Pecan Webworms, Honeydew and Cotton
Those webs in your pecan trees that keep getting progressively
larger are made by the caterpillars of a moth that we know
as the fall webworm. How did they get there? Will they harm
the tree? Can we control them? Will aluminum foil or metal
bands around the tree trunk keep our trees web-free? These
are frequently asked questions from homeowners and they deserve
some answers. Let's consider each question individually.
1. How do webworms get into a tree? The fall webworm spends
the winter as a pupa inside a net-like cocoon. These cocoons
are formed in leaf trash around the base of the tree or hidden
in bark crevices on the trunk. White moths emerge from these
cocoons during the spring months and fly to the foliage of
pecan, cottonwood, mulberry, and fruit trees where they deposit
an egg mass on the leaves. The eggs hatch within 7 to 14 days,
producing the caterpillars that busily begin construction
of their web while at the same time, devouring leaves. As
the caterpillars (webworms) grow, the web is made larger to
surround more leaves that may be eaten. The caterpillar stage
requires 4 to 5 weeks before maturity is reached. When full
grown, they leave the web for the first time and crawl to
a sheltered area to form their cocoon. Moths emerge from these
cocoons in mid- to late summer and the cycle is repeated once
2. Will webworms harm your trees? Fall webworms can harm
trees if they devour most of the foliage. The tree must use
reverse or stored food to maintain life, thus lowering the
overall vigor of the trees. Trees weakened by defoliation
are more susceptible to damage by diseases and other insects.
3. Can webworms be controlled? The answer to this question
is simply, “yes, we can”. Two approved methods
for controlling webworms are outlined below:
A. Small webs can be pruned from the trees and destroyed.
This method is the most obvious, but effective only if the
webs are within reach.
B. Insecticide sprays containing Carbaryl (Sevin), Malathion,
or Bacillus thuringiensis are highly effective if used as
4. Will aluminum foil and other metal bands around a tree
trunk keep webworms out of a tree? Fall webworm moths are
accomplished fliers and can easily fly from tree to tree where
the egg masses are deposited on the leaves. One can easily
see that metal bands will not aid in keeping our trees web-free.
Another problem is honeydew from pecan trees. When honeydew
from a pecan tree begins to fall, you quickly realize that
there are insects above doing naughty, unmentionable things.
"Honeydew" is a classy word meaning "a sweet
substance secreted by aphids and other juice-sucking insects."
"Secrete" means to be "excreted as a waste"
and the last thing that I will tolerate is to be excreted
upon by aphids –I don't care if the secretion does have
a fancy sounding name like honeydew!
Aphids build to high populations in mid-summer. Although
the actual severity of damage from this pest is widely debated,
the normal level of leaf performance can be impaired. The
primary mode of damage is from leaf shading by a mold that
grows over the upper leaf surface of affected leaves. As the
aphid feeds on the undersides of leaves, the pest excretes
"honeydew" or sugar water. This sticky liquid falls
down through the tree and is deposited on the upper leaf surface
of leaves over which the aphids fed. This "honeydew"
is a perfect substrate for "sooty mold" fungus.
Under high humidity, the fungus mold grows on the "honeydew"
to form a dark gray to black covering over the upper leaf
surface. This effectively blocks the sunlight from the leaf,
Unfortunately, pesticides do not reduce the numbers significantly,
so the best thing to do is to ignore these devils and hope
that beneficial insects decrease the aphid population as soon
For more information about webworms, see:
For pictures, see:
Cotton Root Rot
You live in the greatest state in the Union. What can you
expect down here in Texas? Fresh watermelon! Large tomatoes
and sweet corn! Cotton root rot!
Cotton root rot? That is not a very pleasant topic to discuss
when considering Texas expectations. It may not be a pleasant
topic, but if you live in this area, sooner or later you will
encounter this killer.
Cotton root rot, caused by the fungus Phymatotrichum omnivorum
root rot, Texas root rot and Ozonium root rot. It is one of
the most destructive plant diseases and attacks more than
2,000 species. However, either the fungus infects but does
not kill monocotyledonous plants (grasses, etc.) or these
plants are all highly resistant. The fungus is prevalent in
calcareous clay loam soils with a pH range of 7.0 to 8.5 in
areas with high summer temperatures. Therefore, the disease
is limited to the southwestern United States.
Cotton root rot has been reported in Texas counties from
the Red River to the Rio Grande and from Tom Green County
to the Neches River. It has been a problem since the first
pioneers came to Texas. In fact, the Texas A&M plant pathology
department was founded to find an answer to the root rot problem.
We are not much closer to finding the answer today than we
Disease symptoms are most likely to occur from June through
September when soil temperatures reach 82 degrees F. The first
symptoms of the disease are slight yellowing or bronzing of
leaves followed by wilting. Plants die suddenly after the
first symptoms of wilting. Leaves remain firmly attached to
the plant. Affected plants die suddenly, often after excellent
growth. Large trees and shrubs may die more slowly.
Usually, the fungus has done extensive damage to the roots
by the time plants have wilted. When roots are pulled from
the soil, root bark is decayed and brownish, and wooly strands
of the fungus frequently are apparent on the root surface.
Affected plants can be pulled from the soil with little effort.
Under moist conditions, spore mats sometimes appear on the
soil surface. These mats, 2 to 16 inches in diameter, are
first snow?white and cottony, then later, tan and powdery.
On large roots and tubers, there are numerous small cushion-like
sclerotia, or resting bodies, about the size of a pinhead.
At first, they are light tan but later appear dark and warty.
The fungus generally invades new areas by a continual slow
growth through the soil from plant to plant. Occasionally,
it spreads more rapidly on the roots of infected transplanted
plants. The fungus can survive in the soil for many years,
and often it is found as deep in the soil as roots penetrate.
Affected areas often appear as circular areas of dead plants
in fields of infected crops. These areas gradually enlarge
in subsequent years as the fungus grows through the soil from
plant to plant. Infested areas may increase 5 to 30 feet per
Cotton root rot is one of the most difficult plant diseases
to control. Fungal behavior in different crops and soils,
and its activity from year to year in the same area, are so
erratic that it is ineffective to rely on one approach. Development
of resistant plants using conventional breeding concepts,
has been difficult due to the pathogen's wide host range.
Also, in ornamental plants, we have to depend on observations
that are not always dependable. For example, some plants that
seem immune in the native state may become susceptible to
root rot under the continual moisture found around homes.
However, a safe index is the behavior in nature under the
same conditions that you have where you intend to plant them.
However, there is a list of woody and herbaceous plants
which shows resistance or tolerance to cotton root rot and
should be considered by the homeowner where the disease is
There are no confirmed varieties of peaches, apples or pears
that are resistant to root rot. Grapes such as Thompson Seedless
and the French?American hybrids are very susceptible. Many
susceptible grapes can be grown on native vines such as Dog
Ridge and La Pryor and resistant varieties such as Champanel
and Black Spanish (Le Noir). The wild mustang is immune.
So if you have had plants die of cotton root rot and you
didn't know why, now you know. Don't feel like the Lone Ranger.
You are not the only one who has had the experience, nor will
you be the last.