The rhododendron is a commonly used landscape plant
that has a specific complex of pests. Many of these insects also cause
damage to azalea and mountain laurel. The discussion of the following
insects will focus on recognition and life cycles, damage, monitoring
and control options:
Azalea Bark Scale, Erlococcus azalene
Recognition and Life Cycle
Females are approximately 1/8-inch long and covered with a white, waxy
egg sac. Eggs are laid in the spring and hatch from late-June to mid-July.
These mobile immatures are known as "crawlers". The crawlers
like to attach to the bark and feed in crevices and branch forks. This
pest over-winters as a nymph (immature). There is one generation per year.
Damage
Scales insert their piercing-sucking mouthparts into the plant and feed
on the juices. A large population will cause leaf discoloration, some
dieback and an overall unthrifty plant appearance. These scales excrete
a sticky, sweet liquid called "honeydew" which promotes the
growth of black sooty mold fungus on contaminated surfaces.
Monitoring
Look for discoloration of foliage and signs of sooty mold to indicate
the presence of the scales. Check inner branch forks in May and June for
the white egg sacs and for reddish crawlers in late June into July.
Control
Beneficial insects often control this pest. Dormant oil spray can be applied
to kill over-wintering nymphs prior to new plant growth. Summer oil or
insecticidal soap can be used in mid-June or early July for the crawlers.
Examine the bark of host plants to determine when or if the crawlers are
active to help in the timing of any needed treatments.
Black Vine Weevil, Otiorhynchus sulcatus
Recognition and Life Cycle
Adult snout beetles are 3/8-inch long, black, with small yellow spots
on the back. All adults are females capable of laying as many as 500 eggs.
These eggs are laid in the soil. Adults are seldom seen because they feed
on the foliage at night and hide in plant debris or in the soil during
the day. The larvae are inch long, white and legless grubs. This pest
over-winters as larvae in the soil; the adults emerge in early June. There
is one generation per year.
Damage
The adults feed at night by notching the leaves. This damage is seldom
serious. The grubs or larvae can be highly destructive since they feed
on the roots from mid-summer into late fall and again in the spring. In
heavy infestations, most of the small feeder roots are destroyed; larger
roots and crowns may be girdled. As a result, there is little or no plant
growth, the foliage is often yellowed and may dry out.
Monitoring
Examine leaves in early June to detect adult leaf notching. Interior leaves
usually have the most damage. Burlap strips wrapped around the base of
the plant will provide a daytime refuge for the nocturnal adults. The
burlap can then be checked to see if adults are present. Also, pitfall
traps can be placed into the ground to collect adults. If notching and
leaf yellowing are evident, examine the soil and roots for presence of
larvae.
Control
This pest is difficult to control since it is nocturnal and every adult
is capable of reproduction. At present, control should be directed at
the adults. The Heterorhabditis bacteriophora nematode is very effective
when applied in mid-May and again in mid to late August.
European Giant Hornet, Vespa crabro
germana
 Recognition
and Life Cycle
The adults are the largest hornets in North America. They vary in length
from 3/4 to 1 1/4 inches and are dark reddish-brown with yellowish-orange
markings on the face and abdomen. Nests are located in building partitions,
tree holes or holes in the ground. The adults are unusual in that they
are active at dusk and are often attracted to porch lights at night.
Damage
The adults strip bark from the plant resulting in girdled twigs and branches.
Researchers believe they do this to obtain nest-building material or to
feed on the sap. The larvae live in the nests and do not cause plant damage.
Monitoring
Examine plants in late August into September for early signs of girdled
branches and for adults.
 Rhododendron
Borer, Synanthedon rhododendri
Recognition and Life Cycle
The adults of this pest are day-flying moths that resemble wasps or yellow
jackets. They are approximately 1/4-inch long, making them the smallest
clear-wing borer. The wings are mostly clear, and the body is black with
three yellow abdominal bands. Adults appear in late-May into June and
females lay eggs on the twigs. Upon hatching, the small grub-like caterpillars,
with white bodies and dark heads, bore into the inner bark, and later
the sapwood, of the main stems and branches. This pest over-winters in
the burrows as an inch long mature larva. There is one generation per
year.
Damage
The boring activities of the larvae cause wilted, off-color foliage, reduced
growth and dead branches.
Monitoring
Prune wilting branches and cut them open and try to locate the larvae.
Pheromone traps can be used to determine if a rhododendron borer population
is present and when it is active. Pheromones are chemicals given off by
the female to attract males in order to mate. The traps only lure males,
which do not lay eggs, and, thus, will not endanger the plants.
Control
Prune and destroy wilting branches in late summer or early spring.
Rhododendron Lace Bug, Stephanitis
rhododendri
 Recognition
and Lifecycle
Adults are about 1/8-inch long. The body is pale yellow. The lacy wings
(very distinctive) are held flat over the back and are transparent with
two dark spots present. The nymphs are black, spiny and smaller than the
adults. The eggs over-winter partially embedded in leaf tissue. The eggs
hatch in May. The nymphs mature into adults in June and lay eggs during
late June and July. The second generation of nymphs appears in August.
The over-wintering eggs will be laid when these nymphs become adults.
Damage
Lacebug adults and nymphs feed on the undersides of leaves by piercing
the leaves with their mouthparts and sucking the plant juices. This causes
a mottled, silvery or white discoloration, known as stippling, on top
of the leaf where the chlorophyll has been removed. The undersides of
leaves are covered with dark brown to black, sticky spots of excrement.
Monitoring
Beginning in May, examine plants that are in full sun for the first signs
of stippling damage. Look on the undersides of the lower, mature leaves
for nymphs, adults and black fecal spots.
Control
Plant rhododendrons in shade to maximize the activities of beneficial
insects. Horticultural oil will give control if thorough coverage is achieved
on the undersides of leaves.
Rhododendron Stem Borer, Oberea myops
Recognition and Life Cycle
Adults are dark-colored beetles that are about 5/8 inch long. They have
long antennae and are pale yellow with two black spots behind the head.
In late June to early July, female beetles lay their eggs in new shoots
several inches below the bud. The larvae emerge, bore into and down the
interior of the twig where they over-winter. The following year, the larvae
continue to bore downward where they over-winter in the roots. Adults
are present every year, but each insect takes two years to complete a
life cycle.
Damage
Adult beetles feed on the undersides of leaves along the midveins, causing
the leaves to curl. Boring by the larvae causes wilting and eventual dieback
of individual canes.
Monitoring
Examine lower branches with curled leaves for the adults and signs of
feeding in June and July. Prune and cut open wilted branches to see if
larvae are present. Also, check branches for sawdust like debris that
push out through small entrance holes in the bark.
Control
Prune and destroy wilting branches.
Two-banded Japanese Weevil, Callirhopalus
bifasciatus
 Recognition
and Life Cycle
Adults are about 3/16-inch long. They are thick bodied and brown to gray
with two darker bands across the back. The larvae are C-shaped, 1egless,
white grubs with brown heads and 1/4 inch long when mature. Eggs are laid
from mid-May throughout the summer. The white eggs are laid on the inside
of a folded leaf. When the larvae hatch, they drop to the ground and burrow
into the soil. All stages may over-winter in the soil or under plant debris.
There is one generation per year.
Damage
The adults notch leaves, causing more damage to foliage than the black
vine weevil. They prefer to feed on new and inner foliage. If there is
a large adult population, smaller plants can be defoliated. Larvae feed
on the roots and can cause discoloration, wilting and death of small plants.
Most damage, however, is caused by the adults.
Monitoring
Check lower, inner leaves for notching beginning in mid-June. The adults
are most abundant in September. Adults feed during the day but will drop
to the ground and play dead when disturbed. Place a tray under the foliage
in order to catch the dropping adults, and then shake the plant. If leaves
are notched and the plant is wilting, examine the soil around for larvae.
Rhododendron Whiteflies, Dialeurodes
chittendenl
Recognition and Life Cycle
The adults are powdery white insects about 1/16-inch long that look like
tiny moths. Clouds of adults may take to flight if an infested plant is
disturbed. Females lay eggs on the undersides of leaves, and when the
nymphs hatch, they attach to the foliage and feed for the duration of
their immature lives. They over-winter as black, flattened, oval immature
insects with a white border. There are two or more generations per year.
Damage
Nymphs remove plant juices with piercing-sucking mouth parts. Heavily
infested foliage takes on a yellow, mottled appearance. The immatures
also produce honeydew which may lead to the development of black sooty
mold fungus growth. This discolors the plant and can reach levels that
interfere with photosynthesis.
Monitoring
Look for early signs of honeydew, sooty mold or leaf yellowing. Then check
the undersides of leaves, particularly the tender terminal leaves, for
the nymphs and adults. Viewing immatures may require careful observation.
Control
This pest can be difficult to control because all of the life stages are
usually present at the same time, and the eggs and pupae are not susceptible
to most insecticides. Thus, a single insecticide treatment will not control
an infestation. Horticultural oil can be used for all stages if thorough
coverage is achieved on the undersides of the leaves. Rhododendron varieties
that have thick, leathery leaves often escape infestation.
INSECT-RELATED PESTS
Southern Red Mite, Oligonychus
illcis
Recognition and Life Cycle
Adult mites are very small, about the size of a period. They are oval
shaped, reddish colored, with eight legs. The immatures appear the same,
only a little smaller and with six legs. The red eggs over-winter on the
undersides of leaves. There are several generations per year. Most activity
takes place during the cooler weather of spring and fall. During the heat
of summer, the life cycle is in the egg stage.
Leaf damage is visible as white stippling damage on both sides of the
leaf. In a heavy infestation, the leaves will turn brown, die and may
drop off the plant. The undersides of the leaves are often coated with
hatched egg shells and shed skins from molted mites.
Monitoring
Look for stippling and mites on the lower leaf surface from April through
June and again from September through November. An efficient and easy
way to detect mites is to tap foliage over a white piece of paper to dislodge
the mites.
Control
When tapping the foliage onto paper, look for the extremely small, light-colored
beneficial mites which move quicker than the southern red mites. They
may be able to control the pest population. Dormant oil can be used if
there is a large number of over-wintering eggs. Summer oil formulations
can be used in the summer
For pesticide recommendations call the UConn Home
and Garden Education Center at 877-486-6271.
References
Childs, R.D. and M. Castonguay. 1994. Insect and Mite Pests of Shade Trees
and Woody Ornamentals. In New England Management Recommendations for Insects,
Diseases, and Weeds of Shade Trees and Woody Ornamentals. Cooperative
Extension System. University of Massachusetts, Amherst.
Davidson, J.A. and M.J. Raupp. 1994. Landscape IPM: Guidelines for Integrated
Pest Management of Insect and Mite Pests on Landscape Trees and Shrubs.
Bulletin 350. Cooperative Extension Service. University of Maryland, College
Park.
Johnson, W.T. and H.H. Lyon. 1988. Insects that Feed on Trees and Shrubs,
2nd. Ed. Cornell University Press, Ithaca.
Johnson, W.T. 1994. Insect Pest Management on Trees and Shrubs. In Pest
Management Recommendations for Commercial Production and Maintenance of
Trees and Shrubs. Cornell Cooperative Extension. Cornell University, Ithaca.
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