A discouraging sight to anyone growing roses is that succulent new cane tip that suddenly wilts and dies. This symptom may have many causes, but one of the culprits is a cane borer. There are numerous borers – they’re generally the larvae of a beetle or moth that bore into canes, feed, and eventually cause the cane to wilt and die. The telltale sign of a borer is an entry hole usually found along the dead cane.
Here are some of the more common boring pests of roses:
Raspberry horntail (Hartigia cressini): the larvae are white, segmented caterpillars up to one inch long. Adults are wasplike, black, or black and yellow. This insect is not really a horntail wasp but a primitive wasp in the stem sawfly family.
Stem boring sawfly (Hartigia trimaculata): the larvae tunnel into a stem often girdling it. Adults are approximately one-half inch long and appear as a black and yellow wasp. They emerge in late April, early May, and insert their eggs under bark at the tips of current season canes. When the eggs hatch, the larvae enter stems to feed on pith, eventually forming a small chamber in upper part of the stem where they pupate and a second season of adults gnaw their way out and emerge in late summer. This second generation lay eggs that hatch into larvae and tunnel downward, passing the winter near the base of the plant.
Rose stem girdler / bronze cane borer (Agrilus aurichalceus): the larvae carves out meandering tunnels under the bark of roses, raspberries and currents. A characteristic slightly swollen area develops around the wounded area of the stem. Canes die back or break at wounded sites, sometimes several weeks after tunneling. They overwinter as partially developed borers under the bark of canes. In spring, they resume feeding and pupate within the plant. Adults are bronze-colored beetles, approximately 3/8 inch long that emerge in mid to late May. You may see them sunning on leaves. Females lay eggs in cracks on stem or base of leaves. Within a week, eggs hatch and larvae burrow into canes. Fortunately, there is only one generation per year.
Flathead borers (family Buprestidae) may kill canes or an entire plant. The larvae are up to one inch long with enlarged heads. Adults do not significantly damage roses. Eggs tend to be laid on stressed roses, especially on bark wounds caused by sunburn or disease.
What can you do if you have damaged canes from the borers? Cut each cane below the wilted portion and examine the pith – continue to make short cuts until the pith is white. Destroy any damaged or dead canes.
While it may be a bit extreme, you can chase a small wire down the cane to kill larvae then plug hole with white glue. No natural controls or realistic non-chemical control methods are presently known for controlling the adult stem sawflies before they lay their eggs on succulent rose growth. Parasitic wasps attack and kill full-grown sawfly larvae as they prepare to pupate.
Your best bet is to keep plants healthy and inspect canes in spring (mid-April – mid June) for egg laying incisions or swelling. If you observe any, remove and destroy infested canes.
By Nanette Londeree, Master Rosarian