One of the best things about hosting an “Ask the Garden Coach” booth at various farmer’s markets around Columbus is that I learn what insects are showing up in area gardens, and lately, a big pest has been sawfly larvae (or rose slugs) on roses. These little &^%$!! are quite capable of defoliating a rose bush in a few weeks if left unchecked. So what should you do if you have severe sawfly damage (see photos)? Consider the following decision flowchart.
Is the rose as healthy as possible, or has this it been struggling to survive? Ask yourself if this rose is a plant you really care about before you spend time, money, and effort on pesticide. Sometimes the best way to deal with an insect problem is to remove their meal-ticket!
If you want to keep the rose, do everything necessary to bring it into full vigor including properly watering, fertilizing, and pruning it at the appropriate time in the appropriate amounts. Hey, no one said perfect roses were easy.
When the infestation is light, non-toxic control methods include hand-picking/squishing, blasting plants with water, and/or cultivating the soil around the base of the plant to expose larvae to hungry birds and freezing winter weather. You can also try spraying insecticidal soap, which kills by physically suffocating the insect.
Recognize that sawfly larvae may resemble caterpillars, but they are NOT caterpillars and spraying them with low-toxicity Bt (is pointless. If you want to use an insecticide, you’ll have to use something stronger like neem oil or a product containing pyrethrins.
Part of the reason higher-toxicity insecticides are effective is that they are concentrated and they persist for a longer time in the environment. Spraying for “bad” insects also kills the beneficial ones. Please be mindful when using any insecticide and precisely follow label directions regarding dilution rates, quantities, time of day to spray, etc.
If you still can’t get sawfly larvae under control and your roses are making you feel like a bad gardener, return to step 1.
Incidentally, there are at least three species of sawfly larvae that dine on roses. The images here do not reflect all of them – your mileage may vary.
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