Ash cambium miner
Phytobia sp.
From: Solomon, J.D. 1995. Guide to insect borers of North
American broadleaf trees and shrubs. Argic. Handbk. 706. Washington, DC:
U.S. Department of Agriculture, Forest Service. 735 p.
Hosts. Ash. White ash preferred in the North, green ash in
the South (Skelly and Kearby 1969, 1970). Other ash species are probably
attacked to a lesser extent.
Range. Previously reported in only eight counties in south
central Pennsylvania (Skelly and Kearby 1970), but the author has found it
commonly in Arkansas, Louisiana, and Mississippi. Probably occurs over
much of the eastern United States.
Description
Adult. No adults have been reared for description.
Larva. Narrow, elongate, cylindrical, slightly flattened,
tapered slightly toward the anterior and posterior ends (Skelly and Kearby
1970). Anterior end small, not retractile, curved. Mandible has two
noticeable, black, clawlike teeth or hooklets. Larvae distinguished from
those of P. setosa by small, oblique tooth on the left side that is
bidentate and other teeth that are similar in size. Body opaque white,
segments almost indistinct without magnification, posterior spiracles consist of
three short bulbs. Mature larvae 20 to 25 mm long and 1.1 to 1.25 mm
diameter.
Biology. Adults apparently emerge during late spring and summer
and presumably oviposit in small twigs and branches (Skelly and Kearby 1969,
1970). Young larvae mine rapidly downward in the cambium of the branches
and bole, eventually reaching the roots. They spend about 10 months in the
lower trunk and roots. Larval burrowing habits differ from other Phytobia
cambium miners. Although young larvae burrow in long, narrow paths and
meander little, they eventually mine in serpentine paths and finally in distinct
zigzags. Once in the roots, many larvae make pectinate or bipectinate
mines, a habit not reported for other Phytobia species. Moreover,
larvae feed at times in layers of the phloem, as indicated by the disappearance
of mines from the cambium and reappearance several centimeters away. In
contrast, detailed studies of P. setosa show that larvae mine in narrow
zones of newly differentating xylem just below the cambium (Gregory and Wallner
1979). Although larvae have been found in roots to 7.6 m from the root
collar, most of their root burrowing is in the nearest 1.5 m of roots.
Second-stage larvae overwinter within the mines in the roots. Larvae are
somewhat active during winter, as indicated by fresh, white or slightly tan
frass deposits in the mines. Feeding is resumed in spring, with larvae
reversing directions in the roots and lower trunk. In Pennsylvania, mature
larvae cut holes in the bark or roots and enter the soil to pupate in May and
June. In Mississippi, the larvae appear to exit the bark much earlier, and
many mine upward from the roots to the root collar or above to cut holes and
exit the bark. This miner has one generation a year.
Injury and Damage. In standing trees, injury is virtually
undetectable (Hardwood Research Council 1987; Skelly and Kearby 1969,
1970). When bark is removed, tan to brown mines on both the white, inner
phloem and surface of the xylem become visible. In the branches and upper
bole, the mines are mostly narrow, threadlike, and straight to serpentine.
In the middle and lower bole, many mines become more sinuate and some distinctly
zigzag. In the roots, in addition to these patterns, pectinate and
bipectinate-shaped mines may be present. As mines fill and heal over, the
wood grain covering the mines becomes distorted and slightly bulging or
swollen. Moreover, much of the brown deposit in most mines fades or
bleaches, becomingly nearly colorless. Consequently, the pith flecks in
cross sections and log ends of ash are hardly noticeable in contrast to those
caused by Phytobia species in maple and other trees. The reason may
be that ashes are ring porous, whereas all other hosts attacked by cambium
miners are diffuse porous. In sawn lumber and sliced veneer of ash, most
mines do not show up as brown streaks and marks as in other host species.
Instead, they are most noticeable as zigzag tracks varying from slightly lighter
to slightly darker than the natural wood. However, the distorted wood
grain gives it a characteristic "gothic arch" grain pattern.
Such tracks viewed from one direction may be almost indistinct, but when the
board is tilted or the angle of light changed, the tracks become distinct.
Grain distortions interfere with the millling and fine finishing process of
infested wood and is objectionable in international markets. These mines
are referred to in the lumber industry as "worm tracks," "pith
flecks," "pith ray flecks," "medullary spots," and
sometimes as "glassworm" or "glass tracks," especially in
Europe. Although not recognized as grading defects in lumber, in face
veneers, they are considered defects by the Fine Hardwood Veneer Association.
Control. Dead larvae have been found in their mines following
unusually cold winters. No other natural controls are known, and direct
controls have not been investigated.
Damage in root [note mine pattern]. James Solomon,
USDA Forest Service. |
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