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Weevils to the Rescue -- Helping to Reduce Spread of Mile-a-Minute!

February 27, 2020 12:00 AM
By: Andrew Rohrbaugh, Botanist, Ecological Services Section, Bureau of Forestry

Weevils to the Rescue -- Helping to Reduce Spread of Mile-a-Minute!

​How can I solve my mile-a-minute problem? This is a question frequently received by DCNR’s Bureau of Forestry.

And it’s no wonder, because this invasive vine can grow up to six inches a day -- quickly smothering native vegetation and climbing up to tree canopies where it restricts light to plants below it.

Its fast growth is one way that the plant spreads, but its seeds are the primary means. Birds and other wildlife eat the fruits and spread the seeds in their droppings.

Mile-a-minute seeds also are buoyant for up to nine days in water and can be spread by streams and floods.

Stopping the spread of this invasive plant is a major concern for DCNR and property owners -- and we’re fighting this battle in three ways: manually removing the plant, chemically attacking the plant, and deploying the help of a small insect -- the weevil.

About Mile-A-Minute

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Mile-a-minute plant

Mile-a-minute (Persicaria perfoliate), also known as devil’s tearthumb, is an invasive plant from Asia.

It is a slender annual vine with downward-pointing sharp outgrowths, bright green triangular leaves, and blue-purple berries.

It closely resembles our native halberd tear-thumb species, which have more arrow-shaped, linear leaves.

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Arrow-leaved tearthumb, a native plant with long arrow-shaped leaves

Mile-a-minute was accidentally introduced into the U.S. with nursery stock in the 1930s and is now a noxious weed in Pennsylvania.

As an annual species, individual plants set seed and entirely die each year. Seeds either drop to the ground or are spread by birds to new areas. The population then re-grows from seed at the start of the next growing season.

Mile-a-minute loves disturbance and full sunlight. When those two factors combine, its seedlings can generally outcompete most of our native plants, sprawling over them and depriving them of sunlight.

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Mile-a-minute infestation, photo by Leslie J. Mehrhoff, University of Connecticut, Bugwood.org

How to Control Mile-A-Minute

Large populations of mile-a-minute are best controlled with pre-emergent treatment, such as Oust or Proclipse, during March before the seed starts to germinate.

However, any pre-emergent treatments will likely affect other plant species that you are not targeting in the application area.

For very small populations, hand pulling and digging may be effective.

Small plants pull out by the roots easily early in the growing season. As the season progresses and plants become larger, root material can be more difficult to remove completely.

Due to the barbs on mature plants, gloves are recommended for pulling plants. Plants can be pulled until fruit begins to appear.

Both tricolpyr (Garlon 3A) and glyphosate (Glyphomate 41) can be applied to the plant during the growing season. Glyphomate 41 will kill all plants; Garlon 3A targets only broadleaf plants, which may be more desirable if mile-a-minute is growing in a grassy area.

A surfactant -- which is used to alter spray solution properties so that herbicides can be more effectively applied to and absorbed by the targeted plant -- should be used as well.

Without a surfactant, some store-bought glyphosate mixes can roll right off mile-a-minute leaves without impacting the plant.

Help From An Insect -- The Weevil

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Photos by Amy Diercks, Bugwood.org, and Kelsey Paras, Oklahoma State University, Bugwood.org

Another method of trying to stop the spread of mile-a-minute is with the help of a small insect, known as the weevil.

A species of weevil (Rhinoncomimus latipes) that feeds on mile-a-minute in Asia was carefully tested to ensure that it would only impact mile-a-minute plants, and not become a new problem or threat.

In 2004, the weevil was approved for release in New Jersey, Delaware, Maryland, and Pennsylvania.

Adult weevils can be black or orange and create feeding holes on the mile-a-minute leaves. The most significant damage to the mile-a-minute plant is from the weevil larvae, which burrow into the stems.

Weevils spread very quickly on their own, and, at this point, have become established across most of southern and eastern Pennsylvania, with more limited releases in the northern and western part of the state.

If you aren’t sure if you have mile-a-minute on your property -- walk slowly up to your suspected mile-a-minute patch (especially on a cool morning) without disturbing the plants.

You can find the adult weevils sitting on the growing tips of the plants, or occasionally in the nodes of the plants.

When scared, they’ll drop to the ground. If you don’t see any adults or feeding holes in the leaves, look for small orange scarring on the nodes of the stems -- that’s where the larvae have burrowed into or out of the plant.

Scarred node.jpg
Scarred node that indicates mile-a-minute weevil larvae

If you don’t have mile-a-minute weevils and would like some, they are available to purchase from the Philip Alampi Laboratory in New Jersey.

Another option is to contact a landowner that has mile-a-minute weevils nearby and collect them for release on your property.

You can readily capture them by stuffing a coffee can with mile-a-minute (don’t spread additional seeds!); and knocking weevils off the growing tips into the can.

They will naturally want to hide in the mile-a-minute at the bottom of the container. You may need to occasionally shake the container to knock them back down.

Only a hundred or so are required to start a new population, but remember to keep the coffee can cool, and if you are not immediately releasing them, poke airholes in the lid with a pin or very small nail.

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Mile-a-minute weevils on the growing tip of the plant, photo by Ellen Lake, University of Delaware, Bugwood.org

Unfortunately, weevils aren’t a silver-bullet. They do help slow mile-a-minute, especially by damaging the nodes and forcing the plant to grow in a bushier, shorter shape.

However, cool or wet summers appear to slow the weevil population growth without slowing the mile-a-minute.

Additionally, weevils take years to build up their population numbers.

In short, weevils help, but mostly to decrease the amount of mile-a-minute, rather than eradicate it.

Learn more about the invasive mile-a-minute and how to treat it from DCNR’s Mile-a-Minute Invasive Plant Fact Sheet (PDF). Learn more about invasive plants at the DCNR website.


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