Willow shoot sawfly
Janus abbreviatus (Say)
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. Willow, poplar. Prefers black willow; two clones of
the interspecific hybrid Salix babylonica x S. alba have been
mentioned specifically as hosts; poplars, including eastern cottonwood, quaking
aspen, bigtooth aspen, and balsam poplar have been recorded (Osgood 1962, Riley
1888, Solomon and Randall 1978).
Range. Southern Canada from New Brunswick west to Alberta and in the
eastern and central United States from Maine west to Minnesota and south to
Virginia, Arkansas, and Mississippi, and in Oregon (Smith and Solomon 1989,
Solomon and Randall 1978).
Description
Adult. Delicate, brown, wasplike sawfly 7 to 10 mm
long, with wingspan of 12 to 16 mm in females and 10 to 12 mm in males (Ries
1937, Smith and Solomon 1989, Solomon and Randall 1978). Head and thorax
shiny black with tiny, white to yellow markings. Abdomen black with
segments 2 and 3 (and sometimes part of 4) red to reddish brown in females; only
venter red in males. Abdomen of females compressed, much deeper than wide;
sharp sawlike ovipositor. Abdomen not compressed in males. Wings
hyaline, without violaceous reflections; base of radial vein atrophied near
base.
Egg. Translucent to whitish, oval to elongate, 0.8 to 1.0 mm
long and 0.3 to 0.5 mm in diameter (Solomon and Randall 1978).
Larva. Cylindrical with thorax slightly enlarged dorsally and
laterally; typically S-shaped; 8 to 11 mm long at maturity; white, except for
pale yellow head, brownish mandibles, and brownish, short, tubular prong at tip
of abdomen (Middleton 1917, Solomon and Randall 1978). Thoracic legs
short, fleshy, and without claws; abdominal prolegs absent.
Pupa. White, 8 to 10 mm long, enclosed in partially transparent
cocoon.
Biology. Adults from the overwintering brood emerge mid-April to
mid-May in Mississippi and May to July in Michigan (Middlekauff 1969, Osgood
1962, Reighard 1985, Ries 1937, Solomon and Randall 1978). Adults are
cautious and take flight after slight disturbances. Females use their
ovipositors to girdle succulent shoots by making a series of punctures
encircling the stem and are selective about shoot diameter and distance from
stem tip. They girdle willow shoots at an average of 44 mm from the tip;
shoot diameter at the point of girdle averages 2.4 mm. Girdle sites on
cottonwood average 50.1 mm from the tip and 3.2 mm in diameter. To girdle
a shoot, females insert their ovipositors, withdraw them, move slightly around
the stem, and puncture again, making 4 to 5 punctures in each of 1 to 3 trips
around the stem for a total of 5 to 16 punctures per girdle. Shoot tips
begin to wilt in 30 to 60 minutes after punctures are made. Females
oviposit in the tender shoots 7 to 26 mm below the girdled site. Single
eggs (rarely two) are deposited per shoot and typically are inserted at oblique
angles into the pith. Eggs hatch in 7 to 12 days. Initially, young
larvae tunnel toward the shoot apex near the girdled site, then turn and tunnel
downward for 15 to 36 cm. In nursery stool beds, the entire length of the
shoot is often tunneled, and occasionally the side of the rootstock.
Larvae pack brownish frass in the gallery as they tunnel; they eject frass from
the shoot only at breaks and girdled points along the stem. Before
pupation, mature larvae cut a hole almost through the bark surface to permit
emergence of the adult. Then they prepare a
thin, somewhat transparent, membranous, cocoonlike structure in which to
pupate. In its northern range, the insect reportedly requires 1 year to
complete a generation; in Mississippi it has three generation per year, with
first-generation adults emerging from mid-April to mid-May, second generation
adults appearing from mid-June to mid-July, and third generation adults emerge
from early August through September. Larvae of the last generation
overwinter in cocoons, transforming to pupate and adults the following spring.
Injury and Damage. The earliest evidence if injury is
wilting and drooping terminals and branch ends (Solomon and Randall
1978). Injured shoot tips wither and turn brown or black. Infested
shoots gradually die back 30 to 60 cm; young shoots sometimes die back to the
rootstocks. Peeling the bark off infested shoots reveals numerous spiral
girdles made by the tunneling larva. Slicing through the center of
infested shoots with a sharp knife reveals galleries filled with brown
frass. Injured plants often produce many new branches just below the
injured part, giving them a bushy appearance. The emerging adults leave
behind round exit holes 1.5 to 2.5 mm in diameter. This insect is a major
factor in the suppression and mortality of sucker shoots in cutover aspen stands
in the Lake States, killing up to 9% of the dominant sucker shoots annually
(Osgood 1962). In Mississippi, injury to cottonwood is usually minimal
because most attacks occur on lateral branches. However, the pest
severely damages nursery-grown willow; 90% of the shoots are sometimes killed by
the first generation sawflies (Solomon and Randall 1978). Damage to willow
plantations in Maryland has been so sever that the trees appeared to have been
damaged by frost or fire. Repeated attacks in young plantations sometimes
adversely affect tree form.
Control. In Mississippi, two hymenopterous parasites-- Bracon
jani Muesebeck and Eupelmus sp.--commonly kill 1 to 12% of the sawfly
larvae in willow shoots (Solomon and Randall 1978). In the Lake States, up
to 22% of the larvae in aspen shoots are parasitized by five species of
hymenopterous parasites-- Bracon sp., Eurytoma sp., Scambus
granulosus Walley, S. pterphori (Ashmead), and Tetrastichus
productus Riley (Osgood 1962). Three species of hymenopterous
parasites-- Eurytoma sp., Microbracon sp., and Tetrastichus
sp.--have been reared in New Jersey (Middleton 1917). Up to 9% of the
sawfly larvae in Mississippi have been killed by an unidentified fungus (Solomon
and Randall 1978). Overwintering mortality of larvae in Mississippi has
been estimated at 56%; mortality is most prevalent in the smallest, least
vigorous shoots (Solomon and Randall 1978). Infestations can be reduced in
nurseries by pruning and destroying infested shoots (Riley 1888). In
Michigan's poplar nurseries and plantations, planting small blocks of willow
(which the insect prefers over poplar) nearby as a trap crop, then annually
coppicing the willow to destroy overwintering larvae and create a crop of
succulent sprouts for next year's sawflies, has been recommended for control (Reighard
1985). Insecticides have reduced girdled shoots by 75 to 90%, but repeated
applications are necessary (Reighard 1985).
Larva(e) in stem (Fig. 235C in USDA Forest Service Ag. Handbook AH-706, 1995). James Solomon,
USDA Forest Service. |
Damage round exit hole in willow shoot (Fig. 235G in USDA Forest Service Ag. Handbook AH-706, 1995). James Solomon,
USDA Forest Service. |
Damage causing shoot dieback. James Solomon,
USDA Forest Service. |
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