By Ed Ricciuti
Locations with shorter and cooler seasons may prevent invasive spotted lanternflies (Lycorma delicatula) from developing into adulthood shielding lucky homeowners in northern and upland areas from the glop and sooty mold left behind as the gaudy but destructive pests suck the sap and life out of myriad trees and crops.
In a study published in September in Environmental Entomology, a two-year project conducted by a team of Penn State University scientists provides strong evidence to support predictions that the spotted lanternfly’s potential spread will be limited by increasing altitude and latitude, saving places like the Appalachians of North Carolina and Green Mountains of Vermont from its depredations. Even in those places, however, a span of not much more than a proverbial stone’s throw or two away can make a big difference in the insect’s survival prospects.
The primary focus of the paper, says lead author Dennis D. Calvin, Ph.D., recently retired from the College of Agricultural Sciences at Penn State, “was to study the seasonal development of spotted lanternfly populations and develop mathematical equations based on seasonal degree-day accumulations to estimate the timing of key life-stage activity periods.”
Degree Days and Spotted Lanternfly Development
In entomology, a degree day (DD) is a measure of the time and extent to which temperatures are within the range that allows an insect to develop. For an insect to develop fully, it needs a particular number of degree days during which the temperature sits at a level that enables its development through each life stage. It is the so-called “Goldilocks Principle” at work—not too hot, not too cold, but just right. If the average temperature on one day is, for example, 10 degrees above an insect’s base threshold temperature for development, then 10 degree days are accumulated that day. The base threshold of development is the temperature at which insect development is essentially zero. Entomologists use accumulated degree days (ADD) to calculate the total heat demands an insect needs to develop through a stage or its entire life cycle. With ADD, scientists can peek into the hidden facets of insect growth and thus figure out the total heat demands required for survival.
The processes described in the new study should produce accurate predictions of the suitable geographic range of the spotted lanternfly by evaluating its potential to complete a life cycle from spring egg hatch to fall egg deposition based on season length, expressed as ADD, at a given location.
Equations that predict the timing of lanternfly life stages could be a critical tool for pest management practices, such as monitoring and activating controls. Using the equations, the Penn State researchers were able to estimate areas lacking enough accumulated degree days for lanternflies to reach the adult stage, lay eggs, and gain a foothold.
“If the timing of key life stages, as seen in field data, is consistent enough across years and/or locations to make sound management decisions, then pest managers can effectively time monitoring, surveillance, scouting, and control tactics,” the researchers write.
Calculations by the research team suggest that areas with fewer than 991 ADD are unlikely to provide enough time for females to emerge, mate, and develop mature eggs for deposition. “Thus, since the northern, higher elevation in the Northern Appalachians, Catskills, Adirondacks, Green, and White Mountains and the Allegheny Plateau region do not accumulate enough ADD for 1 percent adult emergence, these areas may have a very low risk of spotted lanternfly population establishment,” they write. Most of Maine, highland and lowland, is marginal for lanternfly reproduction. Other areas that do not provide enough season length to reach the adult stage include high elevations in the Appalachian Mountains of North Carolina, Virginia, West Virginia, and Pennsylvania and the Northern Allegheny Plateau in Pennsylvania.
The location they chose for their investigation of the lanternfly’s seasonal activity—on red maple and tree-of-heaven host trees—was a housing development in Wyomissing, Pennsylvania and the adjacent woodland. The inch-and-a-half-long insect was first detected nearby in Berks County, Pennsylvania, in 2014. It has now expanded its range to at least 14 states. The study site has similar climate to that of its range in China, where natural foes control it.
Using 10.4 degrees Celsius as a lower threshold of development, the researchers related key life stage activity periods to the day of the year and ADD, starting January 1. They came up with mathematical equations for nymphal instar, adult, and fall egg-mass deposition activity. Another set of mathematical equations was engineered for adult and fall egg-mass deposition periods using the first observation of adults as a starting date. With these equations, they estimated the geographic range where spotted lanternfly can potentially complete a partial or full life cycle, from spring egg hatch to fall egg-mass deposition.
Where Temperatures Do (and Don’t) Suit Spotted Lanternfly
Not surprisingly, the impact of both latitude and altitude seems to have a major effect on potential lanternfly expansion, as average temperature decreases with increases in altitude or latitude. Beyond that, the fact that altitude can offset the impact of latitude can have profound implication on lanternfly habitat, the researchers explain: “In the north, the Hudson, Connecticut, Delaware, Merrimack, and Susquehanna River valleys and the area between the Catskills and Adirondacks Mountains, where the Erie Canal ran, are lower elevation and have longer season lengths than the surrounding area. In very short distances, the seasonal ADD can drop by 500 or more.”
Lanternflies threaten crops such as almonds, apples, blueberries, cherries, peaches, grapes, and hops, as well as hardwoods such as oak, walnut, and poplar. Typical of plant hoppers, they chew into stems and branches of plants to suck out sap, causing wilting, leaf curling, and dieback. To make matters worse, like aphids, the lanternfly excretes sugary honeydew that attracts bees and wasps and feeds the growth of black sooty mold, which discolors and weakens plants and also makes a mess on patio furniture, cars, and anything else on which it grows. The honeydew problem is accelerated when lanternflies congregate, as they commonly do.
Calvin says that the equations developed may make a big difference in lanternfly control. “It is hoped that, by having equations that can be used to predict the timing of key life-stage activity periods, pest managers can use this information to better time monitoring, surveillance. and control activities. It can save time and money by giving a smaller window to time these activities and also show how the timing of these activities varies across geographic locations,” he says.
More Clues for Spotted Lanternfly Management
The Penn State scientists also turned up hints of other aspects of lanternfly behavior that, down the road, could boost control measures. Early in the season, the dominant sex of lanternflies on red maples was male. As fall and the reproductive season approached, however, females began to show up on the maples and then laid their eggs. The significance of egg laying close to the fall equinox suggests that its timing is not driven solely by degree days but possibly also influenced by another environmental signal such as day length that allows the insect to determine the season is drawing to an end.
Co-author Julie Urban, Ph.D., also of Penn State, says that “the movement of adults that we report could potentially have implications for the monitoring that is done at satellite or ‘pop-up’ populations of SLF found in previously uninfested areas.” In other words, monitoring should be extended beyond the initial site of infestation and host species.
Ed Ricciuti is a journalist, author, and naturalist who has been writing for more than a half century. His latest book is called Bears in the Backyard: Big Animals, Sprawling Suburbs, and the New Urban Jungle (Countryman Press, June 2014). His assignments have taken him around the world. He specializes in nature, science, conservation issues, and law enforcement. A former curator at the New York Zoological Society, and now at the Wildlife Conservation Society, he may be the only man ever bitten by a coatimundi on Manhattan’s 57th Street.