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Article Title Date Posted
Webworms - The Summer Pest (12-02)
Springtime Caterpillars (09-02)
Coming Soon...  
Crape Myrtle (09-02)
Adverse Effects of Drought (12-02)
Trees as Pollution Sources? (09-02)
Over-watering (09-02)

Webworms - The Summer Pest
Tree Clinic Quarterly, July, 2002
By Jim Houser

Webworms, often called fall webworms, though they start appearing in late May to early June, are the larval stage of the tiger moth, Hyphantria cunea. The developing caterpillars build large webs, often enclosing an entire limb of foliage, and feed within the web. There are two distinct races, one larva has a black head the other a red head. The former appear one month earlier than those of the red- headed race. The female moths of the black-headed race deposit eggs in mid-March. The larva pass through as many as eleven stages of development. In each stage feeding occurs with a distinct web made of silk produced by the larva. Webworms feed on more than 100 fruit, shade, and forest trees. They are mostly seen on pecan trees in Austin.

Two foliar applications of insecticides usually achieve adequate control. It is possible a third may be required this year due to the extensive early populations and ideal climatological conditions. Synthetic chemicals can be injected into the tree to achieve all season control, though a few webs may still appear.


Springtime Caterpillars

Tree Clinic Quarterly,1st Qtr. 2002
By Jim Houser

Spring nears and soon our trees will be putting forth their bright green leaves. Along with these new leaves, numerous species of caterpillars will hatch out of over-wintering egg masses. Their hatch is timed with the springtime leaf expansion of the tree in order for the caterpillars to feast on the highly palatable leaves. As the growing season progresses, plants produce and move defensive chemicals into their leaves that make them "taste bad" or indigestible. So, as the new leaves emerge, the caterpillars are there to gobble them up.

Loopers, cankerworms, inchworms, and tent caterpillars: all are moth larvae that voraciously feed on the foliage of live oaks and Spanish oaks. Since the caterpillars are so small and spend their time within the canopy of the tree, you initially don't know they are there. The silks or threads that they spin may be encountered as you walk under the trees but you may think it's a spider web. Most people notice too late that their trees are thin or bare after the damage is done.

Caterpillars hatch out in March each year and can severely stress our oaks by defoliation. Often, the oaks take 4 to 6 weeks to put on new leaves. This takes a double toll on the tree by depleting food storage reserves and by not having leaves out to photosynthesize to produce energy for use this year and reserves for next year. This is especially critical for trees that are declining due to problems such as drought stress, over-watering, Phytophthora root rot, construction damage, bacterial leaf scorch, oak wilt, borers, etc. Caterpillar defoliation in spring contributes synergistically to the decline spiral in many of our oaks. Trees defoliated by caterpillars are also more susceptible to secondary attacks by other insects and diseases.

Control by spraying for these pests is simple and very cost effective. We recommend that all homeowners and property managers with oak trees control spring caterpillars each year. It is a proven maintenance activity for the oaks that provides measurable benefits in improved health and vigor.


Ask the Experts: Pruning Crape Myrtles
Tree Clinic Quarterly, 4th Qtr. 2001

By Jim Houser and Jerry Pulley

All pruning activities on plants should be done with an objective in mind. The yearly topping of crape myrtles serves no purpose. Some common alibis for topping include:

"I've always done it that way" or "My neighbors pruned theirs this way."
Topping is improper pruning. Copycat activity is the way it is promulgated.

"It will flower better, or more."
No, it won't. Topping will not promote more flowering. It removes the apical meristem, the point where new cells are added, as well as the hormones that keep lower buds in check. It may even increase the crape myrtle's tendency to send up sucker shoots from their base.

"It's getting too tall."
If a tree or bush is getting too tall, then perhaps it's the wrong variety for its location. Different crape myrtle varieties have different mature plant heights.

Crape myrtles are prized for their vibrant flowers in the middle of a hot summer and their naked, sinewy trunks in the winter. Crape myrtles do need pruning, but if done correctly, it generally goes unnoticed. Remove sucker sprouts from the base as soon as they appear unless you want more trunks; if that is the case, select one of more to leave and remove the rest. Prune to train for form that enhances the natural growth habit of the plant. By doing this, you create a form that is beautiful and easy to maintain, and one that flowers just as much.


Adverse Effects of Drought
Tree Clinic Quarterly, July, 2002

By Jim Houser

Recent rains were welcome news indeed, but the fact remains we are in the midst of a drought. In Texas, drought is a climatological fact of life. Relief comes at intervals but extended dry conditions prevail intermittently. Drought is only broken when adequate reserves of subsoil moisture continually occur.

Effects of drought are both short and long term. Lack of moisture causes leaf wilting, followed by variable leaf browning, leaf drop, and even plant death. Below ground, non-woody root (fine root) dieback occurs. This leads to a reduction in nutrient and water uptake and reverse osmotic pressure in the root causing a severe lowering of the plantís total moisture content. This has happened to the trees in the Rockies where wildfires are raging. It has been said that the moisture content of standing trees is lower than kiln-dried lumber!

Secondary and longer term effects to the plant are a lowering of resistance to insects and disease such as aphids, spider mites, scale, borers, root rot, and canker disease. Since photosynthesis is inhibited by moisture stress, present and future shoot growth and flowering and seed production are reduced. With low starch and energy reserves, next yearís root growth may be limited also.

What can be done to limit damage by drought? Obviously providing water to the root zone in a judicious manner is the key. This water should percolate to the subsoil. With adequate water present, nutrient amendments provided by slow release fertilizer sources will regenerate roots and restore the plant. Additions of mycorrhizae amendments will help provide more root surface area for uptake. Control of secondary pests is key in getting the plant through drought conditions. Targeted sprays and systemic injections of insecticides and fungicides are essential in keeping the plant from being overwhelmed by opportunistic invaders.

Since drought is a fact of life in Texas, people who desire thriving plants in their landscape must take an active role in countering its adverse effects.


Trees as Pollution Sources?
Tree Clinic Quarterly, 1st Qtr., 2002

By Jim Houser

I can remember shaking my head and laughing when he said it. There was President Ronald Reagan on TV saying that trees can pollute! I was working for the U.S. Forest Service at the time, and the other foresters and I held forth on our Chief at a watering hole in Colorado. But here I am now, saying, "Mr. President, you were right!"

Trees and other plants emit naturally formed volatile organic compounds (VOCs) that contribute to the formation of photochemical smog. "Plants release these compounds into the atmosphere in large quantities. These volatile compounds add to the smog in the same way as emissions from human sources. There is no discrimination," says Peter Nelson, a research scientist at Australia's Commonwealth Scientific and Industry Research Organization (CSIRO). Tests by CSIRO measured VOC emissions from trees by enclosing portions of trees in Teflon film bags, or Teflon film chambers and analyzed the air trapped within the bag for concentrations of carbon dioxide, nitric oxide, nitrogen dioxide, and non-methane volatile hydrocarbons. CSIRO says results of the tests show that "plants release highly reactive hydrocarbons that can add significantly to photochemical smog problems. That is, smog caused by the reaction of sunlight with chemical compounds, like those from industry and car exhausts."

Plants differ in their emission rates from one species to another. Some species can release as much as 10,000 times more VOCs than other low emitters. To help reduce potential ozone concentrations, the California Air Resources Board (ARB) recommends considering the potential emissions of each species before planting. According to the ARB, some low emission tree species include elm, ash, peach, redbud, and hackberry. High emitters include live oak, red oak, sycamore, and sweetgum.

What is our view on these interesting findings? Well, in the grand scheme of things it doesn't seem to matter much. The information certainly sounds provocative, but data on total amounts from plants vs. man-made sources are notoriously lacking. We say plant and tend the trees, ornamentals, and turf that you want to! If you want to consider species emissions, then do so: Cal Poly has a website that has information for tree species.

It was good to know that President Reagan was right on this one. Now, what did he say about waste of taxpayer dollars by the government?


Tree Clinic Quarterly
3rd Quarter, 2001
Ask the Experts

Over-watering is the biggest problem we see in caring for trees. Irrigation systems are very efficient in putting out a lot of water in a short period of time. A good rule of thumb is when you irrigate, play like it rained! If you think of it that way, if your lawn is irrigated every day that means you're living in a rainforest! During the summer, watering twice to tree times a week during the hottest periods is sufficient. As we go into the fall, adjust to once a week. No irrigation is usually necessary during winter unless we are in a drought. In spring, go back up to once or twice a week. If it rains ½ inch or more, consider that a watering day and decrease applications accordingly.


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