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 grows larger, the feeding damage begins to go through the leaf, often forming characteristic elongated holes.
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 points towards rose sawfly.
While the damage is unattractive, unless there are a very large 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 actually the larvae of a stingless wasp-like insect. This is important to note because a common 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 the following year so you can treat early.
If you've used Rose Rx Drench within 6 weeks the larvae should be controlled. The larvae can be knocked from the leaves with a strong water spray; once dislodged they will not be able to 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 that you 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!)
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