Tectococcus ovatus is a scale insect that forms leaf galls on strawberry guava trees (Psidium cattleyanum), an invasive tree that disrupts ecosystem dynamics in the Hawai‘ian Islands and many other tropical areas throughout the world. Tectococcus ovatus is effective for biological control of strawberry guava, and researchers in Hawai‘i are using aerial drones to drop T. ovatus into the canopy of the trees. A new study shows drones are advantageous over other methods for getting the insects into the trees, such as poles or slingshots. Shown here is a T. ovatus gall broken open, showing small yellow-green T. ovatus eggs and dark yellow or pink newly hatched nymphs, or “crawlers.” (Photo by J. B. Friday, Ph.D., via Flickr)

By John P. Roche, Ph.D.

Strawberry guava (Psidium cattleyanum) is an invasive tree species that disrupts ecosystem dynamics in Hawai‘i and many other tropical regions throughout the world. Tectococcus ovatus is a scale insect that forms leaf galls in strawberry guava and is effective for biological control of strawberry guava infestations. But distribution of T. ovatus onto strawberry guava is hampered by practical limitations on time required for application and on access to dense stands of vegetation.

To help overcome these limitations, researchers at the University of Hawai‘i (UH) at Hilo and the U.S. Department of Agriculture Forest Service (USFS) tested the use of aerial drones and helicopters to drop T. ovatus gall insects on strawberry guava. They report their findings in an article published in June in the Journal of Economic Entomology.

Strawberry guava is native to Brazil and is a problem invasive species throughout the tropics. It forms dense, monospecies stands and is highly effective at spreading itself via seeds distributed by animals that consume its fruit. Once established, it disrupts ecosystems by increasing erosion, decreasing water recharge, and reducing suitable habitat for native animal species. In Hawai‘i, two of every three seedlings are currently non-native species, most of which are strawberry guava.

Tectococcus ovatus, which also originated in Brazil, uses strawberry guava as its host. This scale insect forms galls in strawberry guava, depriving the guava tree of energy and reducing its survival. Once deployed, T. ovatus spreads to new trees via eggs blowing in the wind and via crawling first-instar larvae.

Researchers in Hawai‘i are using aerial drones to drop the gall insect Tectococcus ovatus into the canopy of invasive strawberry guava trees. A new study shows drones are advantageous over other methods for getting the insects into the trees, such as poles or slingshots. Here, a small drone carrying four bundles of T. ovatus is used to deposit the insects onto strawberry guava trees. (Photo courtesy of Ryan Perroy, Ph.D.)

Tectococcus ovatus was first released for biological control of strawberry guava in Hawai‘i in 2012. Existing methods to distribute T. ovatus are launching leaves containing the insect onto strawberry guava with a sling shot, throwing leaves infested with T. ovatus by hand, or dropping bolas—cords with a bundle of infested leaves tied to each end—using poles. In their study, the researchers—Ryan Perroy, Ph.D., Roberto Rodriguez III, Ph.D., and Olivia Jarvis of UH and Tracy Johnson, Ph.D., of the USFS—developed and tested new methods using aerial distribution of T. ovatus using small drones, larger drones, and helicopters. The investigators used three different sites on the island of Hawai‘i for their research.

Tectococcus ovatus is a scale insect that forms leaf galls on strawberry guava trees (Psidium cattleyanum) and is effective for biological control of the invasive trees. Researchers in Hawai‘i are using aerial drones to drop T. ovatus into the canopy of the trees. Here, T. ovatus galls are visible on strawberry guava leaves, which are tied to the end of a cord that can be dropped from a drone to deliver the insects into the trees. (Photo courtesy of Ryan Perroy, Ph.D.)

In Sept 2017, to document the spread of T. ovatus from an inoculation site, Johnson and colleagues used slingshots to deposit T. ovatus on galled leaves into strawberry guava stands in the Upper Waiākea Forest Reserve. In 2021, Perroy’s group took photos of the 2017 application site using drones carrying high-resolution digital cameras. They found that 23.1% of the photos showed galls. A subsequent photo survey of the same area in 2024 found that 100% of the strawberry guava trees had T. ovatus galls.

The second component of the study was conducted in the Olaʻa Forest Reserve on the slopes of the world’s largest active volcano, Mauna Loa. To spread T. ovatus for their trials, the investigators used 30-centimeter strands of twine with strawberry guava leaves infested with T. ovatus galls tied on each end—dubbed “bolas” for their resemblance to the throwing weapon designed to entangle its target. (See galled strawberry guava leaves tied on the end of bolas in the image at right.)

Perroy and colleagues compared the outcome of T. ovatus bolas dropped by pole versus bolas dropped with small drones that could carry four bolas at a time. (See a small drone used for dropping bolas onto strawberry guava trees in the image above.)

The mean treatment time required was significantly lower with small drones (2.8 minutes) than with poles (12 minutes), but the success rate of the bolas hanging in trees, rather than falling to the ground, was significantly higher for poles (97.6%) than for drones (86.6%).

Inoculation success—i.e., successful infestation of trees with T. ovatus, as evidenced by gall formation—was not significantly different between poles or drones after six months. But after 12 months, inoculation success was significantly higher with drones than with poles.

In the third component of their study, Perroy and colleagues conducted high-capacity deployment trials comparing the results with bolas spread with larger, 16- and 54-unit drones versus bolas that were hand released from helicopters. They conducted these trials outside the town of Keaʻau.

They found that the hang success rate was not significantly different for large drones (71.6%) versus hand throws from helicopters (68.0%). Ninety-three percent of drone drops were within two meters of the target line, and 62% were within one meter. Only 68% of helicopter drops were within two meters of the target line, and only 28% were within one meter.

This study yielded several important discoveries. The photo analysis of locations in which T. ovatus was deployed was successful: All of the strawberry guava trees photographed after seven years that were within 200 meters of the center of the release zone had T. ovatus galls. “The post-release repeat monitoring aspect is critical,” Perroy says, “and since the galls that form are quite small, doing this effectively required the collection of millimeter-resolution imagery. So that meant flying very low and using good cameras.”

Researchers in Hawai‘i are using aerial drones to drop the gall insect Tectococcus ovatus into the canopy of invasive strawberry guava trees. A new study shows drones are advantageous over other methods for getting the insects into the trees, such as poles or slingshots. Shown here are larger drones used in high-capacity deployment trials as part of the study, including a 16-unit system (A) for releasing bolas, a 54-unit payload being prepared (B), and a 54-unit system in flight (C). (Image originally published in Perroy et al 2025, Journal of Economic Entomology)

In the trials of small drones versus poles, they found that using an aerial 4-unit drone was almost five times faster than using a pole from the ground. The authors note, however, that in areas without roads, drones or helicopters are the only way that T. ovatus can reasonably be distributed.

At 12 months, a significantly higher percentage of T. ovatus were observed galling in trees treated with small drones than trees treated with poles. Bolas dropped with drones tended to end up higher in trees than those dropped with poles, and the authors point out that this higher placement in the trees may have helped the scale insects succeed relative to those with lower placement.

“One aspect of this project I think is quite interesting was the iterative process of testing and refining different designs for the release mechanisms,” Perroy says. “3D printing allows this to proceed at a very rapid pace, where you can move from concept to physical testing in a matter of days or even hours, and that was really powerful.”

This study suggests that aerial drones are an effective technique for distributing T. ovatus for biological control and that they can help control invasive strawberry guava in an efficient, cost-effective manner. “Drones and innovative attachments can be effective tools for better managing and protecting our forests,” Perroy says. “We went through a lot of trials and ideas that turned out to not work, but all those provided valuable experience and feedback.”

John P. Roche, Ph.D., is an author, biologist, and science writer with a Ph.D. and postdoctoral fellowship in the biological sciences and a dedication to making rigorous science clear and accessible. He authors books and writes materials for universities, scientific societies, and publishers. Professional experience includes serving as a scientist and scientific writer at Indiana University, Boston College, and the UMass Chan Medical School; as a visiting professor at four tier-one schools; and as editor of periodicals at Indiana University and Boston College.