At least three species of sawflies (Tenthredinidae) are occasional pests of roses in California. The most common is bristly roseslug (Cladius difformis). Adults are stout, 3/8 to 2/3 inch long, and mostly black with orange or yellow, varying by gender and species. Take care to distinguish roseslugs from beneficial syrphid larvae that feed on rose aphids. Roseslugs occur near chewed or scraped leaves, while syrphids occur among aphids and do not chew on plants.
Bristly roseslug. Bristly roseslug in California occurs mainly in cooler areas, such as near the coast. Larvae are pale green or yellowish green with a brown head. They grow to about 3/4 inch long and have many, tiny, hairlike bristles. Young larvae chew off the surface of the underside of leaves, leaving windowpanelike damage.
Curled rose sawfly. Larvae are green with rows of pale, short, stout projections (tubercles). The head is pale brown to yellowish with a black eye spot on each side. The larvae grow up to 3/4 inch long and often coil their bodies while feeding. They mostly chew entirely through leaves along the edges, leaving mostly veins. They may chew half-moon-shaped cuts in leaf edges that resemble the foliage clipping by leafcutting bees.
Roseslug. Larvae are mostly green on top and yellowish along their sides. They have a brown head and grow up to 1/2 inch long. Sawflies develop through four life stages: egg, larva, pupa, and adult. Females insert small eggs in the margin of rose leaves. Larvae hatch and grow through about five, increasingly larger instars (growth stages). Depending on the species and situation, rose sawflies pupate in cocoons on leaves, in organic litter beneath plants, or in a cell in twigs or the stub end of cut roses. Larvae chew and scrape the leaf surface, chew entirely through foliage, or both varying by the age and species.
Natural enemies keep many sawfly populations low and can cause outbreak populations to soon decline. Handpick larvae or clip off and dispose of infested foliage if rose sawflies are on a small portion of the plant. Larvae are relatively easy to control if they are thoroughly sprayed with almost any insecticide, including horticultural or narrow-range oil, insecticidal soap, neem oil, or spinosad. Spinosad can adversely affect bees and certain natural enemies. As of August 1, 2020, pesticides containing the active ingredient carbaryl are restricted use materials in California. A valid pesticide applicator’s license is required for their possession and use.
Adult bristly roseslug, Cladius difformis, a sawfly. Larval chewing damage of bristly roseslug. Larva of bristly roseslug. Larva of roseslug and its chewing damage. Credit: John A.
The pesticide information on this page may become out of date as products and active ingredients change or become unavailable. Some of the pesticides listed are only available for use by licensed pesticide applicators. Rose sawfly (sometimes called rose slug) larvae are a common pest of roses. The early feeding damage causes “windowpanes” of transparent leaf tissue on the leaf as the larvae feed on the surface of the leaf. As the larvae grow larger, the feeding damage begins to go through the leaf, often forming characteristic elongated holes.
Rose sawfly (also known as rose slug) damage. You may initially suspect Japanese beetles as the culprits as the damage looks similar, but the windowpane effect and initially small size of the holes point towards the rose sawfly. While the damage is unattractive, unless there are a huge number of larvae on your roses, the amount of leaf tissue eaten should not cause any permanent harm or weaken your roses if they are otherwise healthy. The larvae are light green with a slimy appearance and tan heads and can be found on the underside of the leaves. While they look like caterpillars, they are the larvae of a stingless wasp-like insect. This is important because a standard pesticide treatment for caterpillars, Bt (Bacillus thuringensis), is only effective on true caterpillars, the larvae of butterflies and moths. Bt will not control sawfly larvae.
If no larvae are visible on close inspection, they may have finished feeding or have been controlled by other insects or birds that feed on the larvae. If no larvae are present and no new damage is seen, no control is needed, as there is only one generation per year. You would want to watch for damage beginning in early June of the following year so you can treat it early. The larvae should be controlled if you've used Rose Rx Drench within six weeks. The larvae can be knocked from the leaves with a strong water spray; once dislodged, they cannot get back to the rose bush. Pesticides containing neem or spinosad are effective, as are insecticidal soaps and horticultural oils. Be sure to cover the bottom side of the leaves well when spraying, as this is where the larvae generally stay. During hot weather, be sure to spray pesticides late in the day when it begins to cool, as spraying in full sun in high temperatures can cause leaf burn. Always read and follow the label!
Here are the common ways your landscape is telling you it needs help. Controlling fall and winter weeds is a constant battle. New Garden Landscaping & Nursery feels that Boxwood Blight could have a major impact on landscapes containing boxwood in the near future. As spring unfurls, plants are not the only things waking up outdoors. Insects are also coming to life. Spring pests that survived winter are hatching, and insects that burrowed into soil or beneath leaf litter to ride out the cold are emerging. Some pests specifically target roses. You can help protect plants by inspecting them a few times a week. Pests of roses reproduce quickly, though, so you want to be sure you recognize the symptoms of a problem before it becomes a major headache. These sucking insects show up early in the season, before the first rose ever opens. Oval-shaped, small and light green or pinkish, Aphids are easy to overlook. Aphids suck the juices out of new shoots, buds and leaves. They produce honeydew, a sweet, sticky substance that hosts Black Sooty Mold and attracts Ants. Two kinds of Inchworms affect roses: the Fall Inchworm and the Spring Inchworm. They're actually Cankerworm Caterpillars. You'll typically spot them on unopened rose blooms, inching along petals, munching as they go. Infested roses are usually located beneath trees. Several types of Sawflies attack roses. These pests look like Caterpillars but aren't - and they're not Slugs, either. That means you can't treat them using traditional Caterpillar or Slug controls. Sawfly Larvae frequently feed along leaf undersides, sucking out the green parts and leaving a transparent, papery spot. Larvae feed for about four weeks before pupating. If you spot damage early and identify the culprit, you can treat. Scale insects lay eggs on rose stems in fall. Eggs hatch in late spring, and the young Scale insects prowl along rose stems, seeking a spot to latch on and start sucking. The surest sign of Scale is white, circular, limpet-looking things on rose stems. Scale feeding produces honeydew, which attracts Ants and may grow Black Sooty Mold. When Thrips are present, you get brown, streaky buds that only partially unfurl. Thrips are tiny (1/16 inch long), slender sucking insects that are usually yellow, brown or black. They're very difficult to see. Thrips typically infest roses when conditions are hot and dry. If an unseasonal spring heat wave arrives, watch for Thrips. Otherwise, they usually appear as summer arrives. You'll also see beneficial insects in spring, including Ladybugs, Green Lacewings, Spiders and Praying Mantises. Don't disturb or discourage these critters.