Common Names Scientific Names In Wisconsin, eggs begin hatching in early spring
Stalk Borer Papaipema nebris (mid-April to early May), and small larvae can cause
Hop Vine Borer Hydraecia immanis pinhole feeding in leaves of seedling corn by late May
Potato Stem Borer Hydraecia micacea in southern Wisconsin. As host plants become dor-
mant or their stems too small for growing larvae, the
larvae disperse to larger-stemmed crops such as
Although the European corn borer is the major stalk- corn. A single stalk borer will damage several corn
boring pest of corn in Wisconsin, the stalk borer (also plants during its larval stage.
called the common stalk borer), hop vine borer and
potato stem borer have caused localized problems in Larvae are fully grown by late August and begin to
recent years. These stalk-boring insects are relatively pupate, usually just below the soil surface near host
common in some parts of the state and may be plants. Occasionally, larvae pupate in tunnels within
spreading to other areas. the host. There is only one generation of stalk borers
per year.
Stalk Borer Damage
Although the stalk borer is known to damage more
Appearance
than 170 plant species, only corn is discussed here.
Stalk borer larvae are off-white with black and white
There are basically two types of stalk borer damage to
stripes running lengthwise. The striped pattern is in-
corn. In the first, the larva enters the corn plant near
terrupted by a dark purple band near the legs. How-
the base and tunnels within the stem. Stem tunneling
ever, this striped appearance is lost during the last
in seedling plants causes unfurled leaves to wilt and
larval stage, at which time larvae are about 1/ inches
produces the symptom called “flagging.” Seedling
long. Inexperienced observers often confuse these
larvae with other insect pests, such as the European
corn borer.
The adult moths have a wingspan of approximately
1 1/4 inches and look like common “millers”,
nondescript tan- to brown-colored moths.
Life Cycle
The stalk borer probably only overwinters in the egg
stage. Soon after emerging from the pupae, female
moths begin to lay eggs on grasses and weeds in
contour terraces, waterways, fence rows, and within
row crops from mid-August through late September,
often in the creases of rolled or folded leaves. Each
female is capable of laying more than 2,000 eggs.
Stalk borer larva removed from damaged corn stalk.
Wilted central leaves due to stalk borer tunneling in seedlings.
Pin-hole leaf feeding injury to seedling corn caused by first-
and second-stage stalk borer larvae.
plants may be killed by this tunneling activity, and the
larva will move to another plant if the food supply is
exhausted. In the second case, the stalk borer larva
enters the whorl and feeds there before tunneling
downward. This results in numerous larval droppings
(frass pellets) and a series of irregular holes in the un-
furled leaves.
Only corn well past the seedling stage is capable of
recovering from stalk borer feeding. Thus, the planting Feeding damage due to full-grown stalk borer larvae in late
June.
date can influence the number of plants killed by
these pests. For example, some growers have tried Increased use of conservation tillage may be partly
replanting damaged patches only to find higher plant responsible for the increase in reported stalk borer
mortality in the new planting because of even larger problems. Conservation tillage leaves numerous over-
larvae on new seedlings. wintering sites undisturbed and allows certain weeds
to escape and become primary egg-laying sites.
Various patterns of stalk borer damage can occur
within fields. The most common pattern is that of lar- Control
vae moving from host plants within contour terraces, Burning or mowing egg-laying sites is not a reliable
alfalfa/grass strips, fence rows or waterways, and in- technique for preventing stalk borer damage to adja-
festing the outer four to six rows of adjacent cent corn. On the other hand, good weed control pro-
cornfields. grams are invaluable to stalk borer control because
they prevent the development of egg-laying sites with-
Poor weed control during the previous year provides in fields.
numerous egg-laying sites for stalk borer moths and
can result in extensive patches of damaged corn the Spray programs must be accompanied by a well-
following year. Planting corn after alfalfa or into old planned scouting program. Even very young larvae
sod fields also can result in extensive damage. may feed deeply enough in the whorl to be protected
from insecticides. Postemergence insecticides will ef- Life Cycle
fectively control this insect pest, but application timing In Wisconsin, hop vine borer larvae begin to hatch in
is critical. During most years, sprays need to be May from overwintered eggs deposited during August.
applied during late May or early June before con- The first three to four larval stages feed on grass
siderable wilting is present. Sprays applied much stems and rhizomes. In southwestern Wisconsin,
beyond the seedling stage are of questionable value. second- and third-stage larvae have been found in
quackgrass stems during late May. The larvae then
If damage is occurring to the outer rows of a field, it move to adjacent corn to complete development
may be necessary to select an insecticide also where they tunnel in the below-ground stem. Larvae
labeled for adjacent vegetation and spray a 6- to 8-ft may complete feeding on corn as early as late June
swath along the edge of the cornfield. Read the label or as late as mid-July. The larvae then pupate in the
before such applications, and be aware of any possi- soil, usually within a few inches of the last plant fed
ble restrictions. For example, non-cropland areas may upon. The pupal stage lasts 4 to 6 weeks, with adults
contain blossoming weeds or other plants that are present from late July until early September. Eggs are
valuable bee foraging areas. Such treatments require deposited on grass stems in or near the cornfield.
careful coordination between beekeepers and There is only one generation per year.
growers.
Damage
Reduced tillage probably favors the hop vine and
Hop Vine and Potato Stem Borers potato stem borers, much like the stalk borer. Damage
is usually confined to the outer four to six rows of
Appearance
cornfields as larvae move into corn from adjacent
The hop vine borer is a native stem-feeding caterpillar
grassy areas. However, damage also can be found
that has caused severe, localized damage in portions
throughout the field if corn follows sod or grass weeds
of Wisconsin, lowa, Minnesota and Illinois since 1975.
were present in the field during previous years.
About 1 1/2 to 2 inches in length when mature, this in-
Damaged corn seedlings seldom survive larval feed-
sect is off-white with transverse violet bands and an
ing by hop vine and potato stem borers; if seedlings
orange to reddish-brown head. The violet bands
do survive, they do not produce normal ears.
disappear just before it enters the pupal stage. Its
European relative, the potato stem borer (first reported
The first indication of a hop vine or potato stem borer
in Wisconsin in 1982), is almost identical in ap-
infestation is wilted corn plants. Unlike the stalk borer
pearance and biology but has a wider host range. Al-
though the potato stem borer has only been reported
from one Wisconsin county, it is possible that mixed
infestations of both have occurred.
Adult hop vine and potato stem borer moths are a
nondescript tan to brown color, much like the stalk
borer moth.
Full-grown hop vine borer larva. Female hop vine borer moth. Note eggs deposited on
quackgrass stem.
which tunnels mainly in the corn stalk above ground, Control
the hop vine and potato stem borers usually feed on Currently, there are no registered insecticides that
the stem below ground. successfully control hop vine and potato stem borers.
Thus, eliminating grassy weeds is the only way to suc-
To check for suspected hop vine or potato stem cessfully control these insect pests. However, it is
borers, remove damaged corn seedlings along with a sometimes difficult to get rid of all weeds, so it is also
3- to 4-inch cube of soil. Look for entry holes in the hard to completely eliminate these stalk borers from
stalk just below the soil surface, split the stalk, and sift corn.
through the soil. You may have to dig and examine
several plants before finding larvae. The only known natural enemy of hop vine and potato
stem borers are skunks, which often cause additional
Some cutworms—such as sandhill, glassy and black damage to injured corn pIants by digging the plants
cutworms—also attack corn below the soil surface. up in their search for the borers.
However, the distinctive color pattern of hop vine and
potato stem borers makes it easy to distinguish them
from cutworms.
“Dead heart” of corn plant caused by hop vine borer larvae
tunneling in underground portions of the stem.
This publication is available from your Wisconsin county Extension office
Extension Service, Charles F. Koval, director, in cooperation with or from:
the U.S. Department of Agriculture and Wisconsin counties, pub- Agricultural Bulletin, Rm. 245
lishes this information to further the purpose of the May 8 and June 30 N. Murray St.
30, 1914 Acts of Congress; and provides equal opportunities in Madison, Wisconsin 53715
employment and programming including Title IX requirements. Phone 608-262-3346
Produced by the Department of Agricultural Journalism, University Editors, before publicizing, contact Agricultural Bulletin to determine
of Wisconsin-Madison. availability.
J. L. Wedberg is a professor and B.L. Giebink is a research assis-
A3354 UWEXnote 5—Stalk-Boring Insect Pests tant in the department of entomology, College of Agricultural and
of Corn (1986) Life Sciences, University of Wisconsin-Madison. Wedberg also
holds an appointment with the University of Wisconsin Cooperative
45¢ I-2-86-5M-s Extension Service.