The Practical Gardener: How To Deal With Borers

Dec 19, 2015


Irene from Chico called, and she said that while she was out pruning in her home orchard, she saw small branches and twigs that have completely died back on her deciduous fruit trees. She said there were small round holes in the bark, and what looks like sawdust. We call that "frass" — or bug poop and wood particles. They were coming out of the holes in some of the trees. She also saw some sap oozing through cracks in the bark, but no sawdust. And finally she noticed lots of small dead shoots, specifically in her peaches and nectarines. What causes all of this? I bet there are some of you out there that have the same problems. 

The problems start with drought. Many of the fruit and nut trees in our home gardens have suffered through four years of reduced water, and the trees are very stressed. The stress opens them up to problems with several types of insect borers that can cause damage or even kill your trees. The holes and the sawdust, or frass, of the peach tree borer and oozing sap from the Pacific Flathead borer are signs of borer infestations.

Another borer that shows up in stressed trees is called the shot hole borer. They make very tiny holes in the bark of sunburned areas about the size of a pencil lead. All of these borers tend to attack either newly planted, stressed, sunburned or unhealthy trees. So the first thing to do is to correct any of those cultural problems. 

Make sure that the tree is properly fertilized and well-watered throughout the year —including the winter if we don't get any winter rains — and properly pruned, making sure that you leave enough branches to provide some shade to help prevent sunburn. 

You should also paint the trunks and lower exposed branches with a half-and-half mixture of white latex paint and water as extra protection against sunburn. This winter, prune out any infested wood and dispose of it or burn it. Don't just put it in the compost pile or leave it in the yard, because adult borers could just move out of the debris into your adjacent trees.

Spraying is not really recommended for shuttle or Pacific flathead borers because once the insects are under the barks of the trees, any pesticides that are applied will not come in contact with them to kill them. The best thing to do with these borers is to improve the cultural conditions and prune out the dead wood or infested wood or even just replace the whole tree.

The small dead shoots that Irene mentioned could be caused by peach twig borer — they bore into the young shoots and cause them to die back. With an integrated pest management program, the preferred management strategy for peach twig borer is a well-timed treatment of environmentally sound insecticides right around the bloom time, such as bacillus or in Gensis or Spinoza's it. Alternatively peach twig borer can be controlled with the spray of a delayed dormant, like late January/early February. To kill the over-wintering larvae, dormant sprays of oil alone or or oil combined with an insecticide have the advantage of controlling some other stone fruit pests well — especially mites and San Jose scale.

Irene, I hope this helps with your home orchard problem!