Cultivating roses has been enjoyed by gardening enthusiasts for centuries. But as any experienced rose gardener knows, the plants attract a wide variety of insects and other pests which can be challenging to eliminate. Caterpillars are usually moth larvae and they can be effectively controlled with the application of several popular types of insecticide.
Bacillus Thuringiensis (Bt)
The most commonly used insecticide for moth larvae is Bacillus thuringiensis (Bt), a soil-dwelling microorganism that is toxic to the caterpillars when ingested. As a naturally occurring pesticide, it is considered safe to both humans and animals. For best results, apply Bt before the caterpillar eggs have hatched. After diluting with water according the product guidelines, spray the leaves of the rose bush, making sure to cover the underside. Repeat the spraying in about two weeks while the young caterpillars are still feeding.
Spinosad is a newer organic insecticide that also kills caterpillars after it has been eaten. According to Dr. Michael Merchant, urban rntomologist with Texas Cooperative Extension, spinosad is a safe and effective way to eliminate a number of garden pests, including caterpillars. Like Bt, spinosad is sprayed onto the plant leaves after dilution with water. It is most effective when applied in the early spring and may be repeated as needed.
It is very important to correctly identify the larvae as caterpillars before applying any insecticide. Sawfly larvae are easily mistaken for leafroller caterpillars. Both are pale green in color and close inspection is needed to tell them apart. The key to identification is counting the prolegs. These stubby appendages are found on the underside behind the true legs. Sawflies always have five or more pairs of prolegs whereas caterpillars have fewer than five. Both Bt and spinosad are ineffective against sawfly larvae.
The most common types of moth larvae (caterpillars) found on roses include the orange tortrix, tussock moth, fruit tree leafroller, tent caterpillar, and omnivorous looper. Pale green leafrollers are particularly common and easily found by looking for the tell-tale rolled up leaves within which they feed. All these caterpillar varieties are easily eliminated using either Bt or spinosad insecticide.
Although caterpillars are big leaf eaters, overall plant damage is usually not severe. They can be easily picked off the plant by hand which may be enough to control small infestations. Many small birds are insect eaters and can be encouraged to feed in the garden by providing birdhouses and water. There are also a number of insect predators such as ladybugs and lacewings that eat moth larvae eggs. Ladybugs also eat aphids and many other types of pests making them one the most beneficial garden insects.