Don’t Confuse Borers in Fruit Trees or the Result Could Be Lethal

Q. Concerning my peach tree, it looks like the peach tree borer is my most significant pest. Being that the Bt pesticide spray is a great organic preventative and killer of the borer, and I already have it and use it in my vegetable garden, shall I just spray the peach tree once a week as a preventative? If I do, how long do I keep this up until the threat is over? Are there any other insects to worry about such as aphids?

A. There are two different borers which we must be careful not to confuse. Control is entirely different between these two borers. The borer you refer to damages the trunk and limbs of many trees including fruit trees as well as ornamentals. This borer is very difficult to control with pesticides.

Flatheaded borer (probably either Pacific flatheaded borer
or Flatheaded apple tree borer) that causes limb dieback

            Bt, an organic spray made of a bacterium, will not kill these deadly borers which we see as damage and dieback of limbs and entire trees. Bt sprays will, however, kill a borer which causes wormy peaches, nectarines and apricots. This borer is called the peach twig borer.

            Peach twig borer kills the soft, succulent growth of peaches and nectarines at the beginning of the growing season. We should start seeing this damage right now. Its lifecycle is very short and so the population of this pest builds very quickly to enormous numbers in a matter of a few months.

Peach twig borer larva or immature form (striped) in the
flesh of peach in upper right hand corner of the picture
causes wormy peaches, nectarines, apricots, almonds

            As peach, nectarine and apricot fruit matures and gets close to harvest, the attacks from this pest switch from soft, succulent growth to soft succulent fruit. These new attacks result in wormy peaches, nectarines and apricots. Instead of shoots which dieback, the fruit becomes infested instead.

            The trick is to get this pest under control as early in the season as possible to prevent this pest’s population from exploding in numbers. We use sprays such as Bt or Spinosad early in the season, starting now, to help keep these populations small and under control.

            I usually apply one of these chemicals when I start to see a few new shoots of peach and nectarine begin to die back. We call this flagging. We also use special traps which catch and monitor this insect when it is hatching and flying. You will see pictures of this on my blog.

Flagging from peach twig borer

            This flagging is one indicator that you need to get out there and start spraying to knock their numbers down and protect the fruit.

            Unfortunately there is not much that you can do to prevent the more damaging borer, a beetle in its mature form, from laying its eggs on the upper surfaces of sunburned branches. These insect eggs hatch and the larvae penetrate the tree through its bark causing extreme damage to the limbs by tunneling just under their surface.

            The peach twig borer adult on the other hand is a moth and its immature form is also a larva but resembling what we might call a “worm” in appearance. The BT sprays do not work on insects which are in the beetle family, only those whose adults are moths.

            Aphids are not a big problem on peach like they can be on plums and plum relatives like pluots. Aphids like cool weather and begin to disappear when it gets hot. Their damage is usually cupped or distorted leaves which are frequently sticky due to feeding by the aphids.

            Because the aphid season is short in our climate, they are usually not worth controlling unless we have a prolonged cool spring.