The operative term here is manage not eradicate. The worms you speak of
are called Canna Leaf Rollers and there are two species that occur in our
area. One is commonly known as the Canna Leaf Roller (Scientific Name:
Calpodes ethlius) and the other is commonly known as the Lesser Canna
Leaf Roller (Scientific Name: Geshna cannalis).
The Lesser Canna Leaf Roller is a yellowish caterpillar
feeding inside rolled leaves tied with short silken treads. The Canna Leaf
Roller is a lime green-colored caterpillar that also 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
read 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.
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
Although the Canna Leaf Roller (Calpodes ethlius)
does occur in our area, the Lesser Canna Leaf Roller is the predominate
pest. The Lesser Canna Leaf Roller overwinters as larvae in the leaves and
stems of canna, and the moths 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!