Many gardeners stop growing cannas because of this pest. They just get tired of having so many “rat-tailed” plants, and unsightly defoliation. This damage is caused by small, green caterpillars that live and feed inside the leaf whorls. They bind the leaf whorls with silk to prevent them from unrolling and feed inside the protected whorl. This is what causes the “rat-tailed” effect. Later, after the caterpillars have pupated, or moved on to a different leaf, the leaves may unroll to reveal the unsightly brown defoliation damage. Some leaves will unroll before they are heavily damaged but will have a row of small holes across the width of the leaf, made when a caterpillar chewed through the rolled leaf.
Stalks that are heavily damaged either do not bloom or produce ragged, unsightly blooms. The overall result of a heavy infestation of lesser canna leafrollers is a planting of cannas that just does not look very good and is more of a liability than an asset. Some varieties are more susceptible than others, but all are subject to some damage.
If there is a lesser canna leafroller, there must also be another leafroller of cannas that is larger.
Larger canna leafroller, Calpodes ethlius, are considerably larger than lesser canna leafrollers, two inches vs one inch for mature caterpillars, and they are much less common through most of the state. This is more of a tropical/sub-tropical insect that is more common in the southern part of the state. These strange-looking caterpillars have clear skin, which allows you to see inside them.
Control: Lesser canna leafrollers can be controlled with insecticide sprays, but because there are multiple generations, it may take several treatments, applied at intervals throughout the summer. Be sure to direct sprays into the unrolled whorls where the insects prefer to feed. Because cannas have slick waxy leaves, it is helpful to add a “sticker” to the insecticide spray. Effective insecticides include: Spinosad, Bt products, Acephate, and pyrethroids such as bifenthrin, permethrin, and zeta-cypermethrin.
Selective pruning and destruction of severely infested whorls can also aid control, and aesthetics, during the growing season. In cases where an infestation has gotten out of hand and the stand has become unsightly, aggressive pruning followed by insecticide treatments applied to new growth, can help restore control and appearance. Don’t forget to destroy those infested stalks. Sealing them inside a plastic lawn bag and sending them away in the garbage is one approach.
One of the most important steps in controlling lesser canna leafroller should be done in the winter. These insects overwinter as partly grown larvae within the whorls of dead leaves (see photo--taken in January). Gather these leaves and burn them, or bag them and send to the landfill, and you can virtually rid the planting of lesser canna leaf rollers. If your neighbor has cannas, be sure they too are aware of this effective cultural control.
Last Article of the Year: This is the last article for 2021, but Bug’s Eye View will resume next spring.
Blake Layton, Extension Entomology Specialist, Mississippi State University Extension Service.
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