Managing a pair of invasives

A Ph.D. student in the department of Forest Resources & Environmental Conservation and an affiliate of the Invasive Species Collaborative at Virginia Tech, Harrison Miles has considerable experience with invasive species.

One troublesome invasive he’s studying is the spotted lanternfly (SLF). Although this leafhopper feeds on more than 70 species in the U.S., its favorite is the tree-of-heaven (ToH).

“We can’t talk about tree-of-heaven without talking about the spotted lanternfly,” said Miles. “ToH is one of the most prolific tree species in the U.S. It’s the fastest- growing tree in North America and can reach six feet in a single growing season.”

ToH can sprout from cut roots (up to 27 m away) and regrow aggressively. Because of this, the best mechanical control solution is “hack and squirt” – cutting followed by immediate application of glyphosate or triclopyr. Mulch from chipped ToH can resprout from intact meristem tissue in chips.

“It’s also allelopathic,” said. Miles. “ToH produces an allelochemical, with highest concentration in its roots. Once it’s in the soil, it’s associated with reduced germination and growth of competitive plant species.”

ToH was introduced in the U.S. in the 1780s by a European colonist who planted it at his Pennsylvania estate. The SLF arrived in Pennsylvania between 2011 and 2014.

“SLF and tree-of-heaven evolved together, which makes them an interesting system to work on,” said Miles. “Not many invasives have this co-evolution story in their native range then find each other on the other side of the planet.”

SLF eggs hatch between October and June, become early nymphs in May, then late nymphs from July through September. Adults emerge between September and December. During SLF development, its affinity for ToH increases. As early nymphs progress to adults, they prefer to feed almost exclusively on ToH. However, they can develop fully without access to ToH.

One of the major issues in the U.S. is SLF’s affinity for grapes. “They don’t feed directly on grapes,” said Miles. “They feed on the vine, so they’re stealing sugars and reducing yield. They can triple the cost of insecticide use and can kill the grapevine.”

Vineyards close to wooded edges where ToH thrive are at highest risk, and SLF that have access to both grapes and ToH become larger and reproduce more.

Biological control involves using living organisms to suppress pest populations. The main benefit of biocontrol over conventional pesticide treatment is once the organism is placed in the environment, it can exist and reproduce.

“In invasion biology, we have explanations for why invasives do so well,” said Miles. “One of them is the enemy release hypothesis, which suggests invasive species have an advantage over native species because they’re released from the stress of their natural enemies of their evolved native range. The benefit of biocontrol in invasive species management is trying to narrow the gap in enemy release.”

Miles outlined several of the most promising biological controls for ToH. The Ailanthus webworm moth, native to Central America, arrived in North America at about the same time as ToH. The moth produces up to four generations each year and damages ToH throughout its growing season. Caterpillars feed on leaves and can completely defoliate trees. It’s possible that the Ailanthus webworm moth is a pollinator of ToH, which is less than ideal, but isn’t a dealbreaker for its use as a biocontrol.

Another biocontrol possibility is the Ailanthus trunk weevil, a serious pest of ToH in China. This weevil was brought to the U.S. in 2005 and has been in quarantine at Virginia Tech for the past 20 years. Adults feed on ToH foliage, lay eggs in the bark and larvae feed under the bark.

“A study … found the Ailanthus trunk weevil is capable of vectoring Verticillium, which is also a natural enemy of ToH,” said Miles. “If we have a biocontrol that could be vectoring another biocontrol, it would be a powerful tool for managing ToH. This species is the second closest to actual release and is currently under review by USDA- APHIS for permitted experimental release.” There’s optimism for field trials on the trunk weevil beginning next year.

The most studied option, and likely the best, is Verticillium nonalfalfae. “ Verticillium is a genus made entirely of plant pathogens,” said Miles. “Some Verticillium species are problems in crop systems, which is an obstacle to people recognizing Verticillium as a positive thing. V. nonalfalfae is found wherever ToH is found in China, Europe and the U.S.” In the U.S. it’s been found naturally in Virginia, Ohio and Pennsylvania.

Verticillium is a soilborne fungus and infects trees through the roots. One of the reasons Verticillium is a superior choice for killing ToH is its ability to spread via roots. It can survive in both roots and soil, but because ToH forms interconnected root systems through functional root grafts, roots from different trees join and share fluids. Because any stand of ToH is fairly connected, Verticillium remains in the environment after it’s been introduced.

The Verticillium strain under study at Virginia Tech is highly specific to ToH and has been tried in many different host plants in the U.S. It only causes disease in ToH and is highly virulent and effective at killing ToH.

The Virginia Tech studies involved springtime inoculation of ToH with liquid spores, after which the xylem takes them up. The current study application rate is 25 ml/0.1 acre (about 400 square feet) to treat every tree in a stand that size.

Symptoms of tree death are evident within four weeks. Symptoms include chlorosis, wilt, vascular staining, defoliation, epicormic sprouting and eventual death in about four months. The tree responds to compartmentalize the infection by blocking water and nutrients and effectively kills itself with its response to the infection.

“Verticillium doesn’t have an obvious means of dispersal,” said Miles. “It doesn’t sporulate on the leaf surface, and we don’t think wind is a big dispersal mechanism for it. Humans and potential insect vectors such as the trunk weevil and maybe even lanternfly can spread it.”

by Sally Colby