Pests with attitude

A recent summer-evening reverie on the porch was abruptly interrupted when a swarming mass of mosquitoes attacked, forcing us to seek shelter behind screens and solid walls. Later, as I pondered the irony of having been put to flight by something no bigger than a freckle and lighter than a hair, I began to wonder whether this is how a plant feels when it’s pushed from its home (or even killed outright) by an aggressive human, insect or disease.

Take a stroll through your garden and you’ll see some version of this scenario played out time and time again. Last season’s seemingly well-behaved perennial is suddenly finding its way into every nook and cranny; a bare piece of ground now harbors a vigorous plant that, if left unchecked, will wreak havoc on its neighbors. And overstressed plants — their natural defenses weakened by some combination of outright neglect, heavy pruning and unnecessary spraying and fertilizing — are left susceptible to invasion. Global commerce and travel (not to mention local growth and development) likewise open pathways and speed up the movement of aggressive and invading plants, insects and diseases.

The effects of developing and manipulating the land aren’t limited to individual plants. Such activities also alter plants’ habitat, creating disturbed, compacted sites devoid of topsoil and organic matter. Ultimately, these changes reduce the availability of nutrients, water and microbial communities, leaving little opportunity for life to thrive or dormant seed to grow, provide cover and renew the soil surface.

And unless these newly created corridors and open spaces are replanted and a proper cover or growing environment is re-established, it’s an open invitation for so-called “opportunistic” plants to establish a toehold. Over time, these newcomers flower, fruit and scatter seed near and far, gradually taking over the surrounding area by shading out, covering up, strangling and smothering the resident species. These invaders show no respect for anything in their way, and if left unchecked, they’ll become a true problem.

In our area, Oriental bittersweet could be considered the poster child of invasive weeds. This woody vine, introduced in the late 1800s from Southeast Asia, is popular for its bright-orange berries, a favorite of crafters as well as the birds and mammals that disperse the seed, helping the plant proliferate. The spread of Oriental bittersweet also threatens the genetic integrity of its native counterpart, American bittersweet, which readily hybridizes with the Oriental variety.

Each of us, on a small scale, can help control this invasive vine. The seeds are viable for several years, so it’s critical to reduce the seed source. Begin by cutting climbing or trailing vines as close to the ground as possible. The cut stump will resprout, requiring repeated cutting until the plant has expended its energies.

Starting early and continuing throughout the season is helpful. You can also pull seedlings and even small vines, but be aware that any roots left behind will resprout. Hand-pulling can be effective after a soaking rain and in more friable soils.

If expanded plant populations seem to call for heavy artillery, chemical applications to the cut stump or foliage are also effective. Again, remember that dormant seeds in the soil will continue to germinate over the years, requiring repeated effort. Happily, a considerable amount of information is available to help gardeners battling this persistent invader.

And if you’d like to get involved with the kind of larger-scale control that’s needed in this area, the Asheville Weed Team wants to hear from you. The group has declared war on the Oriental bittersweet, focusing its efforts along the Blue Ridge Parkway (last year) and in the Bent Creek area (this year). A series of workdays have been scheduled, and volunteers are needed. Training will be provided, and it’s a great opportunity to learn about this pesky vine firsthand.

Another chapter in this story concerns those plants that succumb to the predations of imported insect pests that ultimately threaten entire native populations. At the moment, the hemlock woolly adelgid seems to pose the most urgent threat. This tiny Asian insect was first detected in eastern United States in the 1950s. Since then it has made its way to our region, where it’s known to kill a several-hundred-year-old tree within a matter of years. The adelgid has no known predators in North America, and its incredible reproduction strategy — the entire population is female, and it reproduces asexually — produces tens of thousands of eggs a year. The insect feeds at the base of the hemlock needles, causing them to fall and the tree to slowly starve.

This is a frightening reality, as our native hemlocks (both the Carolina and the eastern species) play a vital role in both the region’s forest ecology and its urban landscapes. Native trout populations depend on this evergreen beauty’s dense shade. And the hemlock’s shade tolerance and resilience make it an important landscape plant, filling niches where other evergreens can’t go. Although the region has experienced other devastating pest invasions (both insect and fungal) in the past, the hemlock woolly adelgid poses an even greater threat, since there’s no substitute for the hemlock in our regional woodlands.

Here too, each of us can help monitor and control this devastating creature on a small scale. Regularly checking the undersides of the lower hemlock branches will disclose the adelgid’s arrival: Look for small white tufts at the base of the needles, which provide a protective cover for the insect as it reproduces and feeds. Depending on your budget and the number and size of the affected trees, you’ll be faced with some tough choices, such as which products to buy and whether to hire an experienced arborist. (To learn more, go to www.saveourhemlocks.org.)

State and federal agencies and nonprofits are conducting research on the adelgid and taking action in the areas of biological control; foliar applications of oils and soaps; and injections of systemic pesticides into trunk and soil. And if what we lived through in the ’80s and ’90s with the arrival of the dogwood anthracnose is any indication, the ornamental nursery industry, homeowners, environmental groups and maybe even the recreational community will get involved and help ensure that hemlocks continue to be part of our gardens and woodlands.

Because the adelgid is so species-specific, efforts to combat it can be more focused. Where the problem and the resulting research efforts have broader implications (as with Oriental bittersweet and other invasive plants), widespread education is needed as well as local action.

Ah, the balance of life. Some people say nature is playing out a divine plan or is finding a balance. If this is so, then it will all work out in time — right? In the meantime, however, it can be very hard to stand back and watch. As my mother recently observed, “If we can do something, we should.”

So whether it’s keeping the bittersweet vines out of the canopy and preventing seed dispersal, or keeping watch and caring for the hemlocks in our gardens, we can learn about these issues (and the many others that are sure to come). We can make an effort to support the survival of the living world immediately around us by acknowledging our detrimental as well as beneficial role, taking action, changing our behaviors — and learning to accept that nature will, in time, have its way.

To learn more about local and regional efforts to combat the invasion of Oriental bittersweet, check out www.samab.org and www.se-eppc.org; to learn more about the hemlock woolly adelgid, go to www.saveourhemlocks.org.

[Alison Arnold is director of horticulture at The North Carolina Arboretum.]