BLACKSBURG, May 2003 -The
17-year cicadas have returned to many areas of Virginia, especially
Southwestern Virginia. For all their fierce looks and annoying noise, they
will not be a serious problem for most people. However, they can do severe
damage to young trees.
Homeowners and farmers may have
to take preventative measures to keep the female cicadas from laying their
eggs in the branches of young trees, or to keep the next generation of
nymphs from burrowing into the ground. Everyone else will find the sheer
number of cicadas the most vexing problem. The insects do not bite and have
no known toxic chemicals.
Though many people call them
locusts, this is misleading, according to Eric Day, Virginia Cooperative
Extension entomologist at Virginia Tech. "The name locust applies to the
grasshopper. The insect that has re-appeared is the periodical cicada. It is
their survival tactic of emerging in hordes to overwhelm their predators
that makes them seem like a 'plague of locusts,'" he adds.
There are two types of
periodical cicadas in Virginia, the 17-year and the 13-year cicadas. This
second variety is usually seen in the southern counties.
It is the mature nymph stage of
the insect that emerges from the soil. They climb onto nearby vegetation or
any vertical surface to molt and become winged adults. In the process, they
leave their exoskeletons behind on tree trunks, walls, and fence posts. The
adult cicadas are not among the prettier insects. They are black with
reddish-orange eyes and legs and about 1.5 inches long.
The loud whirring and
chirping noises that make them so annoying are the mating calls of the
males. The females are silent. As more and more of them emerge from the
ground, the sound can become almost unbearable in some regions.
The females soon make
their presence known by slicing open the wood of twigs 1/4 to 1/2 inches in
diameter and laying their eggs. "Each female lays from one to several dozen
eggs in one branch, and up to 400 eggs in 40 to 50 sites," says Day. The
egg-laying process causes the branch to split, whither, and die, resulting
in a symptom called "flagging."
"Flagging is especially
serious for small trees, four years or younger, because most of their
branches are the size preferred by the cicadas," Day notes. More than 270
species are vulnerable to the cicada, but their favorite trees are the
maple, hickory, hawthorn, apple, peach, cherry, and pear. The pine and
spruce trees escape injury.
After six to 10 weeks the
eggs hatch and the ant-like nymphs drop to the ground to continue the
next stage their life cycle. They burrow into the ground where they suck the
sap from the roots. Prolonged feeding by nymphs on a tree's root system may
reduce plant growth and fruit production.
Day says anyone planning to
plant young trees should check with their local Virginia Cooperative
Extension agent or nurseries about cicada re-emergence cycle first. "By
delaying new orchard plantings for a year, farmers can avoid the damage
altogether," he explains. "Home owners can do the same by planting in the
late summer or in the fall."
For those who already have
young trees planted, hard work and vigilance can reduce the damage. "They
should cover their young trees with cheese cloth or nylon netting with a
mesh no larger than 1/4 inch when the first male cicada songs are heard,"
says Day. The netting should tied to the trunk beneath the lower branches.
It can be removed after adult activity has ended.
"If the eggs have already been
laid in the branches, people need to prune out those branches within four to
six weeks. This will prevent the new nymphs from reaching the ground and
damaging the roots," says Day.
"If none of these cultural
methods have worked, there are chemical controls," he adds. "Orchard
owners can use any of the insecticides labeled for use on cicadas: carbaryl
(Sevin), chlorpyrifos (Dursban, Lorsban, esfenvalerate (Asana), and
resmethrin. Fruit orchards using methomyl (Lannate, Nudrin), oxamyl (Vydate),
or permethrin (Pounce, Ambush) in a cover spray do not need to add a special
cicada insecticide," Day adds. He cautions that Sevin or Vydate within 21
days after full bloom on apple trees may cause thinning. The use of
pyrethroids or Sevin on older trees could lead to problems with increased
densities of mites.
"Homeowners can use carbaryl
(Sevin) once a week to protect their trees and shrubs. However, they
need to avoid those plants that are blooming so they do not destroy the
insects that pollinate," Day cautions.
"It is the long life cycle of
the cicadas that confounds us," Day concludes. "After a few years we forget
all about the 17-year cicada. The best defense would be to ask about the
local cicada cycle any year you plan to plant young trees in the spring or
early summer."
Trade names are used only for
identification and do not imply that Virginia Cooperative Extension endorses
any product.
-
provided by Eric Day, Extension entomologist, Department of
Entomology, Virginia Tech
Insect Pests to lookout for by Month
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