Holes in Your Rose Leaves? It Might be Rose Sawfly
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!)
