Question: A few of my rose bloom stems have curled downward and I wonder what causes this?
This is not an uncommon phenomenon to see in the rose garden and is usually caused by a pest, the raspberry horntail, Hartigia cressoni. Rose cane tips wilt, droop, discolor and die back. This problem can also be caused by other rose culture factors such as over-irrigation and lack of soil aeration, but a close look can identify if the problem is caused by the horntail. If you see a small, circular discoloration of the cane, especially where the second and third leaves from the tip attach to the cane, the raspberry horntail is the problem. This area is where female wasp inserts her egg to develop and where the larva feeds inside the cane. Canes may also become slightly swollen when a larva has fed inside. Cutting open the affected area will expose a tunnel with brown, granular frass (excrement) and sometimes an immature raspberry horntail or its cast skin if the wasp has left. The horntail can also be a pest of other cane-producing plants, such as blackberries and raspberries, hence the name.
A description of the eggs, larvae and adult horntail as well as other information is provided in this publication of the UC Integrated Pest Management program: