Update: I’m pinning post this to the top of the page because this can be such a frustrating problem. The rose slugs (aka rose sawfly larvae) seem to be worse some years than others an it’s still a little early and cool for the bugs to show up, but when they do come here’s what you need to do.
Summer garden doldrums — I’ve noticed that I’m a little less enthusiastic in my garden lately. I’m pretty content to wander about doing nothing more than a little grooming here and there. But that’s all right because besides deadheading and keeping the garden tidy there’s not that much to do. The exception is keeping a sharp eye out for garden pests, because the sooner you deal with them the better off you’ll be. One of the pests that bedevils us this time of year is the rose slug.
Rose slugs are tiny, green worms that are the larvae of the rose sawfly. Heaven knows why they’re called slugs. They don’t look like a slug and they don’t leave a slime trail. Both the name and their appearance cause a great deal of confusion when you want to find a way to get rid of them. The most important thing to remember is that rose sawfly larvae are not caterpillars and you’ll see why this is an essential bit of knowledge in a minute.
When you’ve got rose slugs, you know it. These little creepies will skeletonize your rose leaves seemingly overnight (they don’t eat the buds or flowers) — one day you’ve got beautiful green, glossy leaves and the next day the plant looks like it was hit by a bomb. It is not pretty!
In Southern California the rose slugs have hit in a big way in most of my neighbor’s and client’s gardens. They are voracious and can make a mess of a rose bed in just a few days. As with most garden pests it’s important to treat for them as soon as you notice any leaf damage. Here, following the principles of Integrated Pest Management, are methods of control in ascending order of potency and potential harm to the beneficial insects in your garden.
Remember that the rose slug feeds on the underside of the leaf, so this is where to look for them and where to spray.
- Search & destroy — a great release for your aggressive tendencies. Flip rose leaves every morning and squish, or pick off the little worms. Not for the squeamish. This method can work, but you’ve got to be persistent and if you’ve got a big infestation this is a stopgap at best. So the next step is…
- Water — this is a safe and effective method for many garden pests (works especially well for aphids). Dislodge them with a strong stream of water. Frankly, I haven’t found this to be effective for rose slugs, but it’s worth a try.
- Insecticidal soap. You should spray in the early morning or in the evening when the wind is calm so that you don’t get drift and it won’t harm the good bugs who are not out and about at these times. Aim your spray on the underside of the leaves, it needs to hit the bugs to work.
- Neem Oil works by suffocating the pest, so be sure you cover the underside of all the leaves.
- Spinosad works by excitation of the bug’s nervous system. It must be ingested, so it affects only chewing and sucking insects. That said, be aware that it is toxic to honeybees for 3 hours, so spray in the evening when bees are back in the hive. It will take a couple of days to see any results and you may have to spray more than once.
Now for the bit about sawfly larvae not being caterpillars. The reason this is important is that it means that Bt (Bacillus thuringiensis) will not work. I can’t tell you how many times otherwise knowledgeable gardeners have recommended Bt for this problem. Even our local nursery swore that it would work. It won’t and you’ll be wasting your money if you use it for this purpose.
If you have exstensive leaf damage it might stress your plant, but it’s not fatal. Simply strip off all the leaves and they’ll grow back again in a few weeks. Also, it helps to know that if you’ve just planted some new rose bushes this year they were probably raised in a nursery using non-organic methods. The transition into an organic garden will make them more susceptible to pests than your other plants for the first season. Don’t despair, just give them time to get adjusted.
Here’s where I extol the benefits of organic gardening — again. Boosting your soil with compost and feeding your plants with rose tea (click here for the recipe) will make them healthy enough to resist most pests. And organic gardens will attract all kinds of good creature who will help you with your gardening — beneficial bugs and birds especially. (Every afternoon the birds come by to pick bugs off my plants.) So I’m not aiming to have a completely pest-free garden. After all there needs to be a little bit of bad stuff to feed the good guys.