What Are the Causes of Round Holes in Hydrangea Branches?

Symptoms of Cane Borers

• To determine if your hydrangea bush is suffering from cane borer, there are a few symptoms to watch out for. Sawdust-like frass (excrement) can be seen around the hole, in piles on the ground or on the foliage. The holes in the branches can be either round, oval or D-shaped and can be seen randomly on the plant. Plants that are severely infested may show signs of stress in reduced growth rate and poor form.

Life Cycle of Cane Borers

• There are several types of borers that could be infesting your hydrangea. Adult borers are generally beetles, moths, flies or wasps. The adult borer lays its eggs on the branches. The larvae hatch and bore into the branch, tunneling as they feed. As they grow and mature, cane borers will emerge as adults, leave the shrub, mate and lay eggs on another branch. Understanding the basic life cycle of a cane borer will help you in your efforts to control the infestation on your hydrangea.

Non-Chemical Controls

• Once a plant has been infested with borers, there are few non-chemical solutions that are effective. The most effective non-chemical control in hydrangeas is to prune all infested wood and destroy your clippings by chipping or burning. If left intact, the adults can still emerge and infest other plants. Another option would be to take a small, flexible wire and insert it into the hole to try and puncture the borer. Hanging pheromone traps around the yard will help detect the first sign of adults, and will disrupt the mating ritual, reducing the amount of eggs produced.

Chemical Controls

• There are two different types of chemical controls: contact and systemic. Contact insecticides, such as carbaryl and permethrin, must be sprayed over the entire surface of all branches to kill emerging adults or newly hatched larvae. The residual effect will last three to 10 weeks, and will not kill borers inside of the tree. Systemic insecticides, such as imidacloprid, can be sprayed on the foliage, poured around the drip line of the shrub or injected directly into the branches. Systemic insecticides translocate throughout the plant through the sap, and will only kill borers that feed on the sap and not the hardwood. Keeping your plants healthy and using a combination of both non-chemical and chemical pest management techniques will help you control cane borers in your hydrangea.