|
|
Q. I have
rhubarb, peony and coneflower plants that have been infested with
some kind of borer. I noticed that individual stalks on those plants
were wilting due to insect damage. Can you
tell me what they are?
A. It is unusual that the same insect would
have damaged three very different plants, since most have a limited
host range. It took some research to
learn that this insect is known as the common stalk borer (Papaipema
nebris). Common stalk borer is best known as a corn pest in eastern
North America, but is also reported to feed on over 175 types of
plants, including vegetables, fruits and ornamentals. All of the
plants in question here are known to be susceptible.
Coneflowers
Common stalk borer adults are moths that hatch in late summer
(September). Females lay eggs on unmown dead grass and curled up
weed leaves, or in bark crevices on trees. Common stalk borers
overwinter as eggs that hatch into small larvae in mid-spring. They
immediately tunnel into a host plant for food and shelter. Although
they prefer to tunnel into grass stems, young larvae can also tunnel
into leaf veins and stems (petioles) on many weeds. As they develop,
they outgrow the first host and migrate to the larger stems of
vegetable, fruit, and ornamental plants, or larger weeds. This is
the stage that causes the most damage to crops and ornamental
plants.
They bore their way into the stem, pushing sawdust-like frass from
the hole as they tunnel. They usually move upwards in the stem,
boring holes to the outside as needed to clear the stem of frass.
They also migrate to adjacent stems, with each larva damaging many
plants. (There are species of borers that spend their lives in one
plant, causing less overall damage). Infested stems may wilt as
their feeding destroys the plants' vascular system, or it may break
in the wind since it is weakened from their tunneling activity.
Common stalk borer occasionally infests small twigs on woody plants,
too.
It takes two or three months for the larvae to mature. They pupate
in mid-summer, with adults hatching in three to four weeks. We have
one generation a year. Common stalk borer adults are grayish-brown
with a wingspan of an inch or so. They are not attracted to lights,
so you probably will never see them. Larvae range from
three-quarters of an inch to two inches long. They are
purplish-brown, with a white stripe down the middle and two white
stripes running down either side when young. Mature larvae lose this
distinctive coloration and are purplish-gray and rather
non-descript.
Control options include controlling weeds and cutting tall grass,
especially in mid-August, to reduce suitable habitat for the adult
moths to lay their eggs. Avoid cutting weeds between late May and
late July. This can actually force common stalk borer larvae out of
those weeds and into your garden. You can also try to find the
larvae in infested stems and destroy them. Fortunately, common stalk
borer is not considered a major insect pest in our area. The
mechanical methods described above should provide sufficient
control. The insecticides labeled to control common stalk borer in
agronomic crops are not available to home gardeners.
There are other species of insects - primarily beetles - whose
larvae cause similar damage. I usually see borer damage on
herbaceous plants every year, but it seems to be limited to a few
stems or a few plants here and there. Mechanical controls generally
provide effective control without using insecticides. Contact
insecticides such as insecticidal soap, pyrethrins or Sevin (carbaryl)
are not effective once insects are inside plant stems. Systemic
insecticides such as Bayer Advanced Rose & Flower Insect Killer (cyfluthrin
and imidacloprid) can provide control of borers that are beetle
larvae. It is not effective against moth or butterfly larvae.
Corn Earworm
Eggplant beetles
Four-lined
plant bug
Honeysuckle
aphids
June beetles
Scale insects
Whiteflies
More insect
columns |
