An alien invasion is underway in Westborough, covering native hemlock trees with woolly white fuzz and killing them.

The invader is very tiny, about the size of a pinhead, or the period at the end of a sentence, or a poppy seed. It comes in large numbers and spreads rapidly.

This alien invader is a tiny aphid-like insect, the hemlock woolly adelgid (Adelges tsugae), called HWA for short.

Usually we don't see the invader itself. Instead, we see its protective fortifications – the waxy white woolly stuff that it wraps around itself and its eggs when it takes up a position on hemlock twigs. You can spot an infestation by the white woolliness on the underside of hemlock twigs and between the needles.

To see stands of eastern hemlocks (Tsuga canadensis) in Westborough while we still have them, walk the trails on the WCLT Hero property off Haskell Street near Orchard Swamp. Or, on the other side of Haskell Street, follow a trail near Crane Swamp to a stand of huge hemlocks. These trees are among Westborough's oldest.

Hemlocks are also popular plantings. More than 200 cultivated varieties of eastern hemlock exist, so eastern hemlocks are common in local yards.

What makes the tiny invader so deadly to our eastern hemlocks? Like an aphid, it sucks sap – the lifeblood of a plant. It feeds near the base of the needles. The invader may also "salt the earth," so to speak, by injecting toxic saliva into the tree. The hemlock's needles – its leaves – fall off, and the tree dies from lack of nourishment. Eastern hemlocks typically die within 4 to 10 years after being infested.

This invader is not from outer space, so what makes it an alien? It comes from another continent, in this case Asia. Hemlock woolly adelgid is native to Japan and China. It was accidentally brought to our shores 85 years ago, when it landed in the Pacific northwest in the 1920s. It reached Virginia in the 1950s and has been spreading north and south from there.

What makes the hemlock woolly adelgid invasive? In other words, why is this insect an invader, rather than a homesteader? Many plants in our landscape, for example, arrived from overseas, but not all are invasive. We enjoy aliens such as daisies (Chrysanthemum leucanthemum) and Queen Anne's lace (Daucus carota) that have naturalized along roadsides. Yet we dread the appearance of the lovely but invasive purple loosestrife (Lythrum salicaria) in our wetlands.

The ability to spread quickly and unchecked makes a newcomer invasive. An invasive plant or creature doesn't just settle into a local landscape and become naturalized there, living among the locals. It takes off, often jumping from location to location, dominating and changing environments. It may kill outright, as the hemlock woolly adelgid does, or push out existing inhabitants, as purple loosestrife does.

The hemlock woolly adelgid lived in its native Asian environments with the local plants and animals for countless generations. Over time and many generations, the trees it uses for feeding and breeding developed some resistance to it. Certain beetles and mites developed a taste for the hemlock woolly adelgid itself. These relationships help to prevent the hemlock woolly adelgid from spreading wildly in Asia.

In North America, we have different plants and animals. Out west, native western hemlocks (Tsuga heterophylla) and mountain hemlocks (Tsuga mertensiana) were already somewhat resistant to the hemlock woolly adelgid. The insect is more of a homesteader there. But in the east, our eastern hemlocks and the more southern Carolina hemlocks (Tsuga caroliniana) lack resistance and are being ravaged.

Yet the hemlock woolly adelgid doesn't have all the comforts of home here. A spruce tree that it uses during sexual reproduction doesn't grow here, so the insect can't reproduce sexually. You might think this lack of sexual reproduction would doom the hemlock woolly adelgid, but think again. It simply reproduces asexually ("without sex") on hemlocks, twice a year.

Almost all adelgids in each new generation are female. They are genetically identical, and all lay 100-300 eggs that develop into adults. This asexual reproduction helps the insect to spread rapidly. In the long run, the lack of the genetic mixing that comes with sexual reproduction may spell trouble for the hemlock woolly adelgid, but not so far.

What's to be done? You may be able to save favorite hemlocks in the yard by treating them regularly. With one or a few relatively small hemlocks, experts say it's possible to protect the trees by spraying horticultural oils to smother the insects, or by injecting systemic insecticides into the trees to poison the insects.

But in forests, where hemlocks grow in stands and may reach 100 feet tall, chemical treatments aren't practical or economical.

Some hope lies in the use of biological controls – other insects that prey on the hemlock woolly adelgid. Few insects in eastern North America do much of this, so known predators would have to be imported from elsewhere. Importing such predators can be tricky. For example, they might not work as expected. Or once released into our landscape, they might find something else to eat that they like better than the hemlock woolly adelgid.

Scientists are currently investigating certain beetles and testing them as biological controls. A Connecticut researcher discovered that a particular ladybug beetle (Pseudoscymnus tsugae) helps to keep the hemlock woolly adelgid in check in Japan. It has been released in infected stands of hemlocks in several places in Massachusetts, including Hemlock Gorge (in Newton, Needham, and Wellesley), the Blue Hills Reservation, the Arnold Arboretum, Walden Pond, and Sturbridge.

Researchers are also studying beetles from China (Scymnus ningshanensis) and western Canada (Laricobius nigrinus), as well as a fungus.

In the meantime, the hemlock woolly adelgid may change our landscape forever, just as the chestnut blight purged our forests of mighty American chestnut trees a century ago and Dutch elm disease eliminated American elms – Massachusetts' state tree – from America's main streets more than 50 years ago.