Knockout Roses & Weed Control #458329 - Ask Extension

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Knockout Roses & Weed Control #458329

Asked May 31, 2018, 9:04 AM EDT

Good Morning: I have a little black bug swarming around and feasting on my knockout roses . . . help! I have attached a photo of the culprit and the damage they are causing. Please let me know what to do go stop this. Also, we have large mulch beds that we spend hours and hours weeding so we are considering using something like Preen weed control, does that really work?

Calvert County Maryland

Expert Response

The small black insect looks like it might be an adult sawfly. The chewing damage and browning on the foliage of the rose are caused by the feeding of the sawfly larvae. This is a very common problem on roses. You can learn about rose slug sawflies and how to manage them, here on our website. https://extension.umd.edu/hgic/rose-slugs-shrubs. With regard to using Preen, it will not control existing weeds. It can be used as a weed preventer to control annual weeds (not perennial ones). You have to read the product label to see which weeds it will be effective in controlling. If you need help identifying the weeds you have, feel free to submit us a new question with photos. You can send up to three photos at a time. We also have a gallery of common weeds on our website, with information on how to control each type of weed.  https://extension.umd.edu/hgic/problems/weeds

ckc

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