Artistic Arborist, Inc. - Complete Tree Health Management

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Tree Borers

Types of Tree Boring Insects

There have been quite a few incidents with tree borers this year in Arizona. Most noteworthy are the large infestations of pine bark beetles which have devastated huge tracts of ponderosa pines in northern Arizona. Fortunately, these insects do not invade the hot, low desert regions of Phoenix and Tucson. However, other types of tree borers do infest trees in the low desert and they are most active in the hot summer months.  Most notable of these are the flatheaded and round headed borers that destroy desert and non-desert hardwoods and palm borers that attack date palms. These insects are difficult to eradicate and the best defense against them is to regain your tree's health so that they can "out grow" the damage that the insects cause.

Flatheaded borers (above left) are the larvae of wood boring beetles. The adult female beetles lay up to 100 eggs, singly in bark crevices where they hatch as legless white larva. They try to enter the tree by feeding through bark. If the tree is healthy and vigorous, the larva may be killed by heavy sap flow. If the tree is weak from water stress, from being planted too deeply or if the bark is damaged, the larva enters and develops quickly by eating its way through the phloem, creating long winding tunnels just under the bark. The tunnel just behind the larva fills with powderlike sawdust or frass. One larva can girdle the branch or trunk of a young tree, killing it above the girdle. The most effective treatment is to remove the infested branch containing the borer. In general, treatments with chemical insecticides are not effective in eliminating these types of borers because they do not come in contact with the chemical. The best treatment is to create a healthy environment for the tree to grow vigorously and expel the invading insect.

Roundheaded Borers

Roundheaded borers (above) are the larvae of leopard moths. The adult female moths are attracted to weakened or stressed trees. From there they send out pheromone signals that attract males which fly to females where they mate. The adults cause no feeding damage to trees because they do not have the strong, jaw like mouthparts of the larval caterpillars. The adults only live for 8-10 days. However, the hatched larva can survive for up to 2 years, eating their way into the trees and forming long tortuous tunnels under the bark and into the woody portion of the tree. Again, if trees are vigorous, these larva can be expelled by heavy sap flow. However, trees suffering from poor nutrition, soil compaction or water stress, are more likely to experience greater damage. Infested trees, such as ash, exhibit loose bark that produce large amounts of sawdust, which is the frass of the larva. Trees with heavy infestations usually cannot survive. However, if trees are vigorous and have a dense crown, it is possible to save them with a treatment of a systemic insecticide, such as Bidrin. Another important aspect for saving infested trees is to allow as much sucker growth as possible. We have observed that trees stripped of their suckers are more susceptible to roundheaded borer attack and mortality than tress with lots of healthy, vigorous suckers.

Palm Borers
 
 

Palm borers (above right) are the larvae of large beetles. These devastating grubs can live up to 10 years in a tree and cause spectacular damage to date palms. If you consider that mice live only 2 years and cat and dogs live only 10-20 years, this is a remarkable amount of time for a lowly insect to live. The adult beetles are attracted to weakened or stressed trees, usually trees that have been heavily pruned or undergone severe transplant shock. There is no treatment for trees that are infested with palm borers. They have to be taken down and destroyed, or else surrounding trees will be attacked as the pupating adults emerge from large, quarter-sized holes in the trunks of the infested palm trees. The signs of a palm borer infestation begin with the wilting and drooping of large fronds. Gradually, the heads wither and die and eventually the trunks fold up. The grubs are large and ... tasty. Yes, they are prized as a delicacy by humans through most of the world. They are said to have a taste like coconut shrimp. If a single tree in palm grove becomes infested it is important to test all of the trees by "sounding" them with a rubber or wooden mallet. This requires you to climb up the palm and strike the trunk a listen for a telltale thud which indicates grub damage, versus a "bong" type sound which indicates healthy wood.  To guard against new trees getting attacked keep them healthy and vigorous. Do not prune any green fronds and let the tree crowns grow into a globe shape. Feed heavily with fertilizers and water trees frequently, especially if they are growing in a sandy soil. Trees suffering from poor nutrition and water stress will be attacked. Chemical insecticides are not effective against palm borers.

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