Iris Borer
Larva
of iris borer tunneling in iris rhizome
Iris borers are serious pests that
damage the iris directly and also create entry points for bacterial soft rot
disease. Though mostly found in the eastern parts of the United States, the
iris borer could still pose a serious problem for iris growers in the South.
They will attack all kinds of irises. Recently, there have been isolated
reports of borers in other regions, including Texas, so it is important to be
familiar with the symptoms and treatment. To begin, margins are chewed.
Irregular tunnels appear near leaf bases, and pinholes can be seen on foliage
a few inches above the ground. As borers create their pinhole entry
points, you may notice small piles resembling sawdust around the base of the
plant. Leaves can develop dark streaks and be wilted or rotten. Borers can
hollow out the entire rhizome, causing the plant to collapse. Damaged areas
are then open to secondary infection and begin to rot.
Borers begin as eggs, laid by
moths on garden debris in the late summer and fall. Borer larvae begin to
hatch after just a few days of temperatures greater than 70 degrees. Only as
a larvae do they eat and do damage. At this stage they are most vulnerable to
control efforts, so recognition is key. Later in the summer, the borer larvae
change into pupae with a dark brown spindle-shaped cocoon. These pupae stay
dormant in the soil for about a month and then moths emerge to lay eggs and
begin the cycle again. You may notice these cocoons as you divide clumps.
Borer Control
Prevention: Keeping the garden clean
is the first step in minimizing borer problems. Cutting back iris leaves and
removing debris will reduce the number of borers that hatch each spring. As
the weather begins to warm, watch for signs of borer infestation. If you spot
the borer prior to rhizome entry, it is easily removed with minimal damage to
the plant.
Pesticides: Imidacloprid, which is
currently marketed under the name Merit, is a systemic pesticide that has
proven effective in iris borer control. Merit has exhibited minimal toxicity
to mammals. It is an analog of nicotine sulfate, the pesticide naturally
found in tobacco and petunias.
Because Merit is a persistent pesticide, it only has to be applied once each
spring to effectively control borers. Use a granular applicator to sprinkle
the Merit granules in the soil, ideally before the borers hatch. Water deeply
after application so the granules can dissolve quickly. Once the imidacloprid
enters the soil, it will be taken up into the iris through the roots. As with
all pesticides, read the directions carefully and follow them. Do not apply
near vegetables or plants that will be eaten. Wear rubber gloves and long
pants during the application.
Overuse of any product, including Merit, can lead to resistance. Some
whitefly species have already developed resistance to Merit. Therefore, a
rotation between Merit and the systemic pesticide Orthene may prove most
effective in combating iris borers in your particular garden.
Alternative Treatment: If you
are reluctant to use harsh chemicals in your garden, or if your irises are
near plants that will be consumed (underplanted with herbs, near the
vegetable garden, etc.), you can combat iris borers with Murphy's Oil Soap.
Use a mixture of one half cup of Murphy's Oil Soap to one gallon of
water. Saturate the iris leaves weekly (or after a rain) from early May
to late July. While not as effective as chemical treatment, soap application
does show positive results.
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