The Rose Curculio is a deep cinnamon red rose weevil that drills holes in rose buds and the stems that hold them, causing distinctive damage. On buds, the holes are punched through every layer with meticulous regularity, as if with a drill. On the peduncle which supports buds, one or more holes will cause the bud to droop or die. Possible hosts for rose curculio include other rosaceous plants such as thimbleberry and raspberry.
Merhynchites bicolor is the name of this pest found over much of the United States and Canada. Curculios feed on buds and peduncles. They deposit eggs in the receptacle, where their young incubate. After they hatch, they overwinter in the soil. In the spring, they emerge to eat, breed and ruin roses. They apparently produce only one generation per year. Reports vary as to their color preferences. Curculios and their damage have been observed on roses of all colors, classifications and shapes.
Rose curculios can by controlled, not eliminated, by hand-picking them and by fastidious deadheading and clean-up during their active season (early May through June in Northern California). The goal is to disturb their breeding cycle by making reproduction difficult. If disturbed, they will drop and play dead. Hold a container of soapy water underneath them, poke at them, and they will fall into the container. If damaged open flowers are found with many shot holes, curculios are sure to be nearby. Examine the affected rose and every adjacent roses. Curculios often hide under the sepals and are best detected from ground level, looking up. Remove every damaged buds and stem from the garden and do NOT compost them. Some rose growers report that rose curculios prefer light-colored roses, while others report curculios prefer deep mauvey purple-red roses.