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Historic, archived document

Do not assume content reflects current
scientific knowledge, policies, or practices.

Ee BRAS

OF THE

U. S. Department of Agriculture

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Class ae ish aoe : fg
Book - En 42 C
. Nos } \- 86

CIRCULAR No. 59.

United States Department of Agriculture, f

BUREAU OF ENTOMOLOGY. _ <2 Pp
L. O. HOWARD, Entomologist. | 1e nem Peate

atc igta
NY UN ata oN
Pe ae eae ENN
THE CORN ROOT-WORMS. ae fe
hal

By F. H. CHITTENDEN, A ger™
In Charge of Breeding Experiments?”

Among the many enemies of Indian corn two forms of slender, soft-
bodied, whitish larve known as root-worms are prominent through their
injuries. When the plants are found to be withering or when the ears
fail to fully develop, forming ‘““nubbins,”’ without any visible cause, the
earth about the roots should be searched. It then frequently happens
that root-worms are found at work. They operate somewhat in the
manner of wireworms, and the two forms vary somewhat in the nature
of their operations, as well as in the territory which they ravage. They

are the young of two species of leaf-beetles of the family Chrysomelidee
and of the tribe Galerucini. ,

The species which inhabits the middle western region is the western
or northern corn root-worm (Diabrotica longicornis Say). Its injuries
are practically confined to the middle West, where it would be a very
serious pest were it not that progressive farmers in that region have
adopted as a part of a sound agricultural routine a system of rotation
which greatly reduces losses that might otherwise be sustained.t In
spite, however, of the best methods of tillage, losses due to this insect
are reckoned in millions of dollars annually. Thus Prof. F. M. Web-
ster estimated the damage accomplished by this pest to corn in 24 coun-
ties of Indiana alone in 1885 at nearly $2,000,000, this judgment being
based on a loss of $16,000 sustained by one farmer and a personal knowl-
edge of the destructive abundance of the insect throughout that State.

The second species is the Southern corn root-worm (Diabrotica 12-
punctata 01). It occurs generally throughout nearly the whole of the
United States, but fortunately its injurious range is somewhat closely
restricted to the southern States, though in recent years it is showing
a disposition to extend depredations into more northern regions.

The former species attacks first the fibrous roots and later the larger
ones which it frequently destroys to such an extent that the infested
plants may easily be pulled up by hand after they are seen to wither.
The latter, at least in the maturing stage, eats directly into the stalk at
a point near the upper whorl of roots. If injury by either species is not .
extensive, the ears, as previously stated, do not entirely develop.

1For more detailed information see Forbes, 18th Rept. St. Ent. Il]., 1891-92
(1894, pp. 146-165); Bul. 44, Univ. Ill., May, 1896, pp. 282-296; Webster, Ind.
Agl. Rept., 1885, author’s ed., pp. 6-10; Riley, Insect Life, Vol. IV, pp. 104-108.

THE SOUTHERN CORN ROOT-WORM.
(Diabrotica 12-punctata 01.)

This species is also known in the South as the ““’bud-worm ”’ and less
commonly as the © drill worm.’? In some regions it is recognized only
by the former name, while the latter is of more recent use. The beetle
is familiar to most persons in its occurrence on cucumber, squash, and
other cucurbits. It frequents the flowers, and by peeping into them
one or more beetles and the places where they have gnawed the petals
may be seen, for they are among the most omnivorous of insects and
appear to be able to subsist on any form of vegetation on which they
may happen to alight. They are, in fact, to be found in practically all
fields of corn and in gardens everywhere, but it 1s seldom that the larvee
are associated with the
beetles except by those
well versed in entomol-
ogy or who have had
experience with both
forms of insects. The
species is best known
northward as the
twelve-spotted cucum-
ber beetle.

DESCRIPTIVE.

This insect has four
stages, adult or beetle,
egg, larva, and pupa.

Fria. 1.—Southern corn root-worm (Diabrotica 12-punctata): a, Ti b t] : 1 -
beetle; b, egg; c, larva: d, anal segment of larva; e, work of larva ve veetlé 18 elong-
at base of cornstalk; f, pupa—all much enlarged except e, which gate pyriform, yellowish
is reduced (reengraved after Riley, except f, after Chittenden).

green in color, and the
elytra or wing-covers are marked with twelve black spots (fig. 1, a).
The head is black, as are also the legs, except portions of the femora
or thighs. The length is one-fourth of an inch or a little longer.

The egg (b) does not differ materially from that of related species,
such as the striped cucumber beetle, except in size. It is broadly oval
in shape, measuring in the average 0.025 inch in length. The color is
dull yellow, and the surface is closely and somewhat regularly faceted.

The larva (c) is slender, even thread-like. delicate and soft-bodied,
and white or yellowish in color. The body is smooth and cylindrical,
the head short and rounded, and brownish in color, and the first thoracic
segment is also slightly brown, while the anterior extremity as a rule
ends in two points.?

' Less acute than in the striped cucumber beetle (Diabrotica vittata) and some-
times obtuse or wanting.

3)
oO

The above characters, with the aid of the illustrations (fig. 1, c, d),
serve to distinguish this corn root-worm from young wireworms! with
which they are often associated in injury to corn and other cereals,
grasses, and other plants. Betore changing to pupa this root-worm
attains a length of about half an inch.

The pupa (fig. 1, f) presents no special features worthy of remark
that are not shown in the accompanying illustration. It is white like
the larva, and similar to that of the striped cucumber beetle.

GEOGRAPHICAL DISTRIBUTION.

The twelve-spotted cucumber beetle occurs commonly throughout
that portion of the United States lying between the Atlantic seacoast
and the base of the Rocky Mountains, and from New England to Florida.
It also inhabits Canada in the North and extends in a southwesterly
direction through Texas into Mexico. Throughout practically all this
region, which includes portions of twenty-nine States, it is really abun-
dant, though not nearly so destructive in the northern as in the south-
ern States, where year by year more or less injury is accomplished by
the larve or root-worms as far northward as Maryland, Virginia, and
southern Ohio.?

FOOD HABITS.

The adult, as stated, is practically omnivorous. Its known food
materials are legion and include, besides the blades, green ears, silk,
and pollen of corn, the partly matured kernels of wheat, corn, and oats,
the foliage of alfalfa, corn, clover, crimson clover, cotton, rye, tobacco,
beets, cabbage, kale, cauliflower, turnip, mustard, cucumber, canta-
loupe, watermelon, pumpkin, squash, okra, potato, tomato, rhubarb,
and asparagus, the leaves and pods of beans, as also the buds, flowers,
and sometimes the leaves of fruit trees, including apple, pear, quince,
apricot, cherry, and peach, the buds and flowers of roses, dahlia, sun-
flower, aster, canna, chrysanthemum, grapevine, sweet pea, cosmos,
cultivated Bidens, and raspberry, the fruit of apple, melons and other
curcurbits. To all of these more or less serious injury is inflicted.
Among wild plants, or such as are of little practical utility, frequented
by the beetles for food are golden-rod (Solidago), wild sunflower, New
Jersey tea (Ceanothus), pokeweed (Phytolacca), milkweed (Asclepias),
groundsel (Senecio), horse nettle, Nelumbo, Impatiens, and Amorpha.?

‘The latter, it should be stated, are usually more or less flattened, especially
at the head, while the posterior extremity is usually toothed, notched, or pointed.
They also serve to separate this root-worm from maggots of several small flies,
such as the seed-corn maggots, which also infest corn roots. The root-feeding
maggots, however, are footless, while the root-worms a six legs nenge near
the head.

2In the Pacific States it is replaced by a similar very closely related species,
MNabrotica soror Lec., which is sometimes quite injurious.

3Some of the food plants above enumerated have not hitherto been recorded.

~

Larve or pupe have been observed at the roots of corn, wheat, rye,
millet (Panicum miliaceum and Bromus unioloides), beans, Rud-
beckia, and sedges of the genera Cyperus and Scirpus. The favorite
larval food plants are evidently Graminee, including various cereals
and grasses, and the Cyperacez or sedges.!

The presence of this root-worm in a field of corn gives origin to the
loss of the roots, injury varying according to the age of the corn and the
severity of attack, and somewhat also upon the condition of the weather
and of the soil infested. Injury is manifested in various ways, from
the death of a plant to slight retardation in its growth or what is some-
times termed “‘spindling,”’ or a yellowish and unhealthy look. In
plants 6 inches or less in height the perforations of the stalk (fig. 1, ¢)
are characteristic, and usually show just below the surface of the ground.
This is accompanied by withering of the plant, which is frequently
killed outright. It is this appearance which has given the insect the
name of “ bud-worm.’’ Quite frequently plants are destroyed almost as
soon as sprouted. As plants grow, the signs of injury vary according to
the degree of development. If plants suspected of harboring this
species are pulled up and shaken over a sheet of cloth or paper, the
root-worms can be dislodged, and it is not difficult to distinguish them
from any other forms of insects to be found in the same locations, with
the exception of the western corn root-worm, which, however, seldom
occurs in the same regions.

LIFE HISTORY.

This beetle is one of the earliest as well as latest species. About the
District of Columbia the beetles have been seen abroad by the last week
in March, and eggs have been obtained from April 18 to May 9. Beetles
have been seen freely feeding until the middle of November, when the
flowers which had furnished them with food began to wilt and die.
Just when the first new generation appears seems not to have been
recorded, but several beetles, still immature, have been seen in the
open by the writer on June 12, others were reared the same day and in
July, and still others were seen, newly transformed, late in September.
Although the writer can not speak with positiveness as yet as to the
number of generations, these indications show the probable production
of two and perhaps three generations annually in the District of Colum-
bia, and there is the possibility of four in the insect’s more southern
range.

Eggs are laid, as has been stated, at the base of the insect’s food
plant, and have been observed by the writer to hatch in six and seven
days in cool May weather. One individual deposited 202 eggs

MiAtues nee ie ieee a te ad by the writer from abou the roots of
Jamestown weed (Datura) and pigweed (Amaranthus), and it is not improbable
that they feed on these plants.

5

NATURAL ENEMIES.

The beetle is parasitized by a Tachina fly, Celatoria diabrotice Shimer
(fig 2), which develops in the abdomen of its host. It is preyed upon
by the wheel bug, Arilus (Prionidus) cristatus Linn., and many birds
feed to a considerable extent on the beetles. It is also subject to a
bacterial disease.!

REMEDIES.

For the root-worm the use of insecticides on growing corn is imprac-
ticable. In fact, we can not reach the root-worm stage to any extent
with poisons. Therefore we must have recourse to farming methods.
Injury in corn fields is most abundant when the seed has been planted
in bottom lands, and if planting is necessary in such locations, it should
be done late, in Georgia by the first of May, according to Quaintance;
or attack may be so distributed that dam-
age will be inconsequential by dropping
eight or ten grains of seed corn in each
hill. Of still more importance, however,
is judicious crop rotation. Numbers of
crops are not injured to any extent by the
Southern corn root-worm, and of these are
cotton, buckwheat, the smaller grains, and
various garden vegetables other than beans
and cucurbits. Experience will teach
which of these can best be employed as ace ae
alternates. It is inadvisable to plant Layla ea
squash and pumpkin to any extent in ©?!@rsed (original).
rows between the hills of corn, more particularly in regions where injury
by this species has been previously observed.

For the beetle as it occurs on cucumber, squash, and other cucurbits
the same remedies in use against the striped cucumber beetle should be
pursued.? In its occurrence on most other crops it can be controlled in
the same manner as other leaf-beetles, for example, the Colorado potato
beetle, by applying an arsenical alone or combined with Bordeaux mix-
ture, the latter having a repellent effect. Arsenate of lead is preferable
to Paris green as it adheres to the foliage better and is not washed off
so readily by rains, and there is little or no danger of scorching.

There is little that can be done to lessen the number of beetles for
ensuing years, because of their roving habits and their ability to subsist
on such a vast variety of vegetation. The species appears to have no
particular food material, if we except flowers of cucurbits, goldenrod,
and some others, and thus the beetles are seldom found congregated
where they could be destroyed “at wholesale.”’

1Forbes, 17th Rept. St. Ent. [ll., 1889 and 1890 (1891), pp. 71-73.
2See Circular 31, entitled ‘‘The Striped Cucumber Beetle (Diabrotica vittata
Wain). 7

6

THE WESTERN CORN ROOT-WORM.
(Diabrotica longicornis Say.)
DESCRIPTIVE.

The beetle may be readily distinguished from its Southern congener
by its nearly uniform pale, but usually distinctly grass-green, or green-
ish yellow color. (Preserved specimens usually lose this coloration to
a considerable extent.) It is also smaller, of about the same size as
the striped cucumber beetle, measuring about a fifth, or a little more,
of an inch in length, and is a little more slender and less pear-shaped
(fig. 3,a). In the earlier stage the Western corn root-worm very closely
resembles the Southern species, all stages naturally being smaller, as
would be expected in an insect of smaller size.

The egg. This apples particularly to the egg, which is of similar
form and sculpture. It has only about 20 hexagonal pits in its longest
diameter, compared with 30
to 35 in that of its congener.
It is, moreover, dirty white
instead of yellowish, and
measures only .015 to .025
inch as against .02 to .03 in
the Southern species.

The larva, when fully ma-
ture, attains a length of only
four-tenths of an inch as com-

Fig. 3.—Western corn root-worm (Diabrotica longi- pared with half an inch in
cornis): a, beetle; b, larva or root-worm; c, enlarged the case of the Southern form.
leg of same—all enlarged, ec, more enlarged (original.) In other respects they are very
similar, as will be noted by comparison of the illustration of the West-
ern species (fig. 3, b) with the Southern (fig. 1,¢). The anal segment
in the larva under consideration is always blunt, never armed with tuber-
culous points.

The pupa (fig. 8, d), judging from available material, though smaller,
is proportionately wider, giving it a more robust appearance than in the

Southern species.
DISTRIBUTION.

This species, which was first recognized as a pest in 1874, has a more
limited geographical range than the preceding, and is moreover confined
in its field of destructiveness. We know of its occurrence from central
New York and Canada, including Nova Scotia, westward to Kansas and
Nebraska. Injurious occurrences are limited to the States of Illinois,
where it is most troublesome, Indiana, Lowa, Kansas, Nebraska, Ohio, and
Missouri. It does not occur in destructive numbers in Kentucky; in
fact it is rarely found in that State. It was very injurious in the vicinity
of Chillicothe, Ohio, in 1908, and injuries were reported at Circleville,

ni

just south of Columbus, Ohio, the same year. Reports from southern
Ohio in 1904 indicate that the species is increasing in abundance in
that region.

FOOD AND OTHER HABITS.

It has already been stated that corn is the only known host plant of
the larva. The beetles show a disposition to feed on a variety of
plants, but are more choice than the southern species. In the expert-
ence of the writer and evidently of others also, the beetles, which are
naturally pollen feeders, like other members of this genus, are partial
to the blossoms of thistle (in which they deeply imbed themselves),
sunflower, and goldenrod, and are less frequently found on cucurbits.
It is probable that the beetles do some damage to corn by eating the
pollen and gnawing the silk and tassels, thus preventing cross-
fertilization and causing a partial blasting of ears. Other plants, also,
are frequented for the sake of pollen, and the list includes ragweed and
smartweed growing between hills of corn, clover, and some other plants.
In late fall the beetles have the same habit as the twelve-spotted and
striped cucumber beetles, of gnawing into ripe squash and pumpkin in
the field, and they do some injury in this way. The plants attacked
include bean, sorghum, and cotton, which are visited for pollen, and
ripe apples, which are sometimes injured in the same manner as cucur-
bits; and the beetles have been noticed gnawing the ears of corn, more
particularly where they have first been injured by birds or grasshoppers.
Since the beetles have been observed by Forbes to feed on fungi,
blights, rusts, etc., there is no doubt that they do some harm, which
has been positively ascertained to be the case with the striped cucum-
ber beetle, in acting as conveyors of fungous diseases from unhealthy
to vigorous plants.

The species is credited with being single-brooded. The beetles occur
normally in the field, like the Southern species, until November, sub-
sisting upon such flowers and other vegetation as can be obtained at
that time, and in open winters have been noted abroad as late as the
middle of December, which is, in the writer’s opinion, proof positive
that the beetles hibernate, but the observations of Messrs. Forbes and
Webster indicate that as a rule the species passes the winter in the
egg condition, in the earth. If this hypothesis is true it differs from
the great majority of leaf-beetles. Eggs, so far as known, are deposited
in fields of corn late in the season and hatch the following spring.
Larve have been seen in central Illinois the second week in June, and
the beetles have been reported in southeastern Iowa toward the end of
June. Forbes assigns the first weeks of August and October as the
average times for the first and last deposit of the eggs, and recognizes
a period of eight or nine weeks for the passage of a generation from the
first to the last stage. The egg-hatching period is set from the middle
of May to the middle of July and a little later.

8

Eggs are deposited in the earth in more or less scattered clusters of
three to ten, and at a depth varying from 1,to 6 inches, all being placed
about the roots in a space a few inches around each hill.

The developing larve live entirely beneath the surface, feeding on the
roots, which they mine either upward or downward, and seem capable
of traveling from one root to another. Pupation also takes place under-
eround, and pups may be found in considerable numbers, at the proper
period, about infested corn roots. When the beetles begin to issue,
toward the latter part of August, they are first noticeably abundant on
thistle blossoms, and afterward on other plants blooming at this season.
Like the twelve-spotted cucumber beetle, they show a disposition, late
in the season, to disperse before seeking quarters for hibernation or the
deposit of their eggs, flying from one field to another.

PREVENTION AND REMEDY.

This species is more readily controlled than the Southern corn root-
worm ; in fact, losses can be prevented by simply following crop rotation.
Since the insect, as far as observed, feeds in its larval condition only on
corn, the planting of infested land to any other crop leads to the star-
vation of the young when they hatch in the spring. This is no mere
inference, but has been tested time and again. In Illinois, according
to Forbes, it is ordinarily safe to plant corn two years in succession, but
it is unwise to plant it again without an alternate. It is also impru-
dent to plant corn in fields or meadows in which the beetle has been
observed in great abundance on clover and certain weeds in late Octo-
ber the previous year.

One other measure is to be recommended, as a general farm practice.
It consists in the maintenance of the fertility of the soil by the use of
manures and other fertilizers. Although this does not diminish attack,
it sometimes enables the plants to withstand injury.

Approved : :
JAMES WILSON,
Secretary of Agriculture.

WASHINGTON, D. C., March 23, 1905.

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