successful against some of them will often turn out to be worthless when employed against the remainder. ~The Stalk-Borer~,[A] (_Gortyna nitela_, Guenee.)--This larva (Fig. 2,) commonly burrows in the large stalks of the potato. It occurs also in the stalks of the tomato, in those of the dahlia and aster, and other garden flowers. It is sometimes found boring through the cob of growing Indian corn. It is particularly partial to the stem of the common cocklebur, (_Zanthium sirumarium_;) and if it would only confine itself to such noxious weeds, it might be considered more of a friend than an enemy. It is yearly becoming more numerous and more destructive. It is found over a great extent of country; and is particularly numerous in the valley of the Mississippi north of the Ohio River. The larva of the stalk-borer moth leaves the stalk in which it burrowed about the latter part of July, and descends a little below the surface of the earth, where in about three days it changes into the pupa, or chrysalis state. The winged insect (Fig. 1,) which belongs to the same extensive group of moths (_Noctua_ family, or owlet moths) to which all the cut-worm moths appertain, emerges from under ground from the end of August to the middle of September. Hence it is evident that some few, at all events, of the female moths must live through the winter, in obscure places, to lay eggs upon the plants they infest the following spring; for otherwise, as there is no young potato, or other plants, for them to lay eggs upon in the autumn, the whole breed would die out in a single year. This insect, in sections where it is numerous, does more injury to the potato crop than is generally supposed. ~The Potato-Stalk Weevil,~ (_Baridius trinotatus_, Say.)--This insect is more particularly a southern species, occurring abundantly in the Middle States, and in the southern parts of Missouri, Illinois, and Indiana. It appears to be totally unknown in New-England. The female of this beetle deposits a single egg in an oblong slit, about one eighth of an inch long, which it has previously formed with its beak in the stalk of the potato. The larva subsequently hatches out, and bores into the heart of the stalk, always proceeding downward toward the root. When full grown, it is a little more than one fourth of an inch in length, and is a soft, whitish, legless grub, with a scaly head. Hence it can always be readily distinguished from the larva of the stalk-borer, which has invariably sixteen legs, no matter how small it may be. Unlike this last insect, it becomes a pupa in the interior of the potato-stalk which it inhabits: and it comes out in the beetle state about the last of August or beginning of September. The stalk inhabited by the larva wilts and dies. The perfect beetle, like many other snout-beetles, must of course live through the winter, to reproduce its species the following spring. In Southern Pennsylvania, some years, nearly every stalk of extensive fields is infested by this insect, causing the premature decay of the vines, and giving them the appearance of having been scalded. In some districts of Illinois, the potato crop has, in some seasons, been utterly ruined by this snout-beetle, many vines having a dozen larvae in them. This insect attacks no plant but the potato. ~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_, Linnaeus,) 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 antennae 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. 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 larvae 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 larvae 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 larvae hatching, when full grown descend into the ground, where they transform to pupae (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. Figure 14, _b_, _b_, _b_, gives a view of the larva of the true Colorado potato-bug, in various positions and stages of its existence. Figure 15, _b_, _b_, of that of the bogus Colorado potato-bug. It will be seen at once that the head of the former is black, and the first joint behind the head is pale and edged with black behind only; that there is a double row of black spots along the side of the body; and that the legs are black. In the other larva, (Fig. 15, _b_,) on the contrary, the head is of a pale color, the first joint behind the head is tinged with dusk and edged all round with black; there is but a single row of spots along the side of the body, and the legs are pale. Figure 14, _d_, _d_, exhibits the true Colorado potato-bug; Figure 15, the bogus Colorado potato-bug; each of its natural size. Figure 14, _e_, shows the _left_ wing-case enlarged, and Figure 15, _e_, an enlarged leg of the latter. On a close inspection, it will be perceived that in the former (Fig. 14, _e_) the boundary of each dark stripe on the wing-cases toward the middle is studded with confused and irregular punctures, partly inside and partly outside the edge of the dark stripe; that it is the third and fourth dark stripes, counting from the outside, that are united behind, and that both the knees and feet are black. In Figure 15, _d_, on the contrary, it is the second and third stripes--not the third and fourth--counting from the outside, that are united behind, and the leg is entirely pale, except a black spot on the middle of the front of the thigh. The eggs (Fig. 14, _a_, _a_, and Fig. 15, _d_, _d_) are yellow, and are always laid on the under side of the leaf in patches of from twenty to thirty; those of the bogus are of a lighter color. Each female of the true Colorado potato-bug lays, according to Dr. Schirmer, about seven hundred eggs. In about six days the eggs hatch into larvae, which feed on the foliage of the potato plant about seventeen days; they then descend to the ground, where they change into pupae at the surface of the earth. The perfect beetle appears about ten to fourteen days after the pupa is formed, begins to pair in about seven days, and on the fourteenth day begins to deposit her eggs. There are three broods of this insect every year. Neither geese, ducks, turkeys, nor barn-yard fowl will touch the larva of the Colorado potato-bug when it is offered to them, and there are numerous authentic cases on record where persons who have scalded to death quantities of these larvae, and inhaled the fumes of their bodies, have been taken seriously ill, and even been confined to their beds for many days in consequence. It is also reported to have produced poisonous effects on several persons who handled them incautiously with naked hands. Various plans have been tried to destroy this persistent enemy of the potato plant. Powdered hellebore is said to have been used with effect as a means of destroying the pest. It should be dusted on and under the foliage when the plant is wet with dew. Hellebore, however, is a dangerous remedy on account of its poisonous qualities. A mixture of one part salt, ten parts soap, and twenty parts water, applied to every part of the plants with a syringe, is quite effectual. Several cannibal and one parasitic insect are known to prey upon the larva of the Colorado potato-bug, and the eggs in vast numbers are eaten by several species of lady-birds and their larva.





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