By Larry Hodgson
Do you have plants in your garden—perennials, shrubs, trees, etc.—which present a curious habit? Some of the leaves remain rolled up like a tube while others unfold normally. This is most likely the work of a leafroller.
To be sure, unroll a leaf and look inside. If it’s a leafroller, you will find a small caterpillar, usually of a discreet color: green or brown. And the inner part of the leaf will have been chewed on. Also, you’ll often find brown or greenish frass (the polite word for insect excrement) inside the leaf.
There are a few other insects that can roll up plant leaves to a certain degree, but the specialists—the ones you’re far most likely to see—are moths in the Torticidae family, called tortrix moths or leafroller moths. There are over 11,000 species found all over the world except Antarctica. Some are specific to a single plant genus or family. Others are polyphagous: that means they’ll feed on a broad range of hosts.
Again, given the huge number of species, their life cycle can vary somewhat. Still, typically the female moth lays her eggs at the end of the summer. However, most don’t hatch until spring. In some species, though, the larva does hatch in the fall. If so, it will overwinter in the soil, sometimes weaving a silk shelter for protection.
In both cases, the larva climbs onto its host plant in spring, just as its leaves are opening. It climbs under a leaf and secretes a kind of sticky silk that binds the leaf blade together, keeping it from opening. By doing so, the larva creates a very handy little tubelike shelter where most of its enemies can’t reach it. And neither can pesticides gardeners try to apply. Nor wind or heavy rain. This allows the caterpillar to go through most of its cycle without disturbance.
Most leafrollers continue to feed in their shelter leaf for 3–4 weeks, then spend another 3–4 weeks as pupae, still in their shelter. Around July, the adults emerge and mate, then, at the end of the summer, the females lay their eggs at the base of the host plant or in the soil at its base. And then the cycle starts all over.
Most temperate species have only one generation per year.
What Can You Do?
Honestly, there’s very little you can do, especially once the shelters are visible. As mentioned, commonly available insecticides (soaps, oils, pyrethrum, etc.) can’t penetrate the shelter. And systematic insecticides, which are absorbed by the plant and make it toxic, have almost all been removed from the market, at least for home gardeners. And at any rate, do you really want to make your plants poisonous?
Leafrollers are hard to prevent, too, and that’s partly because they’re often very sporadic. There may be many one year, a few others and often none at yet other years. Where they are an annual nuisance, you might be able to control them by spraying the base of the host plant with dormant oil spray eat the beginning of spring.
If you’re quick enough, you can apply BTK (Bacillus thuringiensis kurstakii), a caterpillar-specific bacteria, to the plant just as the leaves are forming. But BTK is quickly killed by sunlight, is readily washed off the leaves by rain and you pretty much have to apply it the same day the small larva starts to seal the leaf shut. The chances of success are therefore very slim.
Also, at the opposite end of the season, adults are very inconspicuous … and difficult to distinguish from the hundreds of other small, dull-colored moths that live in our gardens.
So, the best thing to do about leafrollers is … do nothing at all! Just ignore them.
There are rarely enough leafrollers on a plant to cause it even minor damage. If their presence really bothers you, pull off and destroy the affected leaves, that’s all.
This is what I call a “15 pace case.” Back up 15 paces. If you don’t see the problem from there, it’s just not worth worrying about!
Leaf-Folders and Leaf-Tiers
Very similar to leafrollers are leaf-folders and leaf-tiers. Leaf-folders, as the name suggests, don’t roll up the leaves to form their shelter, but fold them over, as if making a tent. Leaf-tiers glue 2 different leaves together. In both cases, the larva is then sheltered from its predators. The treatment (or lack of treatment) is the same.
More details on hydrangea leaf-tier can be found in the article What Makes Hydrangea Leaves Stick Together?
A Life Lesson for Kiddies
If you are in contact with children, show them this rather amazing phenomenon. Let them open a leaf, examine the larva, etc. Explain that although this is happening in front of their eyes, most adults don’t even know that leafrollers exist. Perhaps you’ll succeed in instilling in them a positive curiosity about nature that will last a lifetime!