Q: I have caterpillars on my canna lilies. I don’t want to kill any future butterflies, so how do I know if these are butterflies or moths? They are rolling themselves up into the leaves.
A: Sometimes it is tough to know the difference between moth and butterfly larvae but in general butterfly caterpillars do not roll leaves around themselves. Feel free to kill these caterpillar pests as they will become moths. The caterpillar on your plant is probably the Lesser Canna leafroller, Geshna cannalis (Quaintance), which can be a serious pest of ornamental canna. It could be another leafroller which is much larger called the Larger Canna Leafroller, Callpodes ethlius (Stoll). Cannas may be infested with both species at the same time.
Adult G cannalis moths are small, light brown and may be found resting in the shade of a canna plant during the day. Females lay eggs in groups of six to 15 on the upper surface of a canna leaf. Eggs are flat, clear whitish yellow in color. When larvae are approximately one week old, they begin leaf rolling behavior. Five or six larvae may be found within a leaf roll, but usually only one or two coexist.
Cutting dead canna plants to the ground in the late winter and removing debris is a good way to reduce populations. Product that contain Bacillus thuringiensis (Bt) are least toxic to beneficial organisms but beware as this product will also kill butterfly carterpillars. Follow the directions of the label – “The label is the law.”