Educational
Resource Guide #302 
Borers of Shade Trees & Ornamental Plants
compiled by Mary Jane Frogge, Horticulture Extension Associate
Nearly
all shade trees are subject to borer attack. If damage is severe,
young trees are likely to decline or die. The important borers
are worm-like when immature and beetles or moths in the adult
stage. Roundheaded and flatheaded borers are grubs of beetles.
Lilac borers, ash borers, and carpenterworms are caterpillars
of moths. Life cycles and habits vary with the type of borer.
Borer
damage must be prevented because once borers gain access to
the cambium, sapwood, or heartwood, little can be done to control
them. Several cultural practices will help reduce borer infestations.
Trees
in vigorous growing condition are not especially attractive
to borer attack. Trees should be properly watered, fertilized,
and protected from pests, particularly during the first two
or three years of growth following planting and during drought
periods that cause extreme stress.
Plant
trees that are adapted to local conditions and are resistant
to borer attack, such as oaks, lindens, crabapples, and conifers.
Ash, birch, lilac, cottonwood, locust, and flowering stone fruits
are more susceptible.
Roundheaded
Borers
 Redheaded
Ash Borer: This roundheaded borer attacks several species
of shade trees, but causes the most serious damage to green
ash. The adult is a long-horned beetle that is 1/2 to 1 inch
long and reddish brown to black with transverse white or yellow
stripes on the wing covers. The beetles are attracted to weakened
trees where they deposit eggs in cracks in the bark. The newly
hatched larvae (caterpillars) initially feed under the bark
and later tunnel into the sapwood. The redheaded ash borer generally
has a one year life cycle. The larvae feed during the summer
and pupate in the fall. Adult beetles emerge in the spring and
deposit eggs from May until August. For this reason ash trees
must be protected from early spring until autumn.
 Cottonwood
Borer: This borer infests the bases of cottonwood and willow
trees. The adults of this long-horned beetle are 1 to 1-3/8
inch long and black with numerous patches and transverse white
stripes. Adult beetles emerge in late spring and early summer
and feed on tender new shoots of young trees. They deposit eggs
in openings chewed into the bark at the bases of trees below
the soil line. The larvae burrow into the bases and roots of
trees, pushing out frass, a sawdust-like excrement, at the entry
points. Severely infested young trees may be badly damaged.
Larger trees tolerate light to moderate infestations without
apparent serious effects. They have a two-year life cycle.
 Poplar
Borer: This roundheaded borer is a long-horned beetle that
attacks cottonwood, poplar, and willow trees. The adults are
approximately 1 inch long and are dark grey with small orange
spots on the wing covers. They emerge in summer and lay eggs
in slits cut in bark, usually near the middle portion of trees.
Larvae, which are white and about 1-1/4 inch long, bore into
the heartwood. The larvae take three years to mature. Damage
appears as swollen areas on trunks and larger branches. Holes
where larval excrement is pushed out and where adults have emerged
are also signs of an infestation.
Flatheaded
Borers
 Bronze
Birch Borer: This flatheaded borer occurs in most species
of birch grown in Nebraska. Adult beetles are about 3/8 inch
long, slender, and metallic bronze. Adult beetles emerge from
infested trees in late May and are present until July. They
feed on birch leaves for several days before laying eggs. Eggs
are deposited under bark and in cracks in the bark. The larvae
burrow directly through the bark into the cambium layer. Heavy
infestations cause raised burrows that can be detected on the
bark surface. Adults prefer weakened trees for egg laying. Healthy,
vigorous trees usually are less likely to be infested. Cultural
practices are very important in the prevention of bronze birch
borer injury. Birches should be planted where they are shaded
in the afternoon — avoid southern or western exposures. They
are best suited for shaded, damp situations and should be water
regularly. Plant a ground cover over the root area to keep roots
cool and moist. If ground covers are not feasible, do not mow
grass over the root area during summer. Most birches are not
well adapted to Nebraska and are a relatively poor choice for
an ornamental.
Caterpillars
Lilac
and Privet Borers: Adults are day-flying, clear-winged moths
that resemble wasps. They emerge from infested stems in May
and June. After mating, the adult females deposit eggs on the
lower portions of the main stems of lilac or privet. The caterpillars
bore into and feed within stems. Feeding may cause the leaves
to yellow and wilt. Frass is produced by borers and pushed out
from their burrows. Infested canes are scarred, unsightly, and
may eventually die. Severely infested stems should be cut at
the soil level and destroyed in early spring. The borer spends
winter in the pupa stage and there is only one generation each
year.
Ash
Borer: The ash borer is similar in appearance to the lilac
borer. The adult is also a day-flying clear-winged moth that
resembles a wasp. Adults emerge in August or September and deposit
eggs on the bark on the lower trunks of ash trees. The caterpillars
that hatch from the eggs then bore through the bark and feed
within the branches and trunks of the trees. There is one generation
each year.
 Carpenterworm:
Cottonwood and ash are the preferred hosts, but this insect
will attack many of other shade and fruit trees, and shrubs.
Adult moths are active from June through July. Female moths
deposit their eggs on the bark of trees, usually on the lower
trunk. After hatching from the eggs, young carpenterworms tunnel
directly into the inner bark and later bore into the heartwood.
Heavily infested trees are structurally weakened and may be
broken during high winds. The caterpillars require more than
one year to complete their feeding and may be up to 2 inches
long at maturity.
Shade
Tree and Ornamental Plant Borers
Insecticide
Recommendations
The
following recommendations should help reduce borer damage.
Redheaded
Ash Borer: Spray trunks and large branches with lindane
20% EC. Apply the first spray in late April and respray every
three to four weeks until early August. It is not necessary
to spray small branches and leaves.
Cottonwood
Borer: Spray lower trunk with lindane 20% EC in late May
and late June. Borers inside trees may sometimes be killed by
injecting special borer control pastes containing malathion
or lindane into burrow openings where frass is evident.
Poplar
Borer: Spray lower trunk of cottonwood trees with lindane
20% EC in late May and again in late June.
Bronze
Birch Borer: Spray entire tree with lindane 20% EC in late
May and repeat in two to three weeks.
Lilac
and Privet Borers: Spray with lindane 20% EC in early May
and repeat twice in mid-May and early June. Wet lower portions
of stems to point of run-off. Borers inside canes sometimes
may be killed by injecting special borer pastes containing malathion
or lindane, by plugging the openings, or by probing the openings
with a stiff wire.
Ash
Borer: Spray lower trunks with lindane 20% EC, with the
first application in early August and the second in early September.
It is unnecessary to spray branches and leaves. Borers inside
trees may sometimes be killed by injecting special borer pastes
containing malathion or lindane, by plugging the openings, or
by probing the openings with a stiff wire.
Carpenterworm:
Spray the lower ten feet of trunk with lindane 20% EC in late
May and late June. Borers inside trees may sometimes be killed
by injecting special borer pastes containing malathion or lindane,
by plugging the openings, or by probing the openings with a
stiff wire.
(This
resource was added to the site on January 9, 2003)
|
