Thaumetopoea processionea (sometimes known as oak processionary moth) caterpillars (left, with white hairs) and gypsy moth (Lymantria dispar) caterpillars (right, with darker coloration) are both voracious defoliators and frequent targets of forest pest management efforts. (Photo credit: Benjamin Leroy)

By Melissa Mayer

Insects that strip the leaves from trees are a major economic liability for temperate forests around the world. One such insect, the gypsy moth (Lymantria dispar), defoliates to the tune of $3.2 billion every year in North America. And scientists expect an increase in defoliator outbreaks due to climate change.

Melissa Mayer

This may mean an uptick in aerial applications of insecticides, a controversial practice in forests for its potential adverse effects on nontarget insects. Aerial spraying could be refined and improved through field studies, but spraying by piloted aircraft is expensive and covers large areas, which makes it difficult to design experiments, much less replicate them. And many countries tightly regulate aerial spraying, often banning the practice except during outbreaks.

In a new study published this month in the Journal of Economic Entomology, scientists present a promising alternative: targeted spraying using unmanned aerial vehicles (UAVs). The authors believe this tech may enable “resource-efficient” experiments, making it possible to perform many replicates and collect more data to understand the effects of insecticides on forest ecosystems.

A Formidable Pest

Forest pest management is a novel application of drone technology. “Although we knew that UAVs work well for spraying agriculture, using it successfully for forest spraying was a brand-new challenge in 2016,” says Benjamin Leroy, a Ph.D. student in terrestrial ecology in the Department of Ecology and Ecosystem Management at the Technical University of Munich and lead author of the study.

Benjamin Leroy

The research team targeted a notoriously voracious defoliator, Thaumetopoea processionea, sometimes known as the oak processionary moth. Local population surveys forecast high densities for the moth in an oak stand called Greutholz near Willanzheim, Germany.

The oak processionary moth was a particularly pesky choice. The caterpillars have fine, barbed hairs with toxic effects on the skin and respiratory tracts of humans and other animals. Even worse, those hairs can travel long distances on the wind and remain active in the environment for up to 10 years.

“In an outbreak situation, exposure is unavoidable, even with the heavy protective gear that we wear to work in the forest,” says Leroy. “This definitely made the work extremely challenging, as the itching was such that it was difficult to find sleep between working days, and it seriously affected the mood in the team!”

Blocking Out the Design

The team used a full factorial design for their experiment so they could investigate three levels of insecticide application (two pesticides and a control) over two levels of caterpillar density (low and high). For insecticides, they chose two insect growth regulators—diflubenzuron (DFB) and tebufenozide (TBF)—and timed the application to target oak processionary moths.

To figure out pre-spray densities, the team collected and counted frass pellets—i.e., caterpillar droppings—under individual trees. Using this data, they marked trees low density (less than 20 frass pellets per day) or high density (more than 35 frass pellets per day) and assigned trees to 10 experimental blocks of six trees each. Since the UAV could target individual trees, the team was able to test the insecticides at both densities over 10 replicates (blocks) yet only required a sample size of 60 trees.

In a recent study in Germany, targeted delivery of insecticides by unmanned aerial vehicles was effective against oak processionary moths. Researchers say such drones are suitable for aerial spraying during field studies and may open new doors for “precision forestry.” (Photo credit: Peter Eichel)

In a recent study in Germany, researchers conducted targeted delivery of insecticides in trees by unmanned aerial vehicles to manage oak processionary moths. The researchers measured effectiveness by counting numbers of fallen caterpillars caught on tarps placed below tree crowns. (Photo credit: Benjamin Leroy)

Thaumetopoea processionea, sometimes known as the oak processionary moth (for the caterpillars’ behavior of traveling in nose-to-tail processions) is a voracious defoliator and also a public health nuisance, with fine, barbed hairs causing toxic effects on the skin and respiratory tracts of humans and other animals. (Photo credit: Philipp Braumiller)

Take That, Caterpillars

After spraying, the team recovered 15,210 fallen caterpillars from 13 families. Trees sprayed with DFB and TBF had more fallen caterpillars (51 percent and 117 percent, respectively) than the unsprayed controls.

Oak processionary moth mortality was 3.3 to 4.8-fold higher (TBF and DFB, respectively) in sprayed trees versus unsprayed controls, but the difference between the two insecticides wasn’t significant. When it came to nontarget caterpillars, TBF was more lethal than DFB, probably due to the timing of spraying.

The scientists again collected frass pellets to measure feeding activity after spraying. These were 54 percent and 121 percent higher below unsprayed trees versus trees sprayed with DFB and TBF, respectively. Trees with a high density of caterpillars before spraying had higher mortality rates and lower feeding activity for both insecticides.

Droning On and On

This is the first documentation of TBF efficacy against oak processionary moths. What’s more, the findings line up with the results of previous large-scale efficacy trials, which means UAVs may be a good alternative for applying insecticides during field studies.

It’s a first look at deploying UAVs for forest spraying, and it’s hard not to wonder about the future roles for the tech in ecological applications. “We can imagine the development of ‘precision forestry’ with improved population survey methods to allow targeted spraying on the source of the outbreaks,” says Leroy.

“I believe that future research should focus on increasing the spatial resolution of population surveys, to forecast with more precision where outbreaks are likely to start and treat them accordingly,” he says. “UAVs may also play a role in population survey methods, at least in some systems, and may eventually allow real-time forecast of insect density and treatment application.”

Melissa Mayer is a freelance science writer based in Portland, Oregon. Email: melissa.j.mayer@gmail.com.