Strategies for the struggle against spotted lanternfly
by Courtney Llewellyn
Despite stay-at-home orders that stymied some travel in 2020, a lot of traffic still crisscrossed the Northeast and the upper Midwest. Families on road trips and the transportation of goods resulted in certain hitchhikers finding their ways to new areas. Spotted lanternflies (SLF), first reported in Pennsylvania in 2014, have now been found as far north as Maine and as far west as Michigan.
But what makes SLF different from other invasive species? Their high mobility and their highly polyphagous diet (meaning they eat basically anything with sap in it), according to Brian Walsh, Penn State Extension educator.
“We probably went a few years before 2014 without it being noticed because the populations were low,” Walsh said. “As human beings, we keep giving it an advantage it wouldn’t otherwise have.”
Fortunately, SLF only has one lifecycle per year. They overwinter as eggs in our temperate climate. However, where there’s no killing freeze, Walsh thinks we may see multiple lifecycles a year. (It hasn’t been proven yet.) Eggs are laid in autumn, but they’re easy to miss. They’re most commonly found in slightly sheltered areas – under branches, under car bumpers – but egg scraping is not a very effective method of control, because the majority of eggs are laid out of reach, sometimes as high as 29 feet up a tree. It was noted maples are preferred hosts for the egg-laying phase.
Once they hatch in spring, SLF nymphs are often mistaken for ticks, due to their size and black color. Egg hatch is based on growing degree days (which can be tracked through pestwatch.psu.edu/GDD/hatch_expert.html). The first through third instar nymphs feed on tender plant tissues, Walsh said, and they’re constantly on the move when they’re not feeding. He noted roses are great indicators of populations – “they love roses,” he said.
Fourth instars generally begin to appear in July and start to cluster on preferred hosts: the invasive Tree of Heaven as well as black walnut. They molt into adults around late July. Although photos tend to show their colorful adult phase, Walsh said you will not see their bright red hind wings unless they’re flying or they’re startled.
How bad is the struggle against the pest, though? “Are they killing trees? Not really,” Walsh said. “SLF is a plant stressor. The sooty mold that is a result of their honeydew blocks the light required for photosynthesis. Nutrient loss to massive numbers of SLF feeding can kill limbs and weaken plants.” Forced pressure feeding experiments conducted in 2019 and 2020 by Penn State showed suppressed photosynthesis by up to 74% in Tree of Heaven with moderate infestation, and transpiration was suppressed in silver maple as well. “Increasing SLF may be shifting carbohydrate cycling in tree species differently by altering photosynthetic capacity and/or carbohydrate availability in the phloem,” the study found. “This may have implications for species tolerance, tree health and growth.”
“It’s a little bit staggering when you see how they pile up,” Walsh said of infestations. “The number we documented off one silver maple in Berks County was 43,000.”
Walsh said targeted treatments are needed to kill SLF, as there is no way to prevent the pest from moving onto a property. Penn State Extension reports that the most effective trap for SLF is a funnel-style trap, or a “circle trap,” which wraps around the trunks of trees. Nymphs and adults are guided into a container at the top of the funnel as they crawl up the trunks and move upward to feed on the tree.
Another method is using sticky bands, which capture SLF in sticky material as they move up the tree. Sticky materials are not selective, however, and can capture other animals including pollinators, butterflies, birds and squirrels. Penn State recommends against using sticky bands unless an appropriate wildlife barrier is installed around them.
While tree traps (non-chemical methods of killing) are relatively easy to install, it may be difficult to avoid catching unintended, non-target creatures. Penn State Extension recommends only using tree traps where you see SLF feeding or observe SLF crawling up trees. They suggest banding infested trees as soon as SLF hatches (late April – June). The trapping is not effective on bushes or most vines because they don’t have large enough diameters for the traps. The traps should be placed about four feet from the ground and tightly secured against the tree. (Traps will work best on trees with smooth bark, as bark with deep grooves may allow SLF to crawl underneath the trap.) Material should be wrapped tightly and secured by staples or pushpins. To preserve the health of the tree, avoid using nails or wounding the green, living tissue under the bark. Remove all parts of the trap at the end of the season.
While it may seem like removing the preferred host, Tree of Heaven, could be an effective strategy, eliminating all of that species in the U.S. is not feasible or cost effective. However, as a preferred food source for later instars and early adults, using Tree of Heaven as a trap tree should be considered when developing a control program.
“Grapes are at risk, but great strides have been made in developing management strategies,” Walsh said. “I’m going to say apples and peaches are not preferred hosts – they’re not at huge risk unless the insects are able to become a disease vector.”
He added that all general insect predators eat SLF, including many bird species, but not in big numbers. As for introducing a species that consumes the pest in large quantities, he said, “It’s probably going to be a decade before something is identified that can be released here that won’t negatively affect our environment.”
The hitchhikers need to be stopped as well. “Managing SLF when shipping is costly, but it’s necessary,” Walsh said. “They are great at hiding in nursery stock. We don’t have any silver bullets for large populations, but do have control options for ‘pop-ups.’” He noted that spraying with common insecticides often works. He also recommended shipping early in the morning, when they’re least active. Quarantines vary from state to state, so growers and shippers need to make themselves aware of them.
Walsh said the key right now is to pay attention to new research as it comes out. A detailed source of information is extension.psu.edu/spotted-lanternfly, which includes directions on how to build traps.
“Be aware of where you are,” Walsh said. “And if you find it, report it.”
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