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Larch adelgid, Larch woolly aphidIdentification & Distribution Biology & Ecology Damage & Control
Identification & Distribution:Adelges laricis galls on spruce (see first picture below) are waxy, creamy and relatively small. These galls mature in June-July. The winged female of Adelges laricis (see second picture below) which matures on spruce is greyish to blackish, with a body length of 1.9-2.0 mm. The winged female that matures on larch is dark green, with a greyish-green head and thorax and a body length of 1.0-1.5 mm.
First image courtesy AfroBrazilian under a Creative Commons Attribution-Share Alike 3.0 Unported license.
The larch adelgid host alternates between the primary host Picea (spruce) and the secondary host Larix (larch). The larch adelgid was originally found in central Europe, but Adelges laricis is now widespread throughout that continent and has been introduced to North America.
Biology & Ecology:
Females on spruce hatch from fertilized eggs in autumn, and the young larvae overwinter on spruce twigs. In spring they feed at the bud scale bases inducing a gall (see picture below).
Image courtesy AfroBrazilian under a Creative Commons Attribution-Share Alike 3.0 Unported license.
The galls mature in summer to give greyish to black winged females known as gallicolae (see top of page right). These fly to larches, settle on the needles and deposit eggs.
The eggs hatch to give blackish or purplish grey nymphs that overwinter on one-year-old larch shoots near bud. These nymphs mature in April to blackish grey females (sistens) that reproduce parthenogenetically and deposit unprotected clusters of eggs at base of leaf spurs (see picture below).
Nymphs emerging from these eggs spread out on to the larch needles, maturing into wingless or winged adults a month later.
Wingless (and some winged) females continue to reproduce on larch, producing one or more generations and vast quantities of wax wool and large globules of honeydew. The specimen below right has been dewaxed artificially to show the nymphal winged form. The wing buds are just visible laterally.
Winged forms migrate back to spruce, especially sitka spruce, and lay eggs which develop as sexual males or females. These mate, and the females lay fertilized eggs which hatch in autumn and start the cycle again.
Mitchell & Maksymov (1977) have recorded two predators - a syrphid (Cnemodon) and a cecidomyiid Aphidoletes abietis - feeding on developing gallicolae within the tightly closed galls of both Adelges laricis and Adelges abietis. The syrphid larva enters the gall before it is closed, but the cecidomyiid larva enters the gall after it has closed, and causes the gall to open prematurely when the predator reached the last larval instar.
Damage & Control:
The main damage caused by Adelges laricis is to provide an entry route for fungi to larch trees. Skarmoutsos & Millar (1982) found that the fungus Meria laricis invaded needles of Larix x eurolepis at the feeding sites of Adelges and caused premature needle fall. Several other fungi were also isolated from larch needles attacked by aphids. Labanowski & Soika, 1999 achieved good control of Adelges laricis using sprays of acetamiprid, imidclopridand other older insecticides.