Insects Injurious To The Potato
from Gthe $100 Prize Essay On The Cultivation Of The Potato; And How To Cook 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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