(Millbrook, NY) In June, Cary Institute’s hardwood forest — stripped of its foliage by hordes of invasive spongy moth caterpillars — more closely resembled a savanna. Standing in a grassy area doused in sunlight, Kelly Oggenfuss pointed out, “This spot would normally be a lot shadier, and the ground would be covered in leaf litter.”
Oggenfuss has been collecting data at this site for 25 years as part of Cary’s long-term project studying the ecology of tick-borne diseases, and she had never seen it like this before.
Cary’s campus in Millbrook, NY, was hit hard by the invasive spongy moth this spring and summer. Defoliation of oak trees — the pest’s favorite food — reached nearly 100%, and other species, including evergreens, suffered significant damage. Cary was not alone; the caterpillars denuded much of the Mid-Hudson Valley region before they died or retreated into cocoons, allowing the trees to unfold new leaves.
Because Oggenfuss and the tick ecology team — led by Cary disease ecologists Richard Ostfeld and Shannon LaDeau — had already been studying how heat and moisture at the ground level shapes tick survival, the team realized the sudden increase in light due to defoliation might have serious implications for ticks and the diseases they spread to humans. So they applied for, and quickly won, a RAPID grant from the National Science Foundation. RAPID grants are fast-tracked to allow scientists to study unanticipated and fast-changing conditions.
With an award of $179,544, the team sprang into action, scaling up the experiments they had already started. The one-year project is assessing how spongy moth defoliation shapes the survival of blacklegged ticks, the main vectors of the pathogens that cause Lyme disease, babesiosis, and anaplasmosis.
“We know from our previous research that if it's very warm and dry, that's really bad for some life stages of ticks,” said Ostfeld. “So if this defoliation by the spongy moths is changing temperature and humidity conditions on the ground, it could influence their survival, and as a consequence, our risk of getting sick from tick-borne disease.”
To examine the impacts on ticks, the team set up 144 mesh enclosures. The enclosures are bags made of white organza, the same material often used in wedding party favors. However, the contents are definitely not suitable for a party. Each bag contains a small cylinder of the natural layer of soil and leaf litter, two data loggers the size of watch batteries that record temperature and humidity, and a certain number of blacklegged ticks. The crew ties each bag, trapping the ticks inside, then comes back in a few weeks to count how many survived.
Under the RAPID grant, the tick enclosures are evenly deployed in three kinds of conditions: heavily defoliated areas, areas with lower defoliation, and in defoliated locations under a shade cloth to simulate an intact canopy. The team will measure tick survival under each condition, and with each tick life stage. They started with larvae in September. Next they’ll deploy adults in November, and nymphs in May 2025, matching their natural seasonal patterns.
Ostfeld and LaDeau hypothesize that ticks in the most defoliated areas will experience warmer and drier conditions, and therefore higher mortality rates. However, ecological responses to change are notoriously complex and unpredictable, and the team is prepared for nature to throw a curveball. For example, the scientists have already observed that more light coming through to the forest floor has allowed understory plants to thrive in areas where they’re normally not found.
“The crew was blown away by the degree to which grasses and forbs underwent this amazing flush of growth in the understory,” said Ostfeld. “So in terms of the impact on ticks, it could go either way. It could be that the loss of leaves from the trees makes it hotter and drier and kills a lot of ticks. Or it could be that the loss of leaves from the trees makes conditions just lovely for ticks, because of all this flush of greenery.”
In another interesting dynamic, the flush of ground-level plants seems to have welcomed large numbers of meadow voles into the study sites, where they’re not commonly found. This influx could mean that many ticks will feed on voles instead of mice and chipmunks. Voles are less likely to pass pathogens to the ticks, and may be more likely to kill the ticks that bite them, and therefore could influence tick survival and disease risk.
Cary scientists are uniquely poised to keep a finger on the pulse of these dynamics, as they’ve been studying interactions like these for almost 35 years.
The long-term project has revealed connections between acorn production, rodent population size, and ticks that carry Lyme disease bacteria. When oak trees drop a lot of acorns in the fall, for example, the all-you-can-eat acorn buffet can lead to surges in populations of white-footed mice and other rodents the following year. Ticks are then more likely to feed on these rodents, which happen to be very good at sharing the bacteria that cause Lyme disease. Hence, more ticks are then capable of spreading Lyme disease if they bite a human the following year.
Over the years, the team has investigated many other ecological dynamics surrounding these interactions. Predators such as foxes and bobcats can lower Lyme disease risk, they learned, possibly by killing and eating rodents, and/or serving as an alternate food source for ticks. The team also discovered that years with high numbers of mice and chipmunks can be devastating for birds that nest on the ground, as the rodents are more likely to eat their eggs and young.
This isn’t the first time spongy moths have been included in the long-term experiment. Many years ago, the project revealed that when mice are abundant, they have some capacity to regulate spongy moth populations by eating the moth’s cocoons.
“Now we’re coming at it from a new angle,” said Ostfeld. “It’s the same players, but different interactions.”
As part of the long-term project, the team had already deployed soil cores and were tracking microhabitat data when the spongy moth infestation exploded. Those data will no doubt prove useful in understanding how defoliation affects tick survival, but the RAPID grant allowed the team to rapidly scale up their efforts.
“The level of defoliation this summer was a surprise to all of us,” said LaDeau. “It's pretty remarkable that we were able to respond and get funding and start studying it as quickly as we did. And a lot of that wouldn't have happened without the longer-term project in place.”
The new study will clarify how dramatic ecological changes — from spongy moth infestations to the hotter and drier conditions projected in some locations — influence tick survival, and what that means for people. LaDeau and Ostfeld are eager to plug their findings into a custom model they’ve been developing with collaborators to generate real-time forecasts of tick populations and local risk of Lyme disease.
Finding out whether the ticks thrive or “take it on the chin,” in Ostfeld’s words, may turn out to be key to understanding that risk over the next few years.
