Walnut shoot moth
Acrobasis demotella Grote
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. Walnut, hickory, pecan. Black walnut is preferred (Neunzig
1972). Hickories, especially bitternut and pignut hickories, are sometimes
attacked; pecan is infested less frequently.
Range. Throughout the range of its hosts, but scarce to absent
in the warmer regions of the United States even though an occasional host tree
may be present (Nuenzig 1972). Reported from Ontario south to North
Carolina and west to Missouri and Michigan (Martinat and Wilson 1978).
Description
Adult. Brownish gray moth with wingspan of 20 to 24 mm (Heinrich
1956). Forewings have brownish gray background with three reddish brown,
contrasting patches basally, medially, and distally (Heinrich 1956, Martinat and
Wallner 1980). Head and thorax dirty white with pink or reddish suffusion
darker in females than in males.
Egg. Elliptical, ivory white, convex above and flattened below,
with reticular pattern on surface; 0.28 by 0.71 mm (Martinat and Wallner
1980).
Larva. Purplish brown, 17 mm long when mature, with head
capsules width 1.39 to 1.52 mm. Reddish brown to brown head with dark
brown spots. Dorsal body surface purplish brown with greenish undertones;
underside pale, sometimes mostly green. Thoracic shield yellow brown with
dark brown lateral margins. Pinacula pale brown to brown and same color as
or darker than surrounding integument (Martinat and Wallner 1980, Neunzig 1972).
Pupa. Reddish brown and 7.5 to 8.5 mm long. Head small and
rounded distally. Prothorax and mesothorax distinctly wrinkled (Neunzig
1972).
Biology. Adults emerge from late May through mid-June in
Missouri (Kearby 1978), during the first half of June in Massachusetts (Neunzig
1972), and from late June to late July in Michigan (Martinat and Wallner
1980). Females deposit eggs singly on the undersides of expanding
leaflets, usually adjacent to the lead midrib on the basal half (Martinat and
Wallner 1980). Laboratory-reared eggs hatch after 11 days' incubation, but
incubation is probably longer in the field. Newly hatched larvae wander a
short distance from the egg chorions and construct trumpet-shaped enclosures of
frass "plastered" onto a silken framework. The larvae feed on
the lower epidermis, skeletonizing leaflets. First- and second-instar
larvae move to buds in fall and construct gray hibernacula 1.1 to 1.5 mm long on
the terminal buds or in the axils of lateral buds. Larvae overwinter in
hibernacula, emerge in spring about the time of bud swell, and third-instar
larvae feed in expanding buds. As shoots elongate in April and May,
fourth-instar larvae enter shoots, usually where leaf petioles are
attached. Larvae bore downward in the pith of the shoot for 2 to 5 cm, and
small, loose mounds of frass mixed with silk collect at entrance holes.
Shoot tips beyond the larval tunnels usually wilt and die. Mature (fifth-instar)
larvae complete feeding in shoots by late April in Missouri (Kearby 1978) and by
mid-June in Michigan (Martinat and Wallner 1980) and then vacate shoots through
entrance holes. Mature larvae pupate just below the soil surface in thin
cocoons composed of soil particles. One generation per year has been
recorded in Missouri (Kerby 1978) and in Michigan (Martinat and Wallner 1980).
Injury and Damage. In spring, early-stage larvae bore into
swelling, unfolding buds at terminal-bud clusters. From one to all buds in
a cluster may be killed, resulting in blunt shoots or forked tops (Kearby
1978). Wilting and dying foliage on small succulent shoots is further
evidence of attack. Infested shoots may be completely hollow. Small
mounds of frass and silk collect near larval entrance holes in buds and in leaf
axils (Neunzig 1972). If the attacked bud or shoot is not killed
immediately, it is usually hollowed out and subject to breaking by wind (Weber
and others 1980). On young black walnut trees, repeated attacks cause
forking, branching, reduced growth, and ultimately misshape trees and greatly
diminish their value for timber (Martinat and Wallner 1980). The basal 4
to 6 m of a black walnut must be straight and free of forks and excessive
branches to qualify as a prime sawlog or veneer log; thus, damage by this borer
when trees are in the seedling and sapling stage can be ruinous (McKeague and
Simmons 1979).
Control. No parasites have been reared from larvae and pupae,
which is surprising because numerous parasites have been reared from the closely
associated species, A. judglandis (LeBaron) (Martinat and Wallner 1980, Neunzig
1972). To minimize infestation culturally, new black walnut plantings
should be relatively isolated from existing stands of Carya and Juglans.
Where trees are grown for high-quality timber or veneer logs, corrective pruning
is an effective alternative to insecticides (McKeague and Simmons 1978,
1979). Managers should prune soon after the current season's damage has
been done and new shoots have grown enough to make it possible to leave the best
of several shoots. Proper timing will permit pruning wounds to heal by the
end of the growing season. Workers should leave the strongest shoots that
is closest to the original damaged terminal to reestablish apical dominance and
maintain a straight trunk and good form. Insecticides have been used
successfully in Michigan to protect young black walnut plantings (McKeague and
Simmons 1978).
Damage. Minnesota Department of Natural Resources Archive,
Minnesota Department of Natural Resources. |
Larva(e) in stem. Minnesota Department of Natural Resources Archive,
Minnesota Department of Natural Resources. |
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