Sap From Plum May or May Not Be Borers

Q. I have a fruit bearing plum tree approximately 2 years old. I noticed an amber hard substance on the trunk. I think it’s borers. If it is, what can I do to save it and still be able to eat the fruit? 

A. The most effective way to kill borers is to apply a systemic pesticide and let the tree distribute this pesticide everywhere inside it. There is a very popular systemic insecticide available nearly 100% effective at killing borers if they are present.

            This insecticide is distributed everywhere (systemic) inside the tree and lasts for nearly 12 months. And yes, it is labeled for fruit trees that are bearing fruit as well as vegetables. This pesticide applied to food bearing crops makes me nervous for obvious reasons.

            Let me present to you an idea that does not require pesticides but a little work on your part. I have observed it to be about 80% effective. This requires a sharp knife and a method to sanitize it such as alcohol, butane lighter or even Pine-Sol.

Plum sap from a pruning cut in the spring

Sap oozing from the trunk of a fruit tree due to overwatering

            Plums are very sappy trees. Any injury to living parts of the tree cause sap to be produced. The production of sap is a defense mechanism against “intruders”. The tree does not differentiate between damage from boring insects and damage caused by pruning, invading diseases or environmental damage.

Extensive damage to the trunk of plum and bark is peeling from the west side

            To a plum tree, it’s all the same. It reacts by producing sap. If it is an intruding insect like a borer, sap engulfs and frequently suffocates it. If the damage by boring insects is extensive, loose bark easily lifts away from the damaged area because that area is dead. 

Borer damage under the bark leaving behind "debris" in the tunnels just under the bark from eating

            Damage from boring insects most first appears on the west or south facing sides of the trunk and limbs or on their upper surfaces. A tree may have damage for one or two seasons before you see parts of it suffering outwardly. On plums, sap is a good indicator something is going on.

            The only way to find out if a boring insect is involved is to inspect the wood under the sap for damage. It is easiest to do this after the leaves drop in December. You can wait. It’s cold now and they are not active.

            When you are ready, take a very sharp, sanitized knife and remove the sap along with the bark just under it. Look for damage to the trunk or limbs in the exposed wood. Boring insects leave debris from feeding, in tunnels, just under the bark.

Borer damage removal using a sharp knife removing all the damaged bark to fresh wood

            If you do not see damage to the wood under the sap, then this damage is not due to borers. Leave it alone if the limb appears otherwise healthy. If you see insect damage in the wood under the bark, cut and remove ALL bark from the damaged area with your sharp knife and let it heal on its own. If the damage is extensive and the limb is weak, remove it.

            There is no pesticide you can apply to the tree that will kill the borers AND leave the fruit safe to eat in my opinion even if it’s on the label and you can use it for that purpose.