Looking for Natural Enemies of Invasive Species

Rick Bogren

For almost as long as they have been coming to North America, foreign settlers have brought with them plants and animals from their native lands. Although most have been innocuous, others have proved troublesome.

Whether purposefully or by accident, imported plants and animals have been disruptive. Some examples include Chinese tallow trees and kudzu. Other plants that were brought in as ornamentals, like giant salvinia, water hyacinth and hydrilla have pushed out native water plants and have caused problems for wildlife and navigation. People in the South are plagued by red imported fire ants, and Formosan subterranean termites invaded the South after coming in wood materials returned from south Asia following World War II.

Sometimes the best way to control an invasive species is to find its natural enemy in the country where it originated and introduce it here. But because of the nature of unintended consequences, researchers must be careful not to introduce another problem that wasn’t there before.

“The process of bringing a native enemy to control an invasive species is highly regulated to make sure they are safe,” said LSU AgCenter entomologist Rodrigo Diaz.

Diaz has been working with a salvinia weevil used to control giant salvinia, an invasive water plant first discovered in Louisiana in the Toledo Bend Reservoir in 1998. A native of South America, the plant has no natural controls in the United States and has moved into many water bodies in Louisiana.

Diaz’s research team in Cameron Parish has been monitoring areas where salvinia weevils have been released and has identified several areas where heavy infestations of giant salvinia have been extensively controlled.

“Giant salvinia has been reduced as salvinia weevils have been introduced, and natural vegetation has recovered,” Diaz said.

Diaz and his team have been monitoring 50 square miles of marshland below the Intracoastal Canal known locally as the Big Burns to measure the effectiveness of the weevil in controlling the invasive plant. In three months’ time, heavy infestations of giant salvinia have been reduced, and clear water has emerged along with native plants.

“We found that if the weevil damage occurs early in the season — in June or July — submerged vegetation had enough time to recover and provide critical ecological services such as increasing dissolved oxygen levels and food for waterfowl. We suspect some vegetation may take longer to come back,” Diaz said.

The effectiveness of weevil control depends on the dynamics of weather cycles.

“In warm south Louisiana, the weevils do very well except for a really cold winter,” Diaz said.

And even then, the weevil population is able to survive in micro climates and re-emerge as temperatures rise in spring. “Winter temperatures and spring rains can move the timing of success,” he said.

A salvinia weevil from Brazil has been used in biological control programs throughout the world. A population was released in Texas and Louisiana in 2001 through the efforts of retired AgCenter entomologists Seth Johnson and Dearl Sanders.

At densities greater than 18 weevils per pound of salvinia, weevil feeding causes giant salvinia mats to turn brown and eventually die. Populations of the weevil reared in south Louisiana, including in ponds at the AgCenter, are distributed to rivers, bayous and lakes where they are introduced to control the plants.

A series of cold winters has significantly reduced weevil populations in north Louisiana, so Diaz is heading to Argentina, where he will collect weevil populations that have been exposed to colder weather.

“We will bring a population from Buenos Aires for rearing in a quarantine facility,” Diaz said. “It’s the same species as the weevil from Brazil but with greater cold tolerance that will be a new tool for controlling giant salvinia in north Louisiana.”

Diaz also is working with a leaf beetle to control air potato, an invasive weed that’s spreading in Louisiana. Air potato is a perennial vine native to Asia and Africa that is a member of the yam family. The vines climb vertical surfaces and compete with other vegetation for light and nutrients. The larvae and adults of the air potato leaf beetle feed on the plant leaves, causing stunted growth and early death.

The beetles are defoliators that stress plants. “We get less aggressive growth and control with no major problems,” Diaz said. About 10 years ago the U.S. Department of Agriculture looked in Nepal and China for beetles, and the ones they found were first confined for stringent host-range testing before they were released into the wild in the U.S.

Working with the Nature Conservancy and other organizations, Diaz released a population of beetles in Louisiana in 2016. The results appear promising.

AgCenter researchers are also investigating the alligator weed flea beetle and a water hyacinth weevil to determine if they can be effective in controlling the weeds.

“With aquatics weed management, we utilize biological control if aquatic herbicides are not cost-effective or unable to selectively remove the pest,” said Christopher Mudge, a research biologist with the U.S. Army Engineer Research and Development Center and adjunct professor in the AgCenter School of Plant, Environmental and Soil Science.The flea beetle is from southern Brazil and northern Argentina.

Speed and success are limited compared with chemicals or mechanical methods for controlling weeds, but natural enemies are less expensive for long-term success.

One problem with these biocontrol agents, Mudge said, is that some bugs can become food for other species in the food chain.

“For example, an insect that performs well in a research tank may not be as effective in the wild if it becomes food for fish,” he said. “On the other hand, we have the potential to spread millions of weevils per year at a lower cost than using herbicides or mechanical methods.”

Sometimes, however, biological controls don’t work as expected.

The emerald ash borer, which can kill stands of ash trees, found its way into Louisiana from Michigan a couple of years ago. Native to Asia, the borer is a cold-weather parasite. Ten years ago, researchers in the Midwest identified parasitic wasps from China to control the borer.

In 2015, AgCenter researchers introduced wasps from Michigan in north Louisiana. Most disappeared. The preliminary data suggest that they don’t thrive in the warmer, humid climate of Louisiana, Diaz said.

Another attempt as biological control in Louisiana hasn’t been as successful as researchers had hoped.

A phorid fly, also known as a decapitating fly, was released in Louisiana from 1999 to 2006 to control red imported ants. Less than 1.5 millimeters in length, the phorid fly was first released in the United States by the USDA in Florida in 1997. The Louisiana releases were in pastures. A follow-up study in 2009 found flies were still present in 57 of the 64 parishes. Although the flies are still present, they’re not abundant enough to have much effect on fire ant populations, Diaz said.

Minor successes have been encouraging enough for researchers to continue looking for biological controls for invasive species. Diaz is now working with partners in different international agencies in South America and Asia to try to find a natural enemy for the Roseau cane scale that showed up in Louisiana marsh near the mouth of the Mississippi River. Originally from China and Japan, the scale is threatening the integrity of the river channel.

Diaz is going to Asia this summer to use DNA testing to identify the source of the scale and try to find a natural predator there.

“We are building a network of international research teams,” Diaz said. “We are not isolated anymore. We need to be cautious and realistic, but if a natural solution works, it will save millions of dollars.”

Rick Bogren is a professor and science writer in Communications.

(This article appears in the summer issue of Louisiana Agriculture.)

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Adult females of the salvinia weevil lay eggs one at a time in small crevices on the giant salvinia growing tips. Newly hatched larvae feed on external plant parts, then later burrow inside the rhizome to continue their development. Larval feeding activity disrupts the flow of nutrients from the “roots” to the newly growing tips, causing plants to turn brown and die. Adults feed preferentially on nitrogen-rich buds, slowing new growth as they attack growing tips. Photo by Rodrigo Diaz

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In June 2016, giant salvinia was thick in the marshland below the Intracoastal Canal in Cameron Parish in an area known as the Big Burns. Photo by Rodrigo Diaz

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In September 2016, there was open water in the Big Burns, and the giant salvinia was found only on the fringes of the marsh. Photo by Rodrigo Diaz

9/17/2018 9:26:13 PM
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