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Tree Borers Are Here!
Andrew Backhaus, Ph. D. Artistic Arborist Inc.
4519 N. 7th Ave, Phoenix, AZ 85013
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
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 though 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 catepillars. 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 at the pupating
adults emerge from large, quarter-sized holes in the trunks of 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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