A “Canna” Worms or Managing the Canna Cat
The operative term is managing not eradicating. The Canna worms are called Canna Leaf Rollers and there are two species that occur in our area. One is commonly known as the Canna Leaf Roller (Calpodes ethlius) and the other is commonly known as the Lesser Canna Leaf Roller (Geshna cannalis).
The Lesser Canna Leaf Roller is a yellowish caterpillar feeding inside rolled leaves tied with short silken threads. The Canna Leaf Roller is a lime green-colored caterpillar that feeds inside rolled leaves and it eventually gets much larger than the Lesser Canna Leaf Roller. Since they both start out small, size doesn’t help identify them during their early stage of growth.
Both types of caterpillars can destroy the large, beautiful foliage of cannas in short order. These caterpillars are the larvae of the large brown skipper butterfly. The female butterfly lays her eggs on the leaves of canna. After developing into caterpillars, they spin silk thread and roll themselves inside the edge of the leaves, forming a tube. They feed on the leaf inside the tube as they pupate. When they outgrow or out-eat one tube, they spin themselves a larger tube until they reach approximately two inches in length.
There are several ways to manage infestations of the Leaf Roller caterpillars. Since there can be three or more generations of the butterfly in a single season, re-treatment should be anticipated. The first control measure to consider is Bacillus thuringiensis which sold under various trade names including Dipel and Bio-Worm Killer. Applied as a spray, BT is specifically targeted to kill the immatures of moths and butterflies. Foliage should be thoroughly covered with the spray, with particular attention given to the underside of the leaves. After applying BT the caterpillars will stop feeding on the leaves although it may take them a day or two to expire. If BT is applied early enough in the growth of the caterpillars, little damage to the leaves will be evident.
Another alternative is to apply a systemic insecticide in a foliar spray (i.e. Orthene). When applying systemic insecticides by spraying, careful attention should be given to food and herb crops planted in the immediate vicinity to prevent over-spray or drift of the insecticide.
A third treatment option to consider is a carbaryl insecticide such as Sevin. Applied as a liquid, Sevin can help manage the infestation of many varieties of insects, including beneficial ones. The alternative between the recommended treatment options for management of the Leaf Roller caterpillars can help to spare beneficial insects. Alternating insecticides also prevents insects from building up resistance to any one particular method.
The “no chemical” method I choose is to remove the infected leaf by pulling it off the shoot and then destroying the leaf by leaving it in the sun, burning it, or trash bagging. By June 1st, all rolled leaves are gone but the butterfly will return later in the year so a daily inspection is warranted.
Although the Canna Leaf Roller (Calpodes ethlius) does occur in our area, the Lesser Canna Leaf Roller is the predominate pest. The Lesser Canna worm overwinters as larvae in the leaves and stems of canna, and the butterflies emerge to mate and lay eggs after the new growth emerges in spring. One mistake that canna growers make is to leave the old dead growth on the canna bed as a mulch. As canna seems to be the only host plant for this pest, it may be possible to drastically reduce the Lesser Canna Leaf Roller population just by removing dead leaves and stems in the winter after the frost has killed back the foliage.
As an aside, there is a much-debated product call Garlic Barrier Insect Repellent (99.3%) garlic juice) that is a registered pesticide. Studies show that insects, including the skipper butterfly, are not a problem where the malodorous substance has been applied. The use of such a product falls somewhat out of the bounds of Integrated Pest Management, and given some thought, might repel a whole host of creatures from the garden, including people!