Garden Q&A: What’s chewing my rose leaves?

Q: I have noticed rose shrubs in mine and many gardens have holes or pale grayish patches on the leaves, but I don't see any bugs on them. Is this an infection or something I’m just not finding?

A: Although a couple of diseases can cause damage that winds-up looking like insects chewed holes in the leaves, in this case the culprit actually is an insect, just one that's hard to spot.

When leaf chewing creates paler sections that aren't all-the-way-through holes, we call it window-paning, because the feeding removes enough of the leaf layers that the remainder is translucent, like a frosted window pane. Young or small insects sometimes can't chew all the way through a leaf and so they gnaw off the layers on one side, leaving a bit that's almost see-through because it has so little tissue left. When we view this from above, it can look pale gray because it's nearly clear. As the larvae mature, they can then chew distinct holes, so both types of damage can appear together because leaves can't heal prior injuries.

The primary insect responsible for chewing rose leaves this time of year is a sawfly – in particular, one of a few species of roseslug sawfly. Despite their name, they are not relatives of flies, nor are they slugs. The larvae of sawflies look very similar to caterpillars, though there are subtle differences. Not lumping them in with caterpillars is an important distinction when determining how to manage them as not all pesticides that work for caterpillar control will work for sawflies.

The sawfly larvae can be very hard to see, even when you are familiar with what to look for. They are green, smooth-skinned, and short-legged, so they sit nearly flush with the leaf and blend in very well. I see them most often damaging cultivated roses, but occasionally find them on wild native (and invasive) roses as well.

Two roseslug sawfly larvae on the underside of wild rose leaves, next to their characteristic type of feeding damage. (Miri Talabac/Handout)

Roseslug sawfly can have only one or as many as three generations per year in our area depending on which species is present. Fortunately, in terms of suppression, it doesn't much matter which one you have as they are all treated the same way. You can either ignore them if damage is minor or tolerable, remove them by hand or with a garden hose on full blast, or if absolutely necessary, treated with a lower-toxicity insecticide. Here is where the caterpillar separation matters: Bt (Bacillus thuringiensis), a naturally-derived product that can kill young caterpillars and is handy for minimizing risks to other insects, will not work on sawflies.

A rose shrub should not succumb and die due to sawfly attack, so if a sawfly-riddled plant is declining or doing poorly after the sawflies have matured and stopped feeding, something else is probably hampering the plant's vigor. You can browse our web page about typical rose ailments to narrow down likely causes.

Q: I’m still fairly new to vegetable gardening and have learned that my beloved greens of lettuce, spinach, and kale don't like Maryland heat. Are there leafy greens I can switch to during summer?

A: Yes – sweet potato foliage is edible, and varieties like ‘Tokyo bekana’ are good to use for a greens harvest. Swiss chard and three types of "spinach" (I’m using quotes because they’re not true spinaches) are also good candidates: perpetual spinach (botanical name Beta vulgaris variety cicla), New Zealand spinach (Tetragonia tetragonoides), and Malabar spinach (Basella alba or B. rubra). Perpetual spinach is actually a type of chard; New Zealand and Malabar spinach are unrelated to each other as well as unrelated to true spinach. Refer to our growing spinach web page for a couple more spinach-like crop ideas.

Leafy amaranths like callaloo (Amaranthus viridis and others) are foliage vegetables that are actually related to spinach and whose foliage can be harvested and cooked like their cousin. Roselle (Hibiscus sabdariffa) has a spicy-tart foliage flavor. Collards (a subset of Brassica oleracea) can take our summer weather and some varieties of kale, like ‘Red Russian,’ will do okay in our Mid-Atlantic heat.

University of Maryland Extension's Home and Garden Information Center offers free gardening and pest information at extension.umd.edu/hgic. Click "Ask Extension" to send questions and photos.