Adelgids

Adelgids are aphid like insects that suck the sap from conifers. Often covered in a white waxy material some also cause galling.

Larch adelgid ( Adelges laricis ) on larch ( Larix  sp.)

Larch adelgid (Adelges laricis) on larch (Larix sp.)

Quick facts

Common name Adelgids
Scientific name Adelgidae
Plants affected Conifers
Main symptoms Fluffy white wax, some species cause galls
Most active Year round

What are adelgids?

Adelgids are insects closely related to aphids, and like aphids they feed on plant sap. They feed on conifers and can have complex lifecycles including more than one host. Species often found in gardens are listed below.

Spruce pineapple gall adelges, Adelges abietis. Found on spruce (Picea) this species causes green pineapple shaped galls up to 20mm long, usually on young shoots. The galling can cause distortion of shoots, affecting the appearance of trees. Yellow, winged adults (gallicolae) leave the galls in late summer and lay eggs, the nymphs soon hatch and overwinter close to buds. In spring the nymphs feed on plant sap but do not cause galls, maturing into light green wingless females (pseudo-fundatrices). These females lay eggs covered in white waxy threads. The nymphs hatching from these eggs induce the galls when they feed at the base of needles. The galls contain numerous chambers within which groups of pale orange nymphs develop.

Douglas fir adelges, Adelges cooleyi. A native of North America which has become widely established in Britain. It alternates between spruce (Picea) and Douglas fir (Pseudotsuga menziesii). It can be most noticeable on Douglas fir where foliage can become encrusted in white fluffy wax, sooty moulds and become mottled. On spruce the adelgid causes galling which can affect the growth of new shoots. The adelgid lays eggs in the spring on Douglas fir. The nymphs soon hatch and feed on the needles producing large amounts of white wax and honeydew. The adult females are less than 2mm in length and reddish brown to black, both winged and wingless forms occur. In early summer the winged forms migrate to spruce where they produce a generation that overwinters. In spring the overwintered generation induces elongate galls on spruce, this generation matures in the summer producing winged females that migrate to Douglas fir and overwinter, laying eggs in the spring.

Larch adelges, Adelges laricis. This species is only found on larch and spruce. On larch it covers itself in a white waxy material and it can cause the foliage to become discoloured and distorted and a premature loss of needles. On spruce it forms pineapple galls that are up to 15mm long. The blackish nymphs of this adelgid overwinter on young larch shoots, maturing in early spring and laying eggs at the base of leaves. Nymphs from these eggs develop into wingless or winged 1.5 mm long dark green adults. The winged individuals migrate to spruce. Wingless individuals continue to breed on larch and extensive populations can develop, characterised by large quantities of white waxen wool and honeydew. Those that migrate to spruce give rise to a wingless generation that produces gall forming individuals in the following spring. Winged individuals mature in these galls in summer and migrate to larches where they lay eggs.

Scots pine adelges, Pineus pini. This adegid produces a white was which can disfigure Scots pine (Pinus sylvestris) but damage is usually minor. This adelgid overwinters as nymphs that mature into 2mm long dark brown to red adults in early spring. These adults lay eggs which develop into winged or wingless adults by early summer. The winged forms migrate to other Scots pine, whilst the wingless forms produce further generation on the original tree. There can be several generations in a year.

Weymouth pine adelges, Pineus strobi. Very similar in appearance and lifecycle to Scots pine adelges but only affects Weymouth pine, Pinus strobus. Native to North America and now widespread in Europe.

Control

Non-pesticide control

The damage adelgids cause is often minor and can usually be tolerated. Which is fortunate as on large trees they cannot be treated. 

Pesticide control

Adelgids are difficult to control with insecticides as they are protected their waxy secretions and there are no controls for the gall forming stages. In addition it is only feasible to treat adelgids on trees that are small enough to be sprayed thoroughly; infestations on tall trees have to be tolerated.

  • Always read the label use pesticides safely and extra care must be taken near plants in flower due to the danger to pollinating insects. 
  • Organic sprays, such as natural pyrethrum (e.g. Bug Clear Gun for Fruit & Veg, Ecofective Bug Killer), fatty acids (e.g. Solabiol Bug Free, Doff Greenfly & Blackfly Killer) or plant oils (e.g. Vitax Organic Pest & Disease Control, Bug Clear for Fruit and Veg) can give good control of adelgids. These pesticides have a very short persistence and so may require reapplication to keep adelgid numbers in check. Plant oil and fatty acid products are less likely to affect larger insects such as ladybird adults. 
  • More persistent insecticides include the synthetic pyrethroids lambda-cyhalothrin (e.g. Westland Resolva Pest Killer), deltamethrin (e.g. Provanto Ultimate Fruit & Vegetable Bug Killer) and cypermethrin (e.g. Py Bug Killer)
  • The systemic neonicotinoid insecticide acetamiprid (e.g. Bug Clear Ultra) is also available
  • Follow label instructions when using pesticides. On edible plants make sure the food plant is listed on the label and follow instructions on maximum number applications, spray interval and harvest interval
  • Inclusion of a pesticide product does not indicate a recommendation or endorsement by RHS Gardening Advice. It is a list of products currently available to the home gardener

Downloads

Pesticides for gardeners (Adobe Acrobat pdf document outlining pesticides available to gardeners)


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