What’s Eating My Rose Leaves?

roseslug sawfly damage on rose
Roseslug sawfly damage on rose. Photo: UME/Ask an Expert

Q: My rose leaves have white spots and holes in them. What causes this and how do I treat it? Is there a natural remedy that does not involve powerful chemicals?

Answer: It looks like your rose has symptoms of sawfly damage. Check the undersides of the leaves and look for tiny green larvae that look like little worms or caterpillars. These are the juvenile stage of an insect called roseslug sawfly.

Roseslug sawflies are neither slugs nor flies. They belong to the same order of insects as wasps, bees, and ants (Hymenoptera). Adult female sawflies use their unique ovipositor (egg-laying part) to saw a small slit in a leaf or stem where they lay their eggs. When the eggs hatch, the larvae feed on the leaf surfaces and cause an etched or scratched appearance. Some roseslug larvae chew through the leaves entirely. Damaged foliage turns brown and curls up as the season progresses.

bristly roseslug on rose leaf
Bristly roseslug (Cladius difformis). Photo: Whitney Cranshaw, Colorado State University, Bugwood.org
bristly roseslug sawfly adult
Bristly roseslug (Cladius difformis), adult stage. Photo: Whitney Cranshaw, Colorado State University, Bugwood.org

In Maryland, there are three species of roseslug sawflies that cause damage to roses: the bristly roseslug sawfly, the roseslug sawfly, and the curled rose sawfly. Most of the feeding activity on roses in Maryland is seen in May and June, but some sawfly larvae can continue to be active until fall. Other insects, such as Japanese beetles, also cause chewing damage on rose foliage (typically in June-July).

rose-brown-leaves
Browning and leaf curling from rose slug sawfly damage. Photo: UME/ Ask an Expert

The best way to manage roseslug sawflies without chemicals is to monitor your plant(s) for damage symptoms early in the season (start looking in May) and manually remove any larvae (squish them or pick them off and discard them). A forceful spray of water from a garden hose targeted toward the leaf undersides can also get them off. Once dislodged, they cannot climb back up into the plant. Horticultural oil and Spinosad also work well against sawflies. Read and follow the product label instructions for the correct application procedures.

Two sawfly larvae are present on the undersides of rose leaves. Photo: C. Carignan

Predatory insects and birds help to manage sawfly populations naturally. Adding more flowering plant diversity to your landscape will provide food and habitat for beneficial animals that in turn help to reduce future pest problems.

Rose shrubs usually recover from sawfly damage eventually, as long as they are not struggling from other ailments or stressors such as drought.

Additional Resources

Sawflies | UME Home & Garden Information Center

Rosie Defoliators | Bug of the Week, University of Maryland, Department of Entomology

By Christa Carignan, Maryland Certified Professional Horticulturist, Coordinator, University of Maryland Extension Home & Garden Information Center. Read more by Christa.

Have a plant or insect question? University of Maryland Extension’s experts have answers! Send your questions and photos to Ask Extension.

Beyond Broccoli Part Three: Pesky Pests

Welcome back to Beyond Broccoil! Read parts one and two to get caught up. We’ve put our plants into the garden in the cooler weather of spring or fall, and now we need to get them successfully to harvest.

Brassica crops aren’t subject to many disease issues, though (as discussed in the last post) they can be affected by temperature extremes and variations. The biggest problems you’re likely to have, though, are pests. Animals such as rabbits, groundhogs and deer love chomping on cabbage family plants, so you’ll need to exclude them with a fence. These are not plants you want to spray repellents on—after all, you’d be eating the leaves or other parts you covered with hot pepper or rotten egg concoctions. Row covers (discussed below) may be enough to keep browsing animals out, but make sure they’re tightly secured.

Many insects also love to feed on brassica plants. Here’s a list of the most common with links to HGIC pages covering them:

Harlequin bugs in different stages feeding on mustard. Photo by Barbara Knapp.

The simplest way to deal with these pests really is to exclude them using floating row cover. I have talked to many, many gardeners who resisted doing this, thinking it was too much trouble, and then realized that picking caterpillars by the dozen off their hole-riddled harvest was actually a lot more difficult. (Worse: realizing you didn’t actually pick all the caterpillars off before cooking the vegetables.)

Read the page linked above to learn all about the uses of row covers and the different types available. I recommend trying the more durable insect mesh netting for summer crops, and also the heavier weights of row cover if you want to start your plants early in the spring or keep them going into late fall or winter. Brassicas overwinter quite well if given some protection from cold snaps. You could also consider wintering over plants in low tunnels with clear plastic or under cold frames, but remember that you may have to vent them on warmer winter days (which we’re dealing with a lot more often). Also look into shade cloth as a way to cool the soil when you’re transplanting fall seedlings in hot summer weather.

You need to uncover the plants only to harvest and weed, and they will look beautiful! Photo by Erica

The one circumstance where row cover is not appropriate is when you’re incorporating brassicas into a flower bed as part of edible landscaping. This is great in theory, since some of these plants are really attractive. Just keep possible pest issues in mind. Surrounding brassicas with flowers will help attract beneficial predatory insects, and strong-smelling plants like herbs or members of the onion family may keep animals away. Insects might also be confused by a diverse mix of plants. But unless you’re very lucky you’ll probably have to accept some damage.

Other methods to deal with insect pests include:

  • Pesticides. You can read about these at the links for each pest, above. Try to stick with organic pesticides, and use them as a last resort and according to directions.
  • Handpicking. Have a bucket of soapy water handy and drop the pests in, or squish them.
  • Last-minute kitchen intervention. Soak the vegetables in a sink full of water with salt added. Pests should float to the top.
  • Trap crop. Plant a crop early in the season and destroy the insects that visit it. This may at least cut down on the total number of pests.
  • Weed regularly. Insects feed on weeds as well as crops, so keep their food supply low.

Aside from dealing with pests, growing brassicas is not difficult. Water as needed, and incorporate a slow-release nitrogen fertilizer into the soil. Brassicas prefer a pH of 6-7.5. Add compost to your soil on a regular basis to help maintain nutrients and improve drainage. If you have room in your garden for crop rotation, it’s a good idea to move members of different families to new areas each year. Brassicas might leave a few of their insect pests behind this way (it won’t help with the ones that fly around freely) and they will appreciate the nitrogen left behind by bean family plants (including cover crops).

Pay attention to the weather forecast. If temperatures are heading upwards, your brassica plants may react by going to flower. It might be time to harvest even if the vegetables in your garden don’t look exactly as expected. I have harvested a lot of disappointingly runty broccoli heads, but I’ve also learned that small is better than exploding into bloom. (Though you can eat the flowers.)

Next time we’ll start learning about specific plants within this genus.

By Erica Smith, Montgomery County Master Gardener. Read more posts by Erica.

Beach Books for Veggie Gardeners

All right, maybe not the beach. But as we exit spring and enter the “oh maybe I’d rather stay indoors in the AC” season, I’ve got some recently-published books that might encourage you to get out there and make your garden better (but you can read them inside on a hot day and count that as horticultural education). Want to learn how to identify and deal with pests? Want to know if there’s anything to this “companion planting” stuff? And what’s up with “regenerative gardening”—can your soil really feed your plants? Read on!

Continue reading

Q&A: What’s wrong with my cherry laurel shrubs?

cherry laurel
Cherry Laurel with Leaf Scorch Symptoms and Holes. Photo: University of Maryland Extension / Ask an Expert

Q: My cherry laurels do not look good. There are brown spots and holes on the leaves and white stuff on the trunk. What can I do?

A: First, there is a lot to like about cherry laurels (Prunus laurocerasus). They are popular evergreen screening and foundation plants, deer-resistant, and pretty tough once established. Cherry laurels make their best growth in moist, well-drained soil in full sun to partial shade. They even tolerate full shade.

We receive numerous questions about problems with these shrubs. They can have multiple issues that are attributed to environmental, climate, or site conditions. When the plants are stressed, they weaken and become susceptible to diseases and/or insect pests. It is helpful to be aware of these issues before planting them. Continue reading

Allium Leafminer: Possible Ornamental Plant Pest?

allium leafminer
Fig. 1 Adult female Allium leafminer. Photo: E. Agallou and D. Collins

There is a new pest in our area first found in Lancaster County, Pennsylvania, in December 2015. It is the Allium leafminer (Phytomyza gymnostoma) which attacks Allium plant species. Leeks tend to be the most damaged host, but all Allium species (onions, garlic, chives) may be attacked. Adult females have yellow/orange heads and yellow ‘knees’ (fig. 1) and make repeated punctures in a leaf with their ovipositor. These punctures are organized in a straight line going down the leaf from the tip (fig. 2). Damaged leaves can appear wavy, curled or distorted.

Larvae mine leaves, and move into bulbs and leaf sheathes where they pupate. Both the leaf punctures and mines serve as entry routes for bacterial and fungal infections. High rates of infestation have been reported from some Allium fields in Pennsylvania.

allium leafminer damage
Fig. 2 Ovipositor/feeding damage to onion transplants by Allium leafminer. Photo: Sarah May

The reason I am telling you this is because we do not know if this pest will attack Allium plant species in the landscape. Not only would this cause possible landscape plant loss, but these infestations would also act as a breeding area for the pest to build its population early in the spring and later in the fall; the two times of year when it is most active. If you see any similar damage to Allium species or the fly itself on landscape plants, please let me know. You can submit your photos to the Home & Garden Information Center’s Ask an Expert service and we will take a look at them.

By Jerry Brust, Senior Agent and IPM Vegetable Specialist, University of Maryland Extension