Adelgids

What looks like a light dusting of snow may actually be life-threatening pests, called adelgids.

Hemlock Woolly adelgid (Connecticut Agricultural Research Station, bugwood.org) CC BY 3.0

Like their cousins, the aphids, adelgids pierce vascular bundles to suck out sugary fluids. While mature, healthy trees can withstand a mild adelgid infestation, saplings, young trees, and unhealthy trees can be killed by this tiny, soft-bodied pest.

Adelgid species

Scientists are still trying to nail down adelgid classification. There are 50 known species, all of which are native to the northern hemisphere, though several invasive species have made their way into the southern hemisphere. The most commonly found adelgids in California include the invasive balsam wooly adelgid (from Europe), the Cooley spruce gall adelgid (Adelges cooleyi Gillette), and pine adelgids.

Adelgids are commonly found on stone pine and other conifer species, such as pine and spruce. Depending on the host plant, the pests are commonly known as “pine aphids” or “spruce aphids”, respectively, even though they are not actually aphids. [Thanks to my friend, Chuck, I now know that adelgids are also found on apple trees. Thanks, Chuck!]

Balsam woolly adelgid (Giles San Martin) CC BY-SA 2.0

Aphids vs. adelgids

Aphids are significantly larger than adelgids, and they have two structures that adelgids do not: cornicles, and a tail-like cauda. Cornicles are tubes found sticking out of the 5th or 6th abdominal segments. These tubes are used to excrete a defensive chemical wax. Contrary to popular belief, cornicles are not used in honeydew distribution. Adelgids are covered with a dense wooly wax, so it is easy to mistake them for wooly aphids. This white fluff may be found on twigs, needles, bark, or cones.

Aphid excreting defensive wax from cornicles (Sanjay Acharya) CC BY-SA 3.0

Adelgid sistentes feeding at the base of needles (USDA) Public Domain

Adelgid lifecycle

Unlike aphids, which reproduce using both eggs and live birth, adelgids only lay eggs. Adelgids generally live for two years and each female can lay from one to several hundred eggs, depending on the species. Adelgid nymphs are called sistentes, which comes from a Latin that means ‘to stand’. When these sistentes overwinter, they are called neosistens. Some adelgid species require six generations to complete their lifecycle, moving between different tree species. Much like the Monarch butterfly, these insect pests do not live long enough to complete migration as individuals.  Generally, it is only the immature stage that causes damage.

Twig tip dieback due to adelgid feeding (Dan Foster) CC BY-SA 3.0

Damage caused by adelgids

Heavy infestations can cause yellowing, drooping, and dieback of twig tips. As they feed, adelgids release toxins that interfere with the tree’s ability to produce conductive sapwood. Eventually, the tree suffers severe water-stress and dies. These infestations can appear as swollen twigs, galls, or twig dieback. Adelgid galls look like tiny pineapples and can be green, red, or purple. The initial damage is usually seen on the underside of buds, before infestation and damage spread to the entire bud.

Adelgid galls (Dan Foster) CC BY-SA 3.0

Adelgid controls

These pests are easily dislodged with a stream of water from your garden hose, but that only works you see them, which means you have to go outside and look. Beneficial predators, such as lady beetles, green lacewings, and some fly larvae. Horticultural oils can slo be used, but they will discolor spruce tree needles.

Infested twigs can be pruned out while they are still green (before adelgids have emerged) and thrown in the trash. Also avoid applying excess nitrogen, which can stimulate vulnerable new growth.

The National Park Service estimates that adelgids are responsible for the death of 90% of the mature fir trees found in the Great Smokey Mountains National Park, since this pest’s arrival in 1962. If you have conifers on your property, it is a good idea to inspect them periodically for signs of adelgid infestation.