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Leafrollers make a meal of cannas

My cannas were growing well and I thought should be about to bloom when I noticed that the leaves hadn’t opened, and they looked like they’d been stitched together! When I unrolled the leaves, I found a little grub and some other black substance. What is that? What can I do about it?

Your cannas have become host to a fairly common creature throughout the southeast called a canna leafroller, and it can do quite a bit of ugly damage to this summer-flowering favorite.

The grub you see when you unroll the leaves is the larval stage of a little brown moth. The adult moth lays her eggs on the surface of the leaves; when the eggs hatch, they begin to feed on the leaves, and as they grow, fasten the unrolled leaves together with tidy silk stitches.

There are actually two leaf-rolling species that infest cannas; the one you describe is the lesser canna leafroller, most easily distinguished by the mass of dark caterpillar droppings (called frass) inside the rolled leaf. The larger canna leafroller keeps its feeding den clean.

The best way to control canna leafroller is to watch for the evidence of the moth and her eggs (tiny, and a whitish-yellow color), and the neat silky stitches on the leaves before they begin to unfurl. If you unroll a leaf and find a grub inside, flick it out where a bird can enjoy it.

In late winter, cut the dead stalks of canna plants to the ground and dispose of it, which reduces the early, over-wintered population of leaf rollers. Watch for re-infestation as the cannas grow. If you feel you must spray something, horticulturists suggest products that contain Bt – Bacillus thuringiensis – as they are less toxic to beneficial organisms.

The University of Florida extension has a good description of the pest, its life cycle, and management practices here.

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