Renée Meyer's Garden Report: Battling Mile-a-minute
Seeds of the invasive annual Mile-a-minute plant (Persicaria perfoliata) were unintentionally introduced into the northeastern United States in the 1930s. Years later, this pesky plant has spread into at least 10 states and the District of Columbia. While it may be relatively new to Block Island, (it was first noticed around Fresh Pond in 2008) its population is exploding and, as any invasive species may, is crowding out native vegetation.
An invasive plant becomes just that when it encounters favorable growing conditions, and when there are no natural predators to control its presence. Mile-a-minute (or MAM, so named for its ability to grow six meters in a single season) is mainly native to areas in China and Japan.
Three weeks ago I had the opportunity to visit the Biological Control Lab at the University of Rhode Island. But before I went, I was given some homework by Dr. Richard Casagrande, Professor of Entomology, who oversees the lab. He is also coordinator of Integrated Pest Management (IPM) for Rhode Island.
My mission was to learn about controlling the invasive Mile-a-minute vine, and my homework was to read “Biology and Biological Control of Mile-a-Minute Weed” by Judy Hough-Goldstein, Ellen Lake, Richard Reardon and Yun Wu. Hough-Goldstein and Lake are both from the Department of Entomology and Wildlife Ecology at the University of Delaware, and Reardon and Wu are from the USDA Forest Service. I was given this assignment because, while the URI lab assists in the control of MAM, Hough-Goldstein and colleagues were among the team that did the original research.
That research involved several phases and took place over several years. The first phase could be named “Know Thy Enemy.” Genetic tests were conducted to see where the plant originated from, and the conclusion was that the particular strain in the northeast had come from Japan. Other research involved determining the viability of the seeds to persist in the soil. Research showed that the seeds of this plant can live up to seven years in the soil.
How did they determine this? One method was to put a certain number of seeds into a mesh bag and bury it in the ground. Each year the bags were dug up and the number of seeds that had germinated were counted and discarded. The remaining seeds were reburied. While germination rates were highest in the first two years, germination continued into year six and overall, 99.3 percent of the seeds eventually germinated. This is important because it suggests to those trying to wipe out an infestation of MAM that it will take a multi-year effort to be successful.
The MAM seeds didn’t fare quite as well in the digestive tracks of deer, where only 40 percent remained viable after their exit. Still, birds and bambis are a factor in the spread of the plant, especially as both may poop out the seeds far from where they ingested them.
Researchers investigated several methods of potential control including mowing, pulling the plants by hand or rake, using pesticides and even burning. Of these options, hand-pulling the plants before they go to seed may be the most successful, if not the most practical. Mowing can make for a bushier plant, and thus one that potentially can make more flowers and seed. As for burning, work in Australia has shown that to kill a hard-coated seed such as that of MAM may take temperatures of up to 750 degrees Fahrenheit for 20 to 30 seconds. This is not easily achievable even in a burn-barrel.
Next came the hunt for a biological control mechanism. Biological control involves finding a natural enemy of the targeted plant, but it must be one that does no harm to other species of plants. There are many insects that have a long-evolved synergy with particular plants. One example is the Monarch butterfly, which, while it feeds on many plants, successfully lays its eggs only on milkweeds. It takes a lot of research to both find an appropriate natural enemy for an invasive species and to get it approved by federal and state regulators.
With Mile-a-minute, the search for the perfect predator started in the United States with researchers shaking down plants to see what types of insects were already there. While they found many, none were doing any particular harm to the plants, save for perhaps the Japanese beetle. However, as many gardeners know, Japanese beetles aren’t especially friendly to lots of plants.
The search then was extended to China and Japan where researchers looked for natural predators in the plant’s home range. Several species were found, and each one initially evaluated. Did they cause significant damage to the plants? Did they damage other plants? Did they reproduce on the plants? (A successful biological control agent must have the ability to reproduce once introduced in order to sustain itself long enough to be effective. The idea is that if the targeted species completely dies out, so will the control agent, as it won’t be able to reproduce.)
Of the potential predators, one stood out in both China and Japan. Enter Rhinoncomimus latipes, a weevil that we have come to refer to here in the states as the Asian weevil. The bugs were put through a battery of tests in quarantine labs. The authors of “Biology and Biological Control of Mile-a-Minute Weed” write:
“In tests in China, R. latipes did not feed on 28 species of plants in 18 families outside of Polygonaceae. Within the Polygonaceae, adult R. latipes did not feed on any plant other than MAM in choice tests or lay eggs on any species other than MAM, and larvae survived only on MAM.” (Page 19)
Work continued in quarantine labs in Newark, Delaware, where researchers tested the weevils on other plants, including species that are threatened or endangered.
“In choice tests adults almost exclusively ate MAM, and newly hatched larvae placed on other plant species did not survive (Colpetzer et al. 2004). Based on these results, a release permit was granted by USDA-APHIS in July 2004.” (Page 19)
Over the next few years researchers took the weevils into the field. They started in New Jersey with both release and control sites. The numbers of weevils released were tracked, and the impact on MAM was measured using a quadrant system. Some studies tracked how far the weevils would move, or disperse themselves, over time. This trait is considered a good thing, as the weevils will move on to other colonies of MAM. Indeed, some of the control sites were shut down after the weevils found them, simply because they were no longer actual control sites. They were compromised.
The use of R. latipes to control MAM has since spread to other states, including Delaware, Maryland, Pennsylvania and West Virginia. More recently, it is being utilized in New York and New England. In 2013, with the blessing of the local conservation commission, Ridgefield, Conn., became the sixteenth community in the Nutmeg State to start a biological control program for MAM.
Originally the weevils were being grown for release only in the lab in Newark, Delaware, but the demand for them has exceeded the capacity for them to only be grown there. This is where the URI Biological Control Lab comes in. It breeds weevils for distribution in Connecticut, Massachusetts and Rhode Island, and breeds 18,000 of them per year.
Lisa Tewksbury, who coordinates research in the lab, and who also is responsible for maintaining the quarantine facility, gave me a tour. We started inside the greenhouse where MAM is grown. In order to rear weevils, one must have a constant source of food for them, both to eat and to lay their eggs on. So, first she shows me the “mother plants.” From these mother plants, paid lab assistants take 100 cuttings a week in order to grow more plants so they have a steady supply. These plants are “precision pruned” so that they will develop a bushiness, as opposed to their more natural vine habit. Then single plants are placed in ovipositors, with a few mating pairs of weevils. (Ovipositors are sealed, clear plastic containers.)
Once the weevils have had time to lay their eggs, plants are removed to larger sealed containers that contain fresh plants also. When the eggs hatch, the resulting larvae will eat their way into the stems, potentially killing the plant - thus the need to have a constant supply of new plants. Those larvae go through three molts before dropping to the ground to pupate. When the adults emerge, they will crawl up the plants to begin feeding. Now they are ready for the field. The weevils are pumped out of the boxes into vials that will then be sent where needed.
Each batch of weevils takes four weeks, and workers in the lab break down and set up four more each week. In all, 16 batches are being maintained throughout the season for availability from mid May (when MAM plants emerge from the ground) through the end of summer. Each batch yields approximately 250 weevils, so that 1,000 of them will be available per week.
R. latipes has been approved for release in Rhode Island, and although no local conservation groups have proposed using them on their properties, local Block Island homeowners may use them privately. Indeed, purportedly, two or three already have. If it is something you would like to try to help to control an infestation, contact Lisa Tewksbury at (401) 874-2924.
Or if you would like to do your own homework, a copy of “Biology and Biological Control of Mile-a-Minute Weed” may be found at: