Iris borer
Macronoctua onusta
The worst thing that happens to a bearded iris: a caterpillar tunnels down the leaf into the rhizome, hollows it out, and lets bacterial soft rot finish the job.
How to know it
A fat pink caterpillar up to 50 mm with a brown head, found inside a leaf fan or in the rhizome itself. Earlier in the season the signs are all you get: a water-soaked streak or a ragged notched edge on a young leaf, with a pinhole where the caterpillar went in.
What you'll see
- Water-soaked dark streaks running down the young leaves in late spring, sometimes with a wet appearance at the edges.
- Ragged notches chewed in the leaf margins near the base.
- Leaf fans yellowing and pulling away easily from the rhizome.
- Rhizomes hollowed out and turning to a foul-smelling wet mush — the soft rot that follows the borer in.
- A fat pink caterpillar in the rhizome when it is lifted in midsummer.
Where it comes from
One generation a year, and the timing is everything. Eggs are laid in autumn on the old dead iris leaves and debris and overwinter there. They hatch in mid-spring, and the tiny caterpillars climb the new leaves and bore in at the top, tunnelling downward inside the leaf all through May and June until they reach the rhizome in July. They pupate in the soil in late summer and the moths fly in September.
Bearded iris throughout the eastern and central states, and worst in clumps that are never divided and beds where the old leaves are left down over winter. Wet summers make the soft rot that follows far worse.
Dealing with it
- Squeeze the leaf between finger and thumb along the streak in May and June; you will feel and crush the caterpillar inside it.
- Cut affected leaf fans back to a few centimeters and bin them.
- Lift, split and inspect the rhizomes in midsummer, cut out every borer and every rotten section, and let the cut surfaces dry in the sun before replanting.
- Dust cut rhizome surfaces with sulfur or leave them to callus in the air before replanting.
- Bin the rot; it will not compost safely and it smells appalling.
Keeping it out
- Cut all iris foliage down to 5 cm in autumn and clear every scrap of leaf and debris from the bed. This removes the eggs and is the whole prevention.
- Divide bearded iris every three or four years and keep the rhizomes sitting proud of the soil.
- Do not mulch over iris rhizomes; they want sun on their backs.
- Water at the base rather than over the fans.
- Inspect and clean new rhizomes before planting them into an established bed.
What it goes for
Bearded iris above all; Siberian and Japanese irises are affected far less.
Trouble it brings with it
See also
The Cottage Garden Companion — the gardener's bestiary at cottagegardencompanion.com