Weevils have a bad rap. It's deserved, though, because many weevils destroy crops such as wheat or cotton.
But the tiny insects now have a chance to redeem themselves. One specific species is being used to combat the invasive Japanese mile-a-minute weed, which aggressively spreads and chokes out native plants throughout the eastern U.S.
Collect your own weevil day
A few years ago, Rachel Wagoner, a resource management specialist with Pennsylvania's Bureau of State Parks, read about the Rhinoncomimus latipes korotyaev weevil, which was introduced in selected states to combat an invasive plant called mile-a-minute vine.
The article mentioned a "Collect Your Own Weevil Day," in which researchers attempted to collect the insects in order to rid the mile-a-minute weed.
Neither mile-a-minute weed nor the R. latipes weevils are indigenous, or native, to Pennsylvania. The vine is native to parts of Asia and the insect first was found in China.
The annual mile-a-minute weed can be identified by its distinct triangular leaves and recurved barbs, or spines, along the stem. It is considered invasive because it grows well in a variety of habitats, aggressively spreading and displacing other plants.
"I had been working with a number of stakeholders who were interested in obtaining weevils to control mile-a-minute (MAM) on properties they stewarded, so we decided we would attempt to collect our own as well," Wagoner said.
She scheduled collection days in June this year and last year.
Native insects have fed on mile-a-minute, but the plant's numbers have exceeded what the insects could consume.
In 1996, the U.S. Forest Service initiated a biological control program targeting the mile-a-minute plant. Wagoner said the goal was to find host-specific insects.
Research started in China identified the R. latipes weevil as being host specific to the weed. Eventually, the insect began to be identified as the MAM weevil.
Studies continued in the U.S., specifically in a quarantined area of Delaware.
"A key test was when, given a choice between native plants and mile-a-minute, it (the weevil) chose the mile-a-minute. And, when it was offered only native plants to eat, the weevil did not feed at all on the native plants," Wagoner said.
The U.S. Department of Agriculture did an environmental review of the research and a permit for field release of R. latipes was approved in July 2004.
The weevil now is mass-reared at the Phillip Alampi Beneficial Insects Laboratory in Trenton, N.J.
Since 2004, weevils have been released in 10 states, including Pennsylvania.
Weevils typically are small insects and the species that eats MAM is no exception. It is a black, 2 millimeter long insect.
Once they begin to feed on the plant, Wagoner said, they sometimes can be covered with an orange film.
"Adult weevils eat small holes in young leaves of P. perfoliatum (mile-a-minute weed) and lay eggs on leaves and stems. After hatching, larvae bore into the stem where they complete development," she said.
In the field, the insect and the holes it creates can easily be spotted.
In addition to eating the plant, weevils have been shown to delay seed production, stunt plants and cause mortality. Some monitored sites have shown a spring seed reduction within one to three years.
In June 2011, three volunteers from Lancaster County began a weevil collection and distribution.
"They had obtained permissions from local municipalities to introduce the weevil to mile-a-minute infestations in those municipalities' parks," Wagoner said.
They collected them out of Codorus State Park in York County.
"This location was chosen because weevils had been released there for several years and the weevil population was one of the state parks' most established," she said.
The volunteers collected the insects by using a modified 2-liter soda bottle. They removed the top and used it inverted, as a funnel. The funnel then was secured in a glass flask.
"It was low tech, but highly successful," Wagoner said. "The wide mouth of the funnel was held under the tips of the mile-a-minute plant - where the weevils typically congregate and feed - and the plant was tapped or shaken, causing them to fall into the funnel."
In one hour, about 150 weevils were collected, she said.
The Codorus weevils were transported to new locations within the same day. They were distributed in Gifford Pinochot State Park in York County and a few parks in Lancaster County, Wagoner said, and all of the transplants were successful.
"I think there were a couple of reasons (why they worked so well), one because of the location. Weevils that were grown in the lab had been released into that park for several years and the population was very well established," she said. "Another key was we collected from an area that was in an open sunny field on a warm sunny day. The weevils tend to prefer open, sunnier locations, so I think we were able to collect at a faster rate than if the site was shady or overcast."
While nothing is scheduled for more collection days, Wagoner said the weevil has been something that bureau has been releasing for years.
"We don't know how long the lab will be able to provide us with weevils, and these collections are a way to prepare for when that resource is no longer available," she said. "The field collections from established populations in state parks will allow us to continue expanding this biocontrol in additional sites."
Weeds gone wild
The Plant Conservation Alliance has posted mile-a-minute weed on its list of the "Least Wanted: Alien Plant Invaders of Natural Areas."
The first documented establishment of the plant in the U.S. was in the 1930s at Gable Nursery in Stewartstone, York County. In the 70 or so years since, it has spread more than 300 miles, thriving in a variety of habitats, including forested floodplains, streamside herbaceous wetlands and upland forests.
Mile-a-minute initially emerged with a planting of holly seeds that originated from Japan.
"In their natural range, these species are limited by environmental, pest or disease conditions, keeping these species in balance within their ecosystem," Wagoner said. "When native plant communities are disrupted, wildlife, which has evolved with the native species in those plant communities, loses habitat."
The weed has invaded 12 states, so far.
"Its current range extends from Pennsylvania north to Massachusetts, west to Ohio and south to North Carolina. It has been reported in 46 counties in Pennsylvania," Wagoner said.
The rapidly growing vine easily climbs and chokes out other plants. It can grow to lengths of 19 feet and produce more than 2,000 seeds in full sun and about 400 seeds in shady conditions, she said.
According to Wagoner, the mile-a-minute weed can be identified by its flowers, which often are an inconspicuous iridescent blue.
The plant flowers from mid-summer until fall. It generally likes full sun but will grow in shade.
One reason the plant is a dangerous invasive is because of its seed banks.
"Many seeds will germinate underneath dead vines from the previous year, while others are spread by birds, mammals and water," Wagoner said. "Mile-a-minute seeds can survive for up to six years in the seed bank."