Thursday, June 21, 2018
Q. The leaves of my rose plant are tattered like they have been eaten by a bug. What caused this and what can I do about it?
Margaret from Austintown
A. What made this problem easier to figure out was that Margaret brought us a sample of her rose plant.
That’s because several pests can skeletonize rose leaves (where there is nothing left of the leaf but the veins), and since there was no pest found on the sample we had to use other methods to narrow down what caused the damage.
Japanese beetles, rose chafers and sawfly larvae can all cause this type of damage. But which one was it?
One tool we use to narrow down the culprits when the pest is not there to identify is Growing Degree Days – or GDD. GDD is a measure of heat accumulation during the growing season.
Pest development (also plant development) is largely dependent on temperature. Some pests need more exposure to warm temperatures to develop than others. And GDD can tell you how much heat has already acclimated by calendar date.
For example, Japanese beetles need a lot of warmth before they emerge as adults, and GDD tells us that they have not yet emerged. This leaves rose chafers and sawfly larvae as suspects.
Sawfly larvae emerge before rose chafers, and since the damage was noticed early in the spring, the mostly likely cause of the damage are sawfly larvae.
As adults, sawflies are small, nonstinging wasps. It’s when they are in the larval stage that they are a pest of roses. The larvae are also called rose slugs.
There are three types of rose sawflies in Ohio: European sawfly, the curled sawfly and the bristly sawfly.
Without a sample of the pest we can’t tell which Margaret has, but the same control methods apply to all three.
Sawfly larvae look like small caterpillars. Inspect your rose leafs, especially the under sides of the leaves for larvae. If you see them, you can either pick them off by hand or wash them off with a hose.
If these methods are not effective you can use an insecticidal soap or a horticultural oil. As a last resort you can use an insecticide.
If you use an insecticide, be sure to read and follow the directions and keep the insecticide off the flowers because the insecticides can also kill bees and pollinators.
For more on sawfly larvae, go to http://go.osu.edu/sawflies. For more on GDD, go to http://go.osu.edu/growingdegreedays
Today’s answer is by David Sprague, an OSU Extension Master Gardener Volunteer in Mahoning County. You can meet him at the Mahoning Plant and Pest Clinic. Call the office plant and pest clinic at 330-533-5538 to submit your questions. Regular clinic hours are 9 a.m. to noon Mondays and Thursdays.