Hickory spiral borer
Agrilus arcuatus torquatus LeConte
From: Solomon, J.D. 1995. Guide to insect borers of North
American broadleaf trees and shrubs. Agric. Handbk. 706. Washington, DC:
U.S. Department of Agricultural, Forest Service. 735 p.
Hosts. Pecan, hickory. Hickories seem to be preferred, but
pecan is readily attacked.
Range. Throughout the eastern half of the United States (Beal
and Massey 1942, Brooks 1926).
Description
Adult. Moderately elongate beetle, rather robust, strongly
shining, and moderately flattened (Brooks 1926, Fisher 1928). Females
uniformly brownish copper; males have reddish copper pronotum with greenish or
bluish sides and black elytra with violet tinge. Males 8 mm long and
females about 10 mm long.
Egg. Flat and disklike, 0.8 to 1.1 mm in diameter, and glued
firmly to smooth bark of twigs. Egg resembles shield of small scale
insects. Initially, eggs smooth and pale yellowish green, but before
hatching become slightly wrinkled and almost black.
Larva. Slender, flat, and legless, with mature specimens
reaching 15 to 20 mm long and 2 mm wide. Yellowish white except for dark
brown or black mouth parts and anal forceps.
Biology. Adults emerge late April to late June, depending on
location, and feed on foliage, chewing elongate notches and slits in the edges
of the leaves (Beal and others 1952, Brooks 1926). Females begin
oviposition 10 to 14 days after emergence. Eggs are deposited singly on
the bark surface of terminals or laterial twigs, usually near the base of small
shoots of current season's growth and are covered with a transparent secretion
that glues them to the bark. Females lay 2 to 55 eggs each over 6 to 8
weeks. Eggs hatch in 3 to 4 weeks, and the larvae chew through the bottoms
of the chorions and directly into twigs, where they make elongate threadlike
burrows under the bark. Late in autumn, they begin spiral burrows that
sever the twigs by winter or spring. Mining larvae pack their galleries
behind with fine frass. During the second summer, they mine downward under
the bark along the stem for 20 to 60 cm, leaving shallow (but relatively wide)
burrows packed with brown frass. During late fall, they change course
abruptly and cut thin, symmetrical rings around the stem. After the first
circuit, they bore spirally inward in the same plane, encircling the stem until
reaching its center. Larvae then turn and mine upward under the bark for
25 to 76 mm. Here, they form crescent-shaped pupal chambers, with ends
that extend to the bark and whose bottoms curve toward the stem center.
They plug both ends of pupal chambers with frass, then pupate during May and
June for about 3 weeks. A generation requires 2 years.
Injury and Damage. Larvae sever branches and terminals during
late winter and spring (Brooks 1926, USDA FS 1985). The portion above the
girdle usually (but not always) dies in spring before foliage appears, the
injury becoming apparent as the rest of the tree puts forth leaves. Most
severed branches break and fall to the ground just before and soon after
budbreak in spring. The borer severs stems 8 to 40 mm in diameter and 0.5
to 2.5 m long--many are larger than those girdled by Oncideres twig
girdlers and Anelaphus twig pruners but slightly smaller than those
pruned by Psyrassa branch pruners. Examination and removal of bark
from infested branches will reveal the frass-packed galleries and sometimes the
long white larvae. The long winding gallery beneath the bark abruptly
spirals inward to sever the branch. The spiral cut made by a larva is
characteristic; it is a winding concentric cut from the inner bark to the heart
of the branch or stem. The coils of the thin burrow join and completely
sever the wood, except for the bark and sometimes a few slender fibers at the
center. Later examination will reveal curved pupal cells in the wood and
D-shaped emergence holes in the bark. A major portion of the terminals of
young trees is sometimes severed. Serious damage to large trees results in
reduced nut production, ragged appearance, and asymmetry. Repeated attacks
on young trees may cause stunted, misshapen, crooked, and forked stems. In
one study in North Carolina, 7% of 500 young hickories examined had part or all
of their main stems killed by this borer (Beal and Massey 1942). Although
individual trees may be seriously damaged, entire stands, groves, nurseries, and
other plantings are seldom badly harmed. Serious damage is most likely to
occur in plantings adjacent to or close to forested tracts containing many heavily
infested hickories.
Control. Five insect parasites--Labena apicalis Cresson, L. grallator (Say), Monogonogastra agrili (Ashmead), Tetrastichus
rugglesi Rohwer, and Zatropis sp. near nigroaeneus (Ashmead)--help
reduce populations (Brooks 1926, Burks 1979, Carlson 1979). To collect and
destroy the larvae, young trees in heavily infested nurseries and orchards
should be pruned to remove dead branches and terminals as soon as leaves develop
in spring (Beal and Massey 1942, Brooks 1926). Special care should be
taken to cut off the small dead twigs that have been severed by the first winter
larvae. To be sure of getting the tunneling borers, such twigs should be
clipped several centimeters below the dead part. Also, all fallen severed
branches and terminals should be picked up and destroyed before adults begin to
emerge. It had been suggested that old hickories growing near nurseries
should be removed (Beal and Massey 1942).
Damage spiral girdling. James Solomon,
USDA Forest Service. |
Larva(e) and gallery. James Solomon,
USDA Forest Service. |
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