Forest Pests are Exploding

[Paul J Hetzler]

Forest Health Forecast

If you’re tired of hearing about new invasive forest pests, I’m right there with you. Seems they arrive at an ever-increasing pace, and the harm potential ratchets up with each newcomer. At this rate maybe we’ll get a wood-boring beetle whose larvae explode, which would put things in perspective. As distasteful as it is to peer at the cast of ugly new characters, it’s better to know what we’re up against.

Novel pests put everyone on a steep learning curve. For the species below, key questions remain unanswered, and some of what we “know” will undoubtedly be proven wrong. Still, it’s worth the effort to take stock of the near-term threats to our woodlands.  

The spotted lanternfly or SLF (Lycorma delicatula), a large (2.5 cm) plant-hopper native to Southeast Asia, has a literal bright side – its color pattern. Scarlet-red hindwings and a fat, cheerful-yellow abdomen make the adults easy to detect. It has a one-year life cycle, developing through four juvenile (nymph) stages in summer before becoming adults in August. Nymphal stages1-3 are black with white spots, with a body shape similar to snout beetles. Red patches appear on the fourth instar.

October to December is SLF egg-laying season. Females oviposit on smooth-barked trees, rocks, and especially on rusted steel surfaces. They cover egg masses with a camouflaging waxy grey substance that dulls and cracks with time. Egg masses are the most common life-stage transported by humans, with vehicles often serving as SLF mobile homes.

Discovered in Pennsylvania in 2014, SLF found its way to North America as egg masses on a pallet of stone from China. Either Amazon Prime is out of control, or folks are confused about where rocks grow. We could’ve sent stone for cheaper, and avoided a lot of trouble.

Right out of the box, SLF gave foresters nightmares, with good reason. Through sheer numbers, these bugs suck the life out of more than 70 species of woody plants. Their favorites include those in the Acer, Juglans, Malus, and Vitis genera, some of the most economically important.

The main host of spotted lanternfly, though, is tree-of-heaven (Ailanthus altissima), itself an Asian import. Not widely established in northern NYS, it’s found in all urban centers below the Adirondacks. Lanternfly may require a drink of Ailanthus sap to reproduce successfully, but this has not yet been proven. If correct, it would spare most of us from the spotted scourge.

Truly encouraging news broke this summer: Cornell University researchers found two native soil fungi (Beauveria bassiana, Batkoa major) causing widespread SLF mortality at one Pennsylvania site. In a 27 August 2019 article in the Philadelphia Enquirer newspaper, Penn State entomologist Dave Biddinger commented on field trials using B. bassiana. With lanternfly mortality running about 50%, “It is working better than we originally thought,” he said.

At the other end of the Obvious Spectrum is an even greater threat to forests, Asian earthworms, which have cleverly disguised themselves as earthworms. The fact that we are used to seeing worms in the landscape makes them a challenge to find, but it’s worth the effort.

The new pests are three related East Asian species, Amynthas agrestis, A. tokioensis, and Metaphire hilgendorfi. Known variously as Asian jumping worms, snake worms, crazy worms, and a few choice expletives, these super-size (20 cm when mature) annelid cousins look similar to one another. They can only be accurately separated by species through dissection, so it’s fair to consider them as a group.

They can be identified by a smooth band called a clitellum, which in European earthworms is dark, close to their middle, and thicker than the body. In Asian worms it is milky-grey to white, and much closer to the head. It is generally flush with the body as well. Behavior is a clue: when disturbed they scatter snake-like across the ground rather than down into the soil. If touched, they thrash wildly and may shed their tail. They feel drier and more turgid than other worms.

Asian worms have a high reproductive potential: 2 generations per season compared to one for European worms. The latter are hermaphroditic, having both male and female organs, but still require a mate. Asian worms are parthenogenic, females which bypass the need to find a date, and spew out cocoons teeming with baby female worms. It takes but one to start an infestation.

Adult Asian worms die off in winter, but their cocoons, tan to brown spheres about 2 mm in diameter,  are cold-hardy. Some references place the limit around -40, so they’re hardy to much northern NYS. Also, cocoons remain viable for at least 3 years, analogous to a soil seed bank.

Asian worms consume most of the organic matter at the surface and within the top 2-3 cm of soil. Research done at the University of Wisconsin-Madison found they reduced leaf litter in hardwood forests by 95%. I have seen an infested forest in Cortland County, and it was entirely bare ground. Soil also loses its healthy structure, becoming granular, much more vulnerable to erosion and compaction.

Lignin-busting enzymes increase dramatically when Asian worms invade. It is unclear whether they secrete these enzymes, or somehow induce endemic white-rot fungi to ramp up production. We do know that wood breaks down at least 2.5 times faster in the presence of Asian worms. Other forest-ecosystem changes at infested sites include the loss of soil invertebrates, juvenile salamanders, native wildflowers, and ground-nesting birds. Possibly due to toxins on their skin, Asian worms have few predators. Raccoons, moles, and centipedes eat them, but many birds and amphibians refuse.

The stakes are high. Wisconsin DNR Invasive Species Specialist Bernadette Williams, who broke the Asian worm story in the northern US, put it bluntly: “Their introduction into our state poses a huge threat to the future of our forests.”

Asian worms are spread through the horticulture trade, both in containerized plants, and mulch. Think twice about getting fill or other material delivered to your land. Where reforestation is concerned, it’s safer to plant bare-root stock than plugs.

Equipment poses the biggest threat to forest land. Before a logging crew shows up at your place, find out where their last job was. Skidders, forwarders and trucks coming from a known infested site may need to be steam-cleaned to protect your property.

If you’re in an unfamiliar woodlot in summer or fall to mark timber, bid a sale, or for a walk, turn over a log or flat rock. Do the worms dive down, or scoot out in all directions? If the latter, be sure to notify the owners, as well as any crews working on that parcel. Asian worms are most likely to be found at log landings and along skid roads. Clean all dirt and debris off your boots, and preferably disinfect them as well, before getting in the vehicle to return. Worm cocoons are all too easy to take home with you.

And don’t look now, but the sky is falling. Again. It’s poised ominously over our hemlock trees, whose cathedral-like groves provide shade to many a stream and glen in NY State. It’s hard to believe these titans of the forest can be killed by a tiny insect less than 2 mm long. Native to Asia, the HWA or hemlock woolly adelgid (Adelges tsugae) is moving north faster than expected. Closely related to aphids, adelgids insert piercing mouthparts and drain sap from twigs and branches. When populations are high they can kill a mature hemlock in 3 years.

Their name comes from waxy filaments they make to protect themselves from desiccation. In heavy infestations, hemlocks can look gray from the “wool” on branches. HWA can’t fly, but are spread by wind, and hitch rides on the feet of birds, which can carry them long distances.

Experts long thought that low temperatures would limit the range of these hemlock vampires, but it appears they are fast becoming cold-tolerant. In January 2014, a low of -31C only caused 82% mortality. As they survive cold winters, the HWA genome shifts toward an increasing tolerance of northern latitudes. 

Like Asian worms, hemlock woolly adelgids reproduce asexually; adult females produce female offspring without mating. Another unusual feature of HWA is that they’re less active in summer, feeding heavily from October through early spring. In eastern North America, woolly adelgids have no native predators. Left unchecked, infestations are lethal in 3 to 10 years depending on initial tree health.

There is hope for controlling HWA. Chemical treatment to eradicate early infestations in the wild or to preserve landscape specimens is effective and economical. Depending on the insecticide, one application sprayed on the lower trunk can impart protection for up to 7 years.

In the long term, obviously, the answer is biocontrol. Cornell Forest Entomologist Mark Whitmore says “It will likely take a complex of natural enemies to maintain HWA populations below damaging levels.” Since HWA biocontrol efforts began in 1993, six species have been released, three in NYS. Predacious and parasitoid insects include Laricobius nigrinus, two species of Scymnus beetles, and Leucopis argenticollis, a fly. Most are from the Pacific coast.

As with any invasive pest, early detection is critical, and can make a real difference in your woodlot or favorite camping spot. In 2017, a hiker noticed a small HWA infestation in the southern Adirondack Mountains of NY State. He knew about HWA, reported it, and likely averted a major crisis. Today the infestation is believed to be eradicated.

Another threat to our forests is a pathogen called oak wilt (Bretziella fagacearum). First identified in Wisconsin in the 1940s, it is now in twenty-four US States. It may have originated outside North America, but no one knows for sure. What is certain is that it’s a death-knell for red oaks, killing healthy red, black, pin, and other red-type oaks in 2 to 6 weeks. Like Dutch elm disease, it plugs xylem tissue, blocking water uptake. Symptoms are sudden browning of leaves, usually in July or early August, followed by leaf drop. Oak wilt spreads easily, and there is no known treatment.

Infected white-type species such as bur and swamp-white oaks may live from 1 to 3 years before dying. But that’s not good news, as an infected white oak can spread disease to other oaks through wounds caused by pruning or breakage, while not showing symptoms. Curiously, European white-type species succumb just like our red-type oaks.

Oak wilt is transmitted in two general ways; below and above ground. We know a tree’s roots extend 2 to 3 times its branch length, and as they touch roots from related species, they form graft unions. We understand little about this “tree internet,” other than that chemical signals as well as nutrients can be shared. Root grafts also spread blights like Dutch elm disease, and now oak wilt. Such tree-to-tree spread, while important, is more limited than airborne transmission.

Above ground, oak wilt is naturally vectored by insects, mainly sap beetles in the family Nitidulidae which carry spores from infected trees to fresh wounds on healthy oaks. The oak wilt fungus only makes spores on red-type oaks, on which the pathogen makes sweet-scented sporangial pads to attract insects. Any species of oak can be infected by a spore-covered beetle which lands on a fresh wound.

We are the other vector, of course. In 2018, infestations cropped up in NYS more than 400 kilometres from the nearest known source, presumably as a result of firewood transport. Changing a few habits will help keep this pathogen at bay. The first is to not bring firewood home with us when traveling. Between 1 April and 1 July, the danger of spreading oak wilt is extreme. Any exposed, freshly cut (or broken) oak is at high risk. From July through September, the hazard is moderate. Obviously we may need to harvest timber or repair natural breakage during this time frame. It’s critical to spray-paint each wound or stump immediately after cutting. I emphasize right away because nitidulid beetles can find a fresh oak wound in less than an hour. If this pathogen gets established here, our oaks could go the way of American chestnuts. Adopting new behaviors will vastly reduce the chance of having oaks disappear on our watch.

Beech-leaf disease was first noticed in Ohio near the shores of Lake Erie in 2012, and by 2017 it was found in Ontario. The causal agent remained a mystery until 2018 when Dr. Qing Yu of Agriculture and Agri-Food Canada identified a nematode (Litylenchus crenata) within affected beech leaves. It is still unknown if it is the only factor or if other elements are involved as well. In Ohio, the longest-affected stands show 7% mortality this year. Phosphate soil amendments may help slow the disease, and a mite identified in Canada is under study as a biocontrol.

Before your eyes cross permanently I need to mention white pine decline. We’re familiar with Lophodermium and Sphaeropsis as dyed-in-the-wool pathogens of two-needle pines. Beginning about 2010, white pine foliage began to noticeably thin in some regions. By 2018, the problem had spread through most of northeastern US, with significant mortality on less-optimal sites.

Unlike with Scots and red pines, white pines are being killed by non-virulent, native saprophytes. Four species have been identified: Lecanosticta acicola, Lophophacidium dooksii, Bifusella linearis and Septorioides strobi. The latter may be exotic, but that’s a moot point since this organism is absent from many of the sites with white pine decline.

In addition, a native stem canker, Caliciopsis pinea, has begun to act up. It causes pitching, crown thinning, and decline. Think white pine blister rust without the hassle of alternate hosts. This is a case where changed weather patterns may be the real issue. Most years since 2000 see periods of 20 - 30 days of consecutive rainfall, far outside the historical range.

A little prevention goes a long way to protect our forest resources. Let’s all do our part to keep new pests, particularly exploding beetles, out of our woodlands.

Paul Hetzler is an ISA-Certified Arborist and a member of NYS Arborists and the Society of American Foresters.

[Paul J Hetzler]

Thinning to minimum stocking levels may help. Also, research on the benefit of nitrogen fertilizer application is ongoing.