Wednesday, May 3, 2023

Got Garlic Mustard? Look for Aphids

These sap-sucking insects traveled with their invasive plant hosts and could offer natural control. Researchers need help learning where they are and what damage they cause.

A stand of garlic mustard with several tall stems bearing toothed, triangular leaves and small, white flowers.
Garlic mustard, Alliaria petiolata, flowering in mid-May in southern Minnesota.

Garlic mustard, Alliaria petiolata, is an invasive plant of woodlands and woodland edges. Since it was introduced to the US from Europe in the 1800s, it has become widespread, especially in the eastern US. In Minnesota, the plant has spread throughout the southern two-thirds of the state and even to some far northern counties (EDDMaps).

Garlic mustard emerges in spring, and like other invasive plants, it’s bad news for the communities it occupies. The plants can form dense patches that shade out and displace other plants, and they can release chemicals into the soil that inhibit other plant growth. For these reasons, garlic mustard is a restricted noxious weed in Minnesota. That means its propagating parts, such as seeds or live plants, can’t be imported or sold and they can’t be transported without a permit (MDNR, MDA, UMN).

Garlic mustard isn’t reported to be invasive in its home range (CABI Compendium), but here, it behaves differently. There are many possible reasons. One is that it arrived without the insects or diseases that control its populations. What a surprise, then, when garlic mustard aphids (Lipaphis alliariae), also called grenade aphids, were first confirmed in 2021 on plants in Ohio. These garlic mustard specialists have since been documented in several more states, including Minnesota. So far, they’ve been found in St. Louis, Anoka, Washington, Dakota, Winona and Olmsted counties (EDDMapS).

Although it's usually worrisome to find another introduced species that's naturalizing, this one may offer some hope. Garlic mustard infested with these aphids develop deformed leaves and seed pods, and researchers want to know if this could be a natural control for the plants. If it is, it could be more effective than hand-pulling, spraying or other methods of garlic mustard management, especially in large invasions.

To start, we need to know where the aphids are, and if they’re present, what damage they may cause. That’s where citizen scientists come in. The Midwest Invasive Plant Network (MIPN) and its partners need volunteers to look for the aphids during garlic mustard pulls or more casual activities. This is an ideal time to search, so below are resources to identify garlic mustard and its aphids and instructions on how to report what you find. If you find garlic mustard but no aphids, that's important, too, and is helpful to report. 

Garlic mustard identification

Garlic mustard aphid identification

Bugwood’s garlic mustard aphid ID cards (print and cut; especially good for group pulls)

How to report garlic mustard aphids

The MIPN has prepared an informational flyer explaining how to report the aphids if they’re found. Negative reports – reports of their absence – are also important. Volunteers use EDDMapS, an early-detection reporting tool available as an app and online. Users must set up an account, but it’s free. See the flyer for more information. 

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