ORNAMENTALSAND
TURF
ORNAMENTALS
The information and recommendations in this newsletter are applicable to North Carolina and may not apply in other areas.
Judging from inquiries and observations, this may be a good year
for fall webworms. Because fall webworm damage accrues over the
summer, fall webworms usually cause little long term health damage
to the trees they defoliate unless the trees are completely
defoliated year after year. At any one location, the populations
of fall webworms wax and wane so that they are conspicuous and
damaging for a year or two and then the populations seem to
disappear. Fall webworms occur throughout North America, Japan and
Korea. Accidentally introduced into Europe in 1946, there it is
considered to be a much greater pest than the gypsy moth. Fall
webworms feed on over 600 kinds of trees, shrubs and other plants.
In North Carolina they seem to prefer pecan, persimmon, sourwood
and willow. The webs become filled with cast skins, droppings and
dead leaves. The web is enlarged to encompass fresh, green leaves
until the web may become two to three feet long. Small trees
infested with several broods of caterpillars may be entirely
enclosed in webs. After feeding for four or five weeks, the
caterpillars crawl down, spin cocoons and pupate in mulch or soil.
In July and August another generation of moths emerges from the
cocoons to continue the life cycle. There are at least two or
three generations each year in North Carolina depending upon how
early or late in the spring the first moths emerge. They
overwinter as pupae in cocoons in the litter. White fall webworm
moths (some moths have small black spots) emerge to mate and lay
350 to 900 eggs on the lower leaf surface. The hairy caterpillars
spin the webs as they feed. Fall webworms can be destroyed by
pulling down the webs and destroying the caterpillars if the webs
are in reach of a pole. If the webs are within reach of a hose-end
sprayer, several insecticides can be sprayed for control.
Insecticides work best when the caterpillars are young. Thus it is
best to treat as soon as the webs are first noticed. If the trees
are too tall for equipment used by the amateur horticulturist, many
professional landscapers and arborists supply tree spraying as a
part of their services. Bacillus thuringiensis, bendiocarb,
Dursban, diazinon, Orthene, Dibrom, Imidan, and Talstar are
specifically labeled for fall webworm control. Ornamentals and
Turf Insect Note No. 46 has additional information on the biology
and control of fall webworms. Copies of this note are available in the county Extension centers in North Carolina.
The caterpillars of Atomacera decepta feed on hibiscus, rose of
Sharon, hollyhock, mallow and perhaps other members of the cotton
family. This insect could be called the hibiscus sawfly as
hibiscus is the most frequently reported host plant. The adults
are small (3/16 inch), black fly-like insects in the family
Argidae. The thorax is reddish brown on top. The argid sawflies
are interesting because their antennae a split almost at the base
so they appear to have four antennae instead of two! There has
been no detailed study of the life history of the hibiscus sawfly,
but the caterpillars have been found from May to October. It is
speculated there are up to six generations per year. The pale
green worms have black heads and tiny black spines on each body
segment. They are slightly gregarious with up to three larvae
feeding on one leaf. When mature the larvae spin a tough silken
cocoon on the base of the plant or nearby. From that emerges a new
adult to mate and lay eggs. The eggs are inserted into the leaf
tissue one at a time. This insect should not be particularly
resistant to pesticides. Sevin should give adequate control. Be sure not to spray open
blossoms which may kill pollinators.
Bagworms are fascinating insects because in the literature they are
reported to have a very wide host range. However, in landscapes
they are usually
associated with arborvitae, cedar or juniper. It is thought that
the original host plant of the bagworm is black locust and from
that it has spread to numerous other plants. The newly-hatched
larvae spin down on silken threads and are blown about by the early
spring breezes. Upon reaching a suitable host, the worm begins to
spin its bag and as it grows, it incorporates some of the host
plant foliage into the bag for camouflage. Apparently when the
larva reaches a plant which is different from its parents' host
plant, the insect has a hard time adapting to it and may die or
produce only a few offspring. After several years of struggling to
keep from going extinct, the population may hit on the right
combination of genes for the new plant and soon the new plant is
covered with bagworms. I think one reason that arborvitae,
junipers and Leyland cypress are often damaged by bagworms in the
landscape is that they are easy to propagate asexually so that
there are lots of shrubs which are genetically identical. Once a
bagworm population adjusts to a variety, then it can sweep through
the neighborhood and develop successfully all the plants of that
variety. Bacillus thuringiensis, Sevin, diazinon, malathion and
Orthene work well for bagworm control early in the season, but at
this time of year the worms are large enough they can close their
bags to avoid these pesticides. It will probably take multiple
applications of the previously-named pesticides to get control now.
Landscapers should consider pyrethroids such as Talstar, Decathlon
or Mavrik as the pyrethroids last much longer in the environment.
Ornamentals and Turf Insect Note No 81 provides additional
information on the biology and control of bagworms.
Web page last updated on July 7, 1997 by Stephen J. Toth, Jr..
