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Don't worry too much about this common condition on rose bush leaves

"Fielding Questions" columnist Don Kinzler also advises readers on a pesky beetle that is prevalent in gardens again this year and how to prevent deer damage to yards and gardens.

Roseslug sawfly June 25 2022.jpeg
A reader wonders what is causing these holes on rose bush leaves.
Contributed / Special to The Forum
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Q: I am not super familiar with roses and have started to try them recently. This year something new is happening to one of my rose bushes. There are little white or gray spots that eventually turn into a hole in the leaf. Can you help identify what this is? — Jessica D.

A: The gray spots which later fall out are caused by an insect called the roseslug sawfly. The larvae rasp away at the green layer of the leaf, leaving the thin gray tissue that eventually falls out, leaving a skeletonized appearance.

The damage occurs in mid-May through mid-June, when the feeding portion of the insect’s life cycle stops. Damage is often on the lower part of the rose bush, and newer growth is usually normal, as the insect’s activity ceases.

The larvae are a pale greenish color and often go unnoticed. Because the damage stops by mid-June, control is often unnecessary or ineffective by the time the signs are noticed. If damage is caught early, and a young rose bush is being badly affected, a rose spray or dust can be used, or insecticidal soap.

Because beneficial pollinating bees also visit roses, insecticides should be used sparingly, if at all. The damage from roseslug sawfly is usually cosmetic, unless feeding damage is affecting a majority of leaves.

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Click the link above to read more of Don Kinzler's "Fielding Questions" and "Growing Together" columns.

Q: Our string bean plants are all full of holes, and I see a small beetle. What is the best way to control them? — John W.

A: Bean leaf beetles are seemingly everywhere again this year, and I’ve received many questions. The leaves of our own string beans were riddled with holes before I applied insecticide.

Several insecticides will give good control of these beetles, including Sevin, malathion and permethrin (one brand that contains the ingredient is the insecticide Eight.) Check labels for the waiting time between application and safe harvest. An organic insecticide that can be applied close to harvest is spinosad, and the label will indicate the exact interval.

Q: I’ve read your past articles about deer damage and wanted to tell you about a way I’ve found to prevent deer damage to our yard and garden. The method might sound a little crazy, but it really does work and is almost foolproof. — Evelyn B.

A: Evelyn caught my attention immediately when she mentioned deer damage prevention and foolproof. Deer cause millions of dollars of damage to trees, gardens and landscapes every year, and repellents can have mixed results.

Evelyn continues, “Deer were always eating our flowers, shrubs and gardens, and around eight years ago I read about a method using monofilament fish line. I read that if an area was encircled about 4 or 5 feet high with the fish line, the deer see the reflection of the line, don’t know what it is and won’t cross it. Or they feel it when they come up to it, and will go away.

“We tried the method, and it works great. We used PVC pipes or other posts to support the fish line, and surrounded the areas that we wanted to protect. It stopped the deer damage. Once, when the fish line came down, the deer got into the garden and ruined our beets. We put the line back up, and there was no further damage. I thought your readers might be interested in this method.”

Evelyn, your method isn’t crazy at all, because I’ve actually tried it with wonderful results in the past. Like you say, the deer see the glint of the monofilament fish line and won’t cross. The method might not be practical in all locations, but it’s much less expensive than a tall deer fence.

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I’ve used this successfully along a tree row through which deer were entering to consume our garden. In areas that can be encircled, where humans might not accidentally walk into the fish line, this is an option well worth trying. Thanks, Evelyn, for passing along this method.

If you have a gardening or lawn care question, email Don Kinzler, NDSU Extension-Cass County, at donald.kinzler@ndsu.edu. Questions with broad appeal may be published, so please include your name, city and state for appropriate advice.

Don Kinzler, a lifelong gardener, is the horticulturist with North Dakota State University Extension for Cass County. Readers can reach him at donald.kinzler@ndsu.edu.
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