A couple of weeks ago, the roses in the OPC display garden in Rochester were awash in colorful blooms. Sadly, the big show lasted only a short time due to scorching hot weather, lack of rain and wind. Lots of wind.
Nothing makes up for lack of water, and our garden has received precious little rainfall the past couple of months and the irrigation system has been on the fritz.
So although we hand water, I’m sure many of the roses in the garden have struggled a bit this summer due to weather conditions, and unfortunately this year we’ve added insects to the mix. Rose sawfly larvae, to be exact.
Over the years I’ve read about this pest. In the South, they call rose sawflies rose slugs because they start out as very tiny and green worm-like critters and chew holes in the leaves of roses – damage that looks similar to that of a slug.
Though I’ve seen them occasionally, they have never been a real problem until now. Two mild winters back to back have probably allowed them to flourish.
The sawfly is a tiny insect that looks like a housefly. As an adult it doesn’t harm plants, but in the larval stage, in large numbers, it can do serious damage to host plants, most notably pines and roses. There are dozens of varieties of sawflies and they are host plant specific, so the good news is those that attack roses will not go after pines. However, in our OPC garden, the rose sawflies also turned the leaves of hollyhocks and malvas to fine lace.
If the infestation is heavy, as it has been on some of our roses, these critters can exfoliate a plant in no time when left to their own devices. However, if the rose is healthy and well watered, after the attack we cut them back by a third, fertilize with a good organic fertilizer (I like Neptune’s Harvest Fish and Seaweed Fertilizer) and they produce a new flush of foliage and flower again.
The larvae of the sawfly often hang out on the undersides of leaves, and when they first hatch they’re tiny, green and very hard to see, especially on the low-growing shrub roses.
As soon as you notice damage, the larvae can be washed away with a stiff stream of water, being sure to hit the undersides and tops of the leaves. This action should be repeated every few days for a couple of weeks.
Nancy Szerlag is a master gardener and a Metro Detroit freelance writer. Her column appears Fridays in Homestyle. To ask her a question go to Yardener.com and click on Ask Nancy. You can also read her previous columns at detroitnews.com/homestyle.