February 25, 2020
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Invasive ants sometimes thrive in mulch

I cannot imagine life without the garden, yet there are many communities along the Maine coast where the joy of gardening is tempered by the presence of a tiny insect, the European red ant. Wherever this invasive nonnative pest has established its nests, gardeners have given up the garden rather than suffer repeated stings.

Horticulture introduced the European red ant to Maine. Infested potted plants were brought from Europe to coastal Maine estates during the early half of the 20th century and, by 1950, the ants were established in two locations. By 2002, their presence was confirmed at more than 20 coastal sites. European red ants are now permanent residents of Cumberland, Hancock, Kennebec, Knox, Waldo, Washington and York counties.

I live with this ant in Eastport. I take my students across the street from the high school to a granite outcrop ecosystem where they can observe the ecological impact of the invader. We look for signs of native ants, but find none. Native ant species have been displaced by the invader, along with most other native insects.

At least one colony of European fire ants was moved from a coastal Maine nursery to a landscape 20 miles inland, and that colony has persisted through several winters. The ants are native to cold regions of Europe and Asia and it is unlikely that their spread throughout Maine will be limited by winter temperatures.

Gardeners should take precautions against the invasion of their gardens by this pest as a stowaway in containerized plants. Since egg-laying queens move with individuals forming new colonies, just a few ants in a potted shrub will have everything they need to establish a new nest.

Nurseries and garden centers in Maine are working hard to prevent the spread of the European fire ant. Still, gardeners should inspect the soil of containerized plants before making a purchase. If you find ants, notify the nursery staff so that they can follow through with identification and treatment.

The greater risk of importing the European red ant to your garden lies with the purchase of infested topsoil or bark mulch. And along with the ant, any of several persistent weeds, including horsetail (Equisetum), Japanese knotweed, Oriental bittersweet and Norway maple, also could be introduced to your garden in purchased soil or mulch.

There are several questions to ask when buying loam from a landscape contractor and all are better asked on-site. For example, how long has the loam been there? If you see plants growing out of the pile, pass it by.

While you’re there, ask a few more questions to ensure that you are buying quality loam. Where did the material come from? Has it been screened to eliminate debris such as rocks, bricks and roots? Has a soil test been done and, if so, what were the results? Has the loam been blended with municipal waste and, if so, does it have a disagreeable odor?

If you are asking these questions on-site, you can personally dig into the pile, looking for weeds and ants. At the same time, squeeze a fist full of soil, make sure that it is not too sandy (the sample does not hold together) or contains too much clay (the sample does not break apart when dropped).

Before purchasing topsoil, taking the risk of bringing a noxious weed or insect into your garden, consider working with what you’ve got. Construct a raised-bed frame from large rocks or lumber, then dig in lots of composted manure to the existing topsoil, and mulch with compost. In a few years you will develop a garden soil that can grow anything. This is how Marjorie’s garden evolved from a thin layer of soil covering a slab of ledge.

Send queries to Gardening Questions, P.O. Box 418, Ellsworth 04605, or to rmanley@shead.org. Include name, address and telephone number.

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