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!

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What’s Eating My Rose Leaves?

roseslug sawfly damage on rose
Rose slug 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 caterpillars. These are the juvenile stage of an insect called rose slug sawfly.

Rose slug 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 appearance. Some rose slug larvae chew through leaves entirely. Damaged foliage turns brown and curls up as the season progresses.

bristly roseslug on rose leaf
Bristly rose slug (Cladius difformis). Photo: Whitney Cranshaw, Colorado State University,

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

In Maryland, there are three species of rose slug sawflies that cause damage to roses: the bristly rose slug sawfly, the rose slug sawfly, and the curled rose sawfly. Most of the feeding activity on roses in Maryland is seen in May and June, but 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).

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

The best way to manage rose slug sawflies without chemicals is to monitor your plant(s) for damage symptoms and manually remove any larvae (squish them or toss them). Insecticidal soap, horticultural oil, and spinosad work well against these sawflies. These products are environmentally friendly insecticides listed by the Organic Materials Review Institute (OMRI). As with any pesticide, read and follow the label instructions carefully. Avoid sprays when your roses are in bloom, to protect pollinators and other beneficial insects.

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

Additional Resources

Rose Slugs on Shrubs | UME Home & Garden Information Center

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

Rose Insects & Related Pests | Clemson Cooperative Extension

Sawflies | University of Wisconsin-Madison

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

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

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