Which Invasive Insects Will Kill North America's Conifers? A Model to Predict It

The hemlock woolly adelgid, accidentally introduced from Japan. Tom Coleman, USDA Forest Service.

A growing threat

Nonnative insects arrive more and more often in North America, carried by global trade in wood products and nursery stock.

• Cost: damage by nonnative insects worldwide has been estimated at more than $70 billion a year. In the U.S., about $12.7 billion had been spent through 2020 responding to the emerald ash borer, a metallic wood-boring beetle from Asia that kills ash trees.
• Wildlife: losing trees harms the species that depend on them — the hemlock woolly adelgid's killing of eastern hemlock has hurt birds such as the black-throated green warbler and Acadian flycatcher.
• Everyone else: damaged forests offer less shade, beauty and recreation, and do less for biodiversity, water quality and carbon storage.

Yet only a small share of established nonnative insects cause widespread tree death. Predicting which ones will is the hard part.

A tool to predict the next invader

The High-impact Insect Invasions Working Group (HIIWG) — scientists from universities, the USGS, the USDA Forest Service and the Davey Tree Expert Company, funded by the USGS John Wesley Powell Center and the National Urban and Community Forestry Advisory Council — built a way to estimate whether a newly arriving insect will seriously harm North American conifers.

The nine-point impact scale developed and tested by the working group. USGS.

• The scale: nine levels, from low impact (no documented damage; minor damage, such as needle loss or cone damage), through medium (kills a stressed tree; weakens a tree so something else kills it; kills a healthy tree), to high impact (isolated deaths in a tree population; extensive or persistent deaths; a wave of mortality across a region; functional extinction of the host).
• The data: reviews of the literature on conifer specialists — nonnative insects that feed only on cone-bearing trees like pines, firs and spruces — went into the Traits and Factors Catalog (TRAFAC), a USGS database.
• The model: the group found which factors best predicted high impact, and built a statistical model of the chance that a conifer specialist not yet here would cause major damage if it became established.

The 58 already here

North America has 58 nonnative conifer specialists, in five insect orders and five feeding guilds — most of them sap-feeders (order Hemiptera) such as adelgids, aphids and scales.

• Impact: about 69 percent had little or no impact, 19 percent medium and 12 percent high.
• The high-impact ones were all sap-feeding Hemipterans — the balsam woolly adelgid, hemlock woolly adelgid and red pine scale — or needle-feeding Hymenopterans: the European spruce sawfly, pine sawfly and larch sawfly.
• Hosts: forty-nine of the nearly 100 native North American conifers are eaten by at least one; some host as many as 21. Most specialists feed on more than one conifer — some on as many as 16.

What predicts damage — and what doesn't

Insect traits: no help. Eight traits — generations per year, feeding guild, home range, sexual or asexual reproduction, number of offspring, pest status at home, number of tree genera eaten at home, and how it disperses — had no predictive value, alone or together.

Tree traits: some help. Of six conifer traits (foliage texture, growth rate, drought, fire and shade tolerance, wood density), trees highly shade-tolerant but not drought-tolerant were more likely to suffer high-impact damage — many firs, spruces and hemlocks. Eastern hemlock seedlings survive in just 5 percent sunlight, but the tree tolerates drought poorly — and it's dying widely from the adelgid. Scientists had assumed shade-tolerant trees evolved strong defenses; instead, once an insect gets past them, such trees may be unable to withstand the feeding.

Evolutionary distance to the insect's home host: important. Researchers compared how long ago (in millions of years, Ma) the insect's home-range host and the North American tree diverged:

• foliage-feeders: if the trees split 5 to 1.5 Ma ago, there's a 75-percent chance of high impact; much less for longer or shorter splits;
• sap-feeders: the greatest chance of high impact is for trees that split 17 to 12 Ma ago.

One explanation: trees that split recently may still share their relatives' chemical defenses, and hold the insect off; trees that split long ago may have become poor food for it.

A native relative on the tree: protective. If a native insect of the same genus already feeds on the conifer, high-impact damage is less likely — perhaps because the tree's defenses against the native also deter the newcomer, the native's natural enemies attack it too, or the two compete.

The model's verdict

The model worked best combining tree traits, divergence time and the presence of a native congener. It puts the odds that a given conifer specialist will have high impact on a given North American conifer at anywhere from 1 in 6.5 to 1 in 2,858.

What's next

The group is:

1. applying the model to insects not yet established but likely to arrive through trade or by accident;
2. studying what predicts the impact of insects that feed only on single families of broadleaf trees and shrubs;
3. studying generalists that feed on many trees;
4. building a software tool, through the national i-Tree suite, to judge the risk from newly arriving tree-feeding insects.

The High-impact Insect Invasions Working Group at work. Nathan Havill, U.S. Forest Service.

Why it matters

By focusing on the most dangerous insects, managers can direct money to prevention and write policies to keep high-impact insects out — and concentrate on the conifer populations most at risk, keeping forests healthy for future generations.

Sources

Based on A Not So Sudden Impact—Historical Relations Between Conifers and Insects Can Help Predict Damage by Nonnative Insects, USGS Fact Sheet 2020–3039, by Lekeah A. Durden, Ashley N. Schulz, Angela Mech, Kathryn A. Thomas and others, U.S. Geological Survey, drawing on Mech and others (2019), Ecology and Evolution, and Schulz and others (2020), NeoBiota; a work of the United States government in the public domain. Its figures and federal photographs are taken from the fact sheet's PDF; photographs credited to universities and private organizations are not reproduced.