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Information
Center -
Tree Problems
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.
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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.
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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.
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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 plant’s 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 year’s 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.
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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?
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Overwatering
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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