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Rose slugs, a thorn in a rose’s side

July 22, 2017
Jane Scurich

My parents retired to a small suburb of Atlanta. I knew my lengthy drive from the airport was almost over when I spotted the local pest control marquee “Bug of the Month.” Although termites were the perennial favorite, it was always fun to guess what might be the next monthly honoree.

I am now host to my own “bug of the month.” My garden has been invaded by so many pests recently: thrips, leaf miners, curculio and more. Numerous years of drought, followed by this year’s record rain, climate change or a combination of factors, may be related to the increase in pest and fungal diseases.

This spring I was horrified to discover the leaves of my rose bushes becoming skeletonized. The inner sections of each leaf appeared to be transparent, the veins were intact, but the tender green leaf sections had all but disappeared. Severely affected leaves were thin and papery feeling.

My research identified a possible culprit that I never knew existed: rose slugs.

I took some leaves, carefully sealed in a Ziploc bag, to my local nursery whose always helpful staff echoed my analysis. For further confirmation, I took a sample to the UC Marin Master Gardener help desk in Novato, completed the information form provided and waited for a diagnosis. Two days later, my research was confirmed: rose slugs had invaded my garden.

Rose slugs are the larvae of the sawfly, which looks like a fly or wasp. It does not sting or eat and is short-lived. The female sawfly uses its sawtooth egg-laying appendage to form a pocket in the edge of a rose leaf and inserts one egg per pocket. The young larvae emerge and dine on the soft tissue, creating the skeletonized appearance. More mature larvae chew large holes in the leaves. Once fully mature, the slug drops to the ground where it pupates before emerging as a sawfly.

Prevention starts with regular garden monitoring and sanitary maintenance. Be alert to early signs of leaf damage. I attribute my infestation to the fact that I took a truly marvelous trip to Japan in early April. I experienced the renowned cherry blossom festival in Kanazawa, a once-in-a-lifetime experience. Meanwhile, my garden went unmonitored. As the adage advises: you play, you pay. How true! So, what now?

For a light infestation, remove leaves showing damage, squish those tiny green caterpillar-like creatures you find or blast water on the leaves to knock off the intruders, and encourage birds and beneficial insects into your garden by providing a water source and a variety of beneficial plants. You could also spray with insecticidal soap or Neem oil, coating both the top and bottom of the leaves.

Larger infestations are more problematic and potentially weaken the entire plant’s ability to photosynthesize. The weakened plant is then more susceptible to attacks by other pests and diseases. For a major infestation, remove all infected leaves, clean up and discard debris in and around the plant. If these methods are unsuccessful, consider using a product containing Spinosad that is potentially harmful to bees and should be used cautiously, late in the afternoon after bees have stopped foraging. Follow all label directions and do not spray where there are flowers that might attract beneficials.

I used a combination of acceptance of small infestations, total leaf removal from the most devastated plants and water blasts. I think I am slowly gaining control, but I will need to remain constantly alert as the sawfly can produce up to six generations in one season!

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