black vine weevil is a pest of over 100 landscape plants. The adults
feed on a wide variety of evergreen, deciduous, and herbaceous plants.
The black vine weevil is also destructive in the larval form on
yew (Taxus spp.), hemlock, rhododendron, and several other evergreens.
It will sometimes feed on strawberry or impatiens. The black vine
weevil is a pest in both Europe and North America. It is found predominantly
in the northern portions of the United States, but its range extends
vine weevil adults are black weevils with short, broad snouts. The
weevils are approximately 6 mm (1/4 inch) long. The head is narrow,
the thorax is medium and rounded, and the wing covers are broad
and well rounded. The wing covers have fine yellow hairs and conspicuous
corrugations which appear as lines down the back. Adults cannot
fly; their wing covers are fused together. All of the weevils are
female; they reproduce parthenogenetically (by development of an
unfertilized egg). Since the weevils do not fly, they disperse chiefly
by walking, although they may be transported by man along with plants.
The weevils feed at night and hide under leaf litter or in the soil
during the day. When disturbed, the adults feign death. The larvae
have a wrinkled c-shaped appearance. They are legless and white,
with a well-developed brown head.
black vine weevil and damage to leaf (R.A. Casagrande)
of the black vine weevil life cycle (R. Harrison)
Overwintering larvae feed on roots deep in the soil and pupate in
May. Adults dig their way out of the ground in mid-June and crawl
up the plants to feed. Feeding occurs mostly at night and adults
hide in dark places on the plants or on the ground during the day.
When disturbed, they quickly drop the ground. After one to two weeks
of feeding, adults crawl or drop to the ground to lay eggs. For
several weeks, they alternately feed and lay eggs. Eggs are dropped
indiscriminately to the ground under the plants (as many as 500
per female!) over her entire life. If houseplants are placed under
shrubs during the summer, eggs may be deposited on the soil of the
containers. Ten to 14 days later the grubs hatch and burrow down
into the soil searching out roots to feed on. Occasionally, a few
adults can be found in houses during winter, apparently able to
overwinter in this stage. Most die in the fall, and larvae survive
the winter in the soil. There is only one generation per year.
damage by the black vine weevil is the most severe. The larvae feed
on the roots and underground stems, sometimes girdling the root
crown. Larvae even feed on roots of houseplants that spend the summer
out of doors near infested plants. Symptoms include stunting and
yellowing or off-color foliage. Root feeding often results in death
of the plant. Plants may fail to put out new growth in the spring
as a result of the root injury, or may put out the first flush of
new growth, and subsequently die. Unfortunately, home gardeners
often first notice the problem when plants are near death. Adults
feed from the outer margin of the leaf inward, creating characteristic
notches, and these notches can be used as an early indicator of
potential larvae in the soil. Adults cut notches on the margins
only, they never create holes on the center of the leaf. On yew,
needles nearer to the main trunk, at the base of the shrub, will
show notching and feeding scars. While the foliar damage from the
adult weevils is often not severe, it can be unsightly.
Look for symptoms of feeding injury by adults. Injury (notches in
leaves) will be easy to see on broad leaved evergreens (i.e. rhododendron),
but may be harder to find on narrow-leaved plants (such as Taxus
spp.). Weevils may be feeding on leaf edges, especially near the
trunk. Use a flashlight at night (adults are nocturnal) during early
to mid-June to inspect plants for the presence of weevils. Or use
a burlap cloth gathered in folds around the base of plants which
provides a hiding place for adults during the daytime; gently unfold
to see if adults are present. Ideally you want to detect the start
of adult emergence activity so that you can accurately time a spray
for adults when most have emerged, but before egg laying begins.
removal of weevils can be effective, especially in small plantings
or for individual plants. Where possible, quarantine plants with
leaf notches. Some rhododendrons and azaleas show resistance to
weevil feeding. Those that have a rolled edge or indumentum (fuzzy
leaf undersides) may be difficult for weevils to feed upon. Insect
parasitic nematodes in the genus Heterorhaditis are a promising
biological control for larvae if certain conditions are met. Nematodes
require adequate soil moisture and temperatures of 60 degrees F
(or higher) in the top 5 inches of soil. Nematodes, available through
garden supply cataloges, need to be applied when the larvae are
present in mid to late summer. They also have been recommended,
especially in warm greenhouse container culture, for root weevil
control. If houseplant roots are found to be infested with larvae,
discarding the plant may be the best practice. Washing off the roots
and repotting might be an alternative, but success will depend on
the amount of damage already done.
Except for protecting young plants, chemical control is not
generally needed by homeowners. When it is required, homeowner treatments
are best directed against adults during mid-June to kill new adults
before they start laying eggs. Orthene can be applied at that time
with a follow-up treatment 3 weeks later. Insecticide applications
should be made late in the day (weevils are nocturnal feeders).
Be sure to follow the label directions when using any pesticide.
from the Cornell Cooperative Extension and Virginia Cooperative
Extension , 1999