Adelgids are often mistaken for Mealy Bugs or woolly aphids. The Adelgid is a sap sucking insect – closely allied to the Aphid family – and is mainly a pest of conifers, for which reason they are often referred to as ‘Conifer Aphids’ – or ‘Wooly Conifer Aphids’.
The main difference between Aphids and Adelgids being that the Adelgid produces young by way of laying eggs. The true Aphids give birth to live young.
Adelgids are difficult to see on conifers until mid-summer when the normally black aphids, cover themselves in a white woolly fleece for protection. Young conifers can suffer a worrying degree of damage, but the Adelgids can also affect mature specimens if not treated.
As they bore into the bases of the leaflets, this can cause leaf (needle) drop resulting in an overall weakening of the conifer if not controlled. The needle drop is quite sudden – unlike red spider mite, where the needles turn brown as the sap is slowly drained from them.
The conifers affected by Adelgids are the Hemlocks – Tsuga; Spruce – Picea; Pines – Pinus. However there are several different types of Adelgid that inhabit a wide range of conifers.
Conifers are relatively trouble-free plants, suffering little in the way of problems, so any sign of distress should mean an inspection first and foremost for this source of pest. Regular inspection should be carried out on prized specimens to catch such pests before they do too much damage.
They bore through the bark of young conifer twigs, and often leave scar tissue after the infestation.
As with many garden pests, the Adelgids have more than one generation during a calendar year. The first is in early summer then a second batch in late summer early autumn. Both generations consist of female-only bugs that need no male to reproduce - they produce their eggs asexually. Each female Adelgid is capable of laying a minimum of 200m eggs. This normally starts directly after the winter period, and the young crawlers that emerge start to feed instantly on young twigs and bark.
A later batch of eggs is laid in late summer, and these emerge in October normally. The young from this batch, feed throughout the winter – ready to start the whole cycle over. They do not hibernate. The only dormancy period is late summer when the white fluffy coat is in place after the infant development into the adult Adelgid.
They fly to find new trees on which to feed, but then after laying eggs, lose their wings and die.
Systemic insecticides were one of the best control methods, but now generally discontinued. There is an option to inject trees with certain chemicals in a forestry situation. This is not applicable or practical for the amateur gardener.
The best way forward is to apply winter insecticidal oil, or in summer with horticultural insecticidal soap. These are certainly the best option for the late generation.
Systemic insecticides are best applied in early spring, when the foliage is in active growth. This is then taken into the sap stream and ingested by the young emerging Adelgids.