VICKSBURG, MISS. - Scientists at the U.S. Army Engineer Research and Development Center (ERDC) and the U.S. Army Corps of Engineers (USACE) Southwest Division (SWD) are researching the effects of biocontrol on an invasive tree in south Texas.
The Brazilian peppertree, Schinus terebinthifolia, was introduced to the U.S. as an ornamental in the 1840s. This invasive tree causes problems where it grows because it forms dense thickets, shading out native grasses and shrubs.
Dr. Christine VanZomeren, associate technical director for Environmental Engineering and Sciences, oversees the ERDC’s Aquatic Plant Control Research Program (APCRP). The APCRP has many focus areas, one of which is biocontrol — using insects to control invasive plant species.
“The larger program is related to integrated plant management, using many different tools to control invasive aquatic plants,” VanZomeren said. “The program is unique because it is the only federally authorized research and development program for non-indigenous aquatic plant species.”
The Brazilian peppertree is a riparian species, growing along ditches and drained wetlands, among other places. The tree can grow up to 40 feet tall, with a trunk about three feet in diameter. It responds to abrupt changes in its environment with heavy growth, acting as an opportunistic pioneer species, meaning the Brazilian peppertree is one of the first species to grow in disturbed areas.
ERDC researchers are currently in the second year of the Brazilian peppertree project under APCRP. Funding was also provided under the Ecosystem Management Restoration Research Program to develop a related publication. The goal of the project is to determine the best way to incorporate biocontrol into USACE projects to mitigate the Brazilian peppertree problem in prevalent areas across the U.S., including Florida, south Texas and Hawaii.
In USACE restoration projects, particularly in south Texas, the Brazilian peppertree is a major problem. The invasive plant requires significant funding and effort to be removed from the restoration sites.
“The insect we are using, the Brazilian peppertree thrips, is very specialized and only feeds and reproduces on the plant that is being targeted,” said Dr. Nathan Harms, ERDC research biologist. “The idea was to figure out the best way to use the thrips at the different stages of the restoration process.”
One of those stages is the planning stage — before any plant material is removed from the restoration sites. If the thrips get established and begin weakening the Brazilian peppertree beforehand, there could potentially be less biomass, and it could make the job of removing plant material easier.
The other stage is after clearing has been completed and new native species have been planted. If not all the Brazilian peppertrees are removed, there will be sprouts that will grow. Introducing the thrips at this stage could allow the insects to target the new sprouts and begin weakening or killing them.
“Adult thrips feed on the growing tips of plants and, over time as the insects keep feeding, the hope is that the plant will use all its stored energy trying to regrow,” Harms said. “However, thrips larvae don’t need new sprouts. They grow and feed directly on plant stems.”
Field research has been two-pronged. First, researchers have been releasing thrips onto large trees at restoration sites in south Texas and measuring different aspects of the tree, such as the number of branches and leaves and leaf photosynthesis. The goal is to see if the thrips can reduce the photosynthetic ability of leaves and reduce flowering and reproduction on the trees.
Second, researchers are targeting young trees to see if the thrips can kill a young tree. Many times, biocontrol does not kill the plant it targets, but simply weakens it to a point where other native species can grow and crowd out the invasive plant. However, early evidence suggests that the thrips may be able to actually kill young Brazilian peppertrees.
“One of the ways this could be useful is we can get the thrips established at a site, and they go in and cut out 99% of the trees with only a few stragglers left over. If the insects can survive, they might be able to attack young plants as they pop up and kill them outright,” said Harms.
Overall, Brazilian peppertree research is nearing its end. Biocontrol research on the tree has been going on for over 20 years as establishing biocontrol protocols is a long process.
In 2019, the U.S. Department of Agriculture (USDA) Animal and Plant Health Inspection Service (APHIS) approved the use of the Brazilian peppertree thrips for biocontrol. Before USDA APHIS approved their use, the insects were subjected to a battery of host range tests to ensure they would not feed on any other species. If the thrips fed on any other species, they would be out of consideration for use as biocontrol agents.
In 2020, the ERDC began applying for USDA APHIS permits. The ERDC needed the permits to be able to transfer the thrips from Florida, where initial research was being conducted, to Mississippi and then to Texas.
Additionally, the ERDC worked with the Texas Department of Agriculture to get permits to move state noxious plants, like the Brazilian peppertree, from sites around Texas.
The city of Brownsville, Texas, and USACE SWD helped the ERDC team identify priority restoration sites. SWD also identified previous restoration sites so the ERDC researchers could look at areas where the Brazilian peppertree was growing again.
While USACE is responsible for some of these restoration sites, they are eventually handed over to a local sponsor, like the city of Brownsville in south Texas. Once the site is handed over, the sponsor is responsible for maintenance of the site.
Establishing an effective biocontrol program could be groundbreaking for local sponsors. It could drastically reduce the amount of maintenance that has to be done once the sites are restored. Biocontrol could also significantly reduce the amount of money required to perform the maintenance.