Bronze poplar borer
Agrilus granulatus liragus Barter and Brown
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. Poplar. Five species of poplars--including
quaking aspen, bigtooth aspen, balsam poplar, black cottonwood, and eastern
cottonwood--are reported as hosts (Barter 1965).
Range. Newfoundland west to British Columbia; also found in the
United States southward to Pennsylvania and westward to northern California and
Oregon (Barter and Brown 1949, Nelson and Westcott 1976).
Description
Adult. Beetle that is structurally similar to A. g.
granulatus but described as distinct subspecies based on color, morphology
of male genitalia, and preferred host plants (Barter and Brown 1949, Carlson and
Knight 1969). Slightly flattened beetle blackish with green
reflections. Male head greenish. As compared to A. anxius,
coppery reflections nearly always lacking and never distinct. Male adults
7.2 to 10.3 mm long; females 6.8 to 11.5 mm long.
Egg. Creamy white, flattened, oval, and about 1.2 by 0.8 mm
(Carlson and Knight 1969).
Larva. Long and slender--measuring 30 to 40 mm long and 2 to 3.5
mm wide. Prothorax wider than rest of body with median I-shaped line
(Benoit 1965). Body white except for dark mandibles and prominent brown
anal forceps.
Biology. In Canada, adults emerge from early June to late August
(Barter 1965, Ives and Wong 1988). Emergence may begin earlier or later,
with a corresponding change in its end, depending on prevailing environmental
conditions. Adults feed on foliage of hosts for about 1 week before laying
eggs. They deposit eggs in bark crevices in groups of five to eight.
Each female deposits several groups of eggs, most frequently on the sunny side
of trees when the temperature is about 27o C (Barter 1965).
Hatching from eggs in about 2 weeks, larvae bore directly through the bark to
the cambium for most of their feeding, moving occasionally into the phloem and
cortex. First-instar larvae travel about 8 to 16 cm before molting.
Later instars travel much farther. Generally, larvae bore into the xylem
to molt; a completed tunnel shows four such departures from the regular feeding
tunnel, suggesting five larval instars. Mature larvae make pupal cells,
and pupation occurs either in the outer xylem or in thick bark in spring.
Borers in severely weakened trees usually complete a generation in 1 year,
particularly in their southern range; those in more vigorous hosts require 2
years to complete development, especially in their northern range (Carlson and
Knight 1969).
Injury and Damage. Foliage discolors and leaves drop in portions
of the crown after the initial attack (Barter 1965). Branch mortality soon
becomes noticeable. Successful infestations in weakened trees
typically move downward on the bole and often girdle and kill the tree.
Removing the bark from affected branches or trunks reveals the larval
feeding galleries in the cambium. Flattened galleries packed with frass
range from 30 to 60 cm long and 0.6 to 3.0 mm wide. Gallery
characteristics vary with condition of the host (Carlson and Knight 1969).
In severely weakened trees, galleries usually meander without any distinct
pattern. In vigorous hosts, a zigzag or sinuate gallery is the rule.
Galleries weave back and forth across the grain with successive loops closer
together in the most vigorous hosts. Emerging beetles leave D-shaped
emergence holes in the bark (Ives and Wong 1988). Attacks on the branches
and trunks of trees cause deterioration and frequently death of the hosts.
Larval feeding damages the phloem and cambium, interfering with the movement of
plant food (Barter 1965). The insect is most likely to become established
in overmature or injured trees and in young trees released from the dominance of
other trees. Felled, topped, and girdled trees and those damaged by the
poplar borer (Saperda calcarata Say) and hypoxylon canker are
particularly susceptible (USDA FS 1985). Evidence exists that infestation
by this borer increases the incidence of hypoxylon canker (Barter 1965).
Control. Both egg and larval parasites have been reared.
In one study, two egg parasites--Coccidencyrtus sp. and Thysanus
sp. in the family Encyrtidae--destroyed about half the eggs. Five species
of insect parasites--Atanycolus charus (Riley), Doryctes sp., Ephialtes
sp., Phasgonophora sulcata Westwood, and Testrastichus sp.
(probably rugglesi Rohwer)--were reared from the larvae. Range of
from 2 to 20% parasitism by P. sulcata was most common (Barter
1965). Larvae and pupae are frequently prey of woodpeckers. In one
study of standing trees, when larvae were pupating and transforming to adults in
late spring, woodpeckers took up to 40% of full-grown larvae, pupae, and teneral
adults from individual trees (Barter 1965). Maintenance of high tree vigor
helps to reduce the incidence of attack. Because susceptibility to the
borer is enhanced by injuries caused by other insects, wind, animals, and fungi,
measures that prevent such injuries can reduce attacks and their severity
(Barter 1965).
Life Cycle adult. Forrest L. Oliveria,
USDA Forest Service. |
Damage defoliation in first year plantation. James Solomon,
USDA Forest Service. |
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