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The iris borer (Macronoctua
onusta) is actually a caterpillar of a nocturnal
brownish moth. In the autumn, it lays its eggs on old
iris foliage where they spend the winter. In the warmth
of the spring, they hatch and the tiny white
caterpillars seek out the new iris leaves emerging from
the rhizomes. After feeding all summer, they will
transform into the moth stage which starts the process
again by laying eggs.
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Early in the season,
damage is difficult to detect. The first signs may
be feeding damage in the form of notches on the
leaves and some frass. The real damage comes when
they move down into the rhizome and may eventually
hollow it out. There will be small, pencil lead (or
is that graphite) size holes in the rhizomes.
Ultimately, the plant
will begin to wilt and may die from a severe
infestation. However, perhaps an even more serious
side effect is the introduction of a bacterial rot
into the damage caused by the borer. This will cause
the rhizome to turn to a very smelly mush and kill
the plant.
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Iris borer tends to
be more of a problem on iris that are already under
stress from other sources such as crowding, poor
drainage or compacted soils. Preventing these
conditions or more frequent division may help to
prevent infestations of iris borer.
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Of course, preventing the
problem by keeping the iris in otherwise healthy
condition is the best approach.
Insecticide treatment of
iris borer can be difficult. Since the critter is living
inside the rhizome for much of its life cycle, many
standard sprays are not effective. Certain systemic
insecticides may work better.
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Note: We
have provided some general information and
observations on this topic aimed at the home
gardener. Before you take
any serious action in your landscape, check
with your state's land grant university's
Cooperative
Extension
Service for the most current,
appropriate, localized recommendations. |
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