Ask the garden sage

Ask the Garden Sage: Beetle is damaging palo verde tree

2014-07-06T00:00:00Z Ask the Garden Sage: Beetle is damaging palo verde treeBy Peter Warren Special to the Arizona Daily Star Arizona Daily Star

I have a palo verde tree in my front yard and recently I noticed a large number of pencil size branches have been breaking off the tree. When I examine the branches I see a hole in the side near the base, and the stem is hollow at the point of attachment. What could be causing this and what should I do about it?

A: A beetle in the Bostrichidae family called Amphicerus simplex causes the damage. It has no official common name so I am calling it the palo verde stem borer since that is what it does. These beetles are known for their boring activity, so much so that we commonly call the family of beetles to which it belongs the branch and twig borers and powderpost beetles.

This is a common beetle in the desert areas of Mexico, New Mexico, Texas and Arizona. Research from the 1950s lists three hosts: little leaf horsebean (aka yellow or foothill palo verde), blue palo verde and mistletoe. There is not much more info than that about this insect. Since it is a native species, the lack of information usually indicates it is of little concern. It would be interesting to know if these beetles are a widespread problem every year that goes underreported or if they are simply a sporadic problem.

I hear about a few of these cases each year so I will be interested to see if there are more. I would not recommend treating the tree unless it turns out to be a problem every year. A little pruning is good. If we can train the beetles to bore into the right branches it will be all good.

Sometimes I see tumbleweed rolling across the road when I am driving out in the county. Why don’t I see this weed in the city?

A: Tumbleweed (aka Russian thistle) is a summer annual weed native to southeastern Russia and it is very well adapted to desert conditions. It is a species that takes advantage of recently disturbed soil. It doesn’t need much water to germinate and it has a long taproot to access water as it grows. Unlike many weeds, the seed does not last long in the soil so managing young plants before they produce seed is usually enough to keep it under control.

Typically we see tumbleweed growing in roadside ditches, near agricultural fields and in recently cleared land for construction. Well-managed landscapes seldom have a problem because they are not left bare for long and many people manage their weeds. Also these plants spread seed by tumbling and so having some nearby and room to roll makes for a problem that persists unless they are managed. At our Campus Agricultural Center in Tucson, the farm supervisor, Ken Kriederman, says they look for it and treat it when the plant is small so it won’t get out of hand. When it is young the plant is dark green and succulent and barely resembles the dead brown tumbling plant that has become iconic of the Southwest.

I have a mature Aleppo pine tree that is showing some browning on the needles. Could this be a disease? What should I do about it?

A: The Aleppo pine is a heat tolerant species from the Mediterranean region that is commonly used in our area. We often see needles with yellowing and browning and there are few potential causes. The first is a reaction to the summer heat and this is a normal condition that the trees outgrow once cooler temperatures occur. Secondly, this tree, like many mature trees, requires regular watering. In the spring, summer, and fall, every two to three weeks deep watering to 24 to 36 inches is best. This can be reduced to once a month in winter. Watering a big tree can be challenging because the root zone is also big and it is important to place emitters near the drip line of the tree to be most effective. Thirdly, there are mites and aphids that sometimes infest these trees and can cause browning of needles. The mites and aphids can be seen with a microscope and are a good thing to rule out. You can bring branch samples to the Cooperative Extension office for help with the diagnosis.

Peter Warren is the urban horticulture agent for the Pima County Cooperative Extension at the University of Arizona. He works with landscape professionals, urban farmers and homeowners to promote integrated pest management and best management practices for desert horticulture in Southern Arizona. Questions can be sent to plwarren@cals.arizona.edu

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