Pine root collar weevil (Hylobius radicis)
is found throughout the north central and northeastern regions of the
United States and in southeastern
Canada. It primarily attacks Scots,
red, jack, Austrian and eastern white pine.
Weevils require two years to complete their entire life cycle. They
lay eggs from early May to early September on the root collar of living
trees or in the soil surrounding the root collar. Each female adult will
lay 10 to 70 eggs in one season. Eggs hatch in 7 to 17 days, depending
on temperature. Larvae are small (up to 1 cm long) whitish grubs. Larvae
burrow into the inner bark (cambium) of the root collar and large roots,
and feed until the weather turns cold in autumn. During the winter,
larvae are inactive and are protected from cold temperatures in
galleries under the bark or in tunnels in the soil. As the soil warms in
the spring, larvae resume feeding. Pupation occurs in chip cocoons made
of sawdust-like frass; they are found in the soil near the root collar
or, occasionally, under the bark. Adult weevils emerge in 30 to 40 days.
Adult weevils feed on the bark of live pine tree shoots until autumn,
then overwinter in the litter or in bark crevices at the base of trees.
Adult weevils often live and continue to feed and reproduce for two more
years. Up to three overlapping generations of pine root collar weevil
may occur on the same host tree. Adult weevils are not strong fliers and
frequently move only a short distance to attack a new host tree.
Weevils are sensitive to light and temperature. Most adults feed
during evening hours when temperatures are cool. Weevil adults spend the
day resting at the base of the tree, venturing up to the canopy only on
cool, overcast days. However, when pines are grown in shade, beneath
other trees, or in dense stands, conditions are often too cool for pine
lot collar weevil populations to build. Trees growing in open sunlight
in Christmas tree fields or young plantations are most susceptible to
pine root collar weevil.
The principal injury to trees is caused by larvae feeding below
ground in the root collar, root crown and large roots. Small trees (less
than 4 inches in diameter) can be killed in a single year by as few as
two to five larvae. Larger trees may harbor several larvae and may be
repeatedly attacked each year. Soil and bark near the root collar of
infested trees becomes black and soaked with pitch. Trees are weakened at ground level and may fall over
or die within one to four years of the first attack. When trees are
girdled, the entire canopy fades from green to yellow to red. Larger
trees that have been partially girdled have low growth rates and are more susceptible to windthrow and secondary
Feeding by adult weevils girdles small shoots and branches, causing
them to die and "flag" or turn red. Weevil feeding can be
distinguished from other pest damage by the gnawed wounded area, often
covered with pitch, located at the base of the dead shoot.
Cultural: Growers can take advantage of the sensitivity of
adult weevils to temperature and light when developing an integrated
management program for pine root collar weevil. Basal prune young trees
(remove the lowest one to two whorls of branches) to allow more light to
reach the root collar. Where practical, rake away litter from the base
of the tree. If trees are living but show early signs of larval feeding
in the root collar, or if pine root collar weevil populations are in
nearby pine stands, scrape away the surface soil as well as the litter.
Usually, pruning, raking or soil scraping will only need to be done once
in a rotation. Avoid mulching trees in areas where pine root collar
weevil is a common problem. Avoid mixed plantings of different pine
species. Risk of pine root collar weevil damage is greater if no or more
species of pine are grown together.
Weevils typically do not move far to find new host trees. If a tree
is killed by pine root collar weevil, scout adjacent rows or clusters of
trees for signs of infestation. Try to locate new pine Christmas tree
fields 1/2 to 1 mile away from other infested fields or forest stands.
Biological: Eggs, larvae and pupae are protected under the
soil and bark, so they have few natural enemies. However, a wasp
parasitoid (Bracon radicis Shenefelt) is known to attack larvae
feeding near the soil surface. Ants may feed upon eggs or young larvae.
Ground beetles, spiders and rodents prey on adult weevils.
Chemical: Use a
persistent registered insecticide to drench the root
collar of infested trees. Research indicates that this
will control parent adults hiding at the base of the
tree, and newly emerging adults. Apply cover sprays in
mid-May (about 300 to 350 degree days [base 50]) and
again in mid-August (1200 to 1400 degree days [base 50])
to control adult weevils feeding on shoots.