Spruce adelgids trick trees into making galls

By Jeff Mitton

Tiny tangles of white fluff appeared on blue spruces in the last several weeks.

Initially, I had trouble finding them, but once I developed my search image, I found many. Within, or more precisely, beneath the shroud of white threads hides an adult female spruce adelgid, and although the adults are rarely noticed, their offspring's galls are a familiar sight.

Cooley spruce adelgids, Adelges cooleyi, are hemipteran insects that feed on plant juices.

Some trees develop dozens to many hundreds of galls and the loss of all those twigs is not trivial.Hemipteran insects share the trait of a proboscis, mouthparts fused into a beak capable of piercing plant tissues and serving as a straw as the insect sucks juices from the plant.

Spruce adelgids are widespread in the west, from southern Arizona throughout the Rocky Mountains to British Columbia. They have a two-year life cycle and two hosts, a spruce species and Douglas fir.

We associate them with spruces because of their common and conspicuous galls, which they produce only on spruces. They complete several generations per year on Douglas fir without forming galls.

Their life cycle is quite flexible, for if either host is unavailable, they can live solely on the available host. Within Boulder County they are found on blue spruces in town and in the canyons, and on Engelmann spruces at higher elevations. To the north they become most common on white spruce.

Female adelgids spent the winter on the underside of twigs and buds on blue spruces. During the following spring, they produce their white shroud, sometimes called flocculence, to hide from predators.

As blue spruce buds swell, females produce large clutches of eggs that they attach to twigs and hide beneath white flocculence.

In the photo, the female is barely visible as a dark form beneath tufts of white, but her head and thorax are grey and the wing covers over the abdomen are dark with light yellow spots. Most of the white threads have been displaced from the mass of eggs and several newly hatched nymphs, more yellow than the eggs, are visible.

As the buds burst and needles begin to grow, nymphs migrate to the ends of twigs to feed at the base of young needles. As they feed, a molecule in their saliva is injected into the tree. This molecule resembles or mimics a plant hormone that initiates development of a female cone, which would usually develop at the top of the tree. But adelgids focus their efforts low on the tree, and their presence and activity shunt the developmental trajectory of the new growth from cone to gall.

The tiny crawlers get inside the gall and feed from it until mid summer, when they emerge as a winged form that flies off in search of Douglas firs, where they are called wooly aphids. They have several generations on Douglas firs without producing galls and then return to spruces to complete the life cycle.

A gall is a major advantage to a slow, flightless, defenseless nymph that wants nothing more than to insert its proboscis into the plant to suck fluids. The gall hides them from sight and puts them inside a tough fortress where they are virtually free from predation.

When a twig is fooled into developing into a gall, the growth of the twig ends when the gall dries in mid summer. But loss of a few twigs is a trivial loss to a blue, Engelmann or white spruce.

However, some trees develop dozens to many hundreds of galls and the loss of all those twigs is not trivial.

On blue spruces, it has been noted that adelgids preferentially gall the greener trees, passing over those with the intensely blue needles. The bluest of blue spruces have large amounts of wax on the needles — perhaps adelgids avoid the wax.

An animal's synthesis of a molecule to hijack the developmental machinery of a blue spruce is an unlikely adaptation to be produced by natural selection and evolution, which are unguided, undirected processes.

However, given many thousands of species and millions of years, unlikely adaptations become commonplace and widespread.

Jeff Mitton, mitton@colorado.edu, is a professor in the Department of Ecology and Evolutionary Biology at the University of Colorado. This column originally appeared in the Boulder Camera.

May 22, 2015