Earthworms As Introduced Exotic Pests

retiredhippie(4)December 23, 2008

Judging from the replies I received after my original request for information on earthworm control, the overwhelming majority of gardeners are shocked to hear that someone would want to kill earthworms. So I decided to write at length on the topic, and the way that I see it, there are several issues involved. At the very least, this essay will be a very controversial read for gardeners. Throughout the essay, when I mention earthworms I mean collectively the common earthworm and the larger night crawler. In fact, the night crawler is often the worst offender of the two.

I knew it was a long shot to ask moss gardeners the question of earthworm control but I had become frustrated with trying to find an answer and sometimes long shots do pay off. After all, it was a moss garden and surely some other moss gardener must have had the same problem. In fact, I believe they have but apparently had not found a solution either, or they have been conditioned to believe that earthworms are a special gift from the gods, certainly to be treasured. By the time I asked for help on the gardeners forum I had been actively searching for a solution for over a month, and getting nowhere.

First, some basic facts about my situation are in order. The moss garden here is about 5,000 square feet and has had no vegetation under trees for 5 years. Even after all that time there was still an extremely high population of earthworms; I could easily pick out 10 or more of the critters in 1 shovel of soil. There are open areas on three sides of the tree-shaded garden so there is considerable air movement through it, especially in the spring and autumn. Because of this, the mosses most suited and the ones that have been colonizing most successfully are several species in the flat growing Eurhynchium genus. They are feathery and flat, anchor to the soil and grow slowly because the air movement does not allow high moisture around them for extended periods. These species all stay nicely green even when totally dry during hot droughty periods. They do have the potential to form a nice groundcover and can take some foot traffic. The taller growing mosses that IÂve occasionally tried to plant in donÂt make progress because the air movement keeps the tops dry, even when the soil is wet after a rain. Besides that, birds often peck it to pieces and pull it from the soil looking for worms. (I grow my vegetables in a mulched garden, sometimes the birds root around in the mulch for worms so severely that they dig out vegetable seedlings and smother others by covering them with tossed around mulch.)

Eurhynchium species thrive on a compacted soil surface, actually requires it to stay in place. They also grow over rock and wood near or on the soil surface. Earthworms constantly loosen the soil surface, working under the delicate moss and dislodging it. I think worms could work under thick mat forming mosses and a person would hardly notice it, but in this case itÂs fatal. Also, contrary to what many folks seem to believe, the earthworms, and especially night crawlers do eat the live moss, especially when deprived of the usual dead organic debris they like to dine on. They donÂt give up their homes easily; they donÂt leave an area if organic matter is sparse. Instead, they churn through greater amounts of soil and digest out every tiny morsel. Many times I have seen chewed up pieces of moss pulled down into their holes, often still green. The moss disappears for some distance around a night crawlerÂs hole and in return I get a pile of loose castings. How much can they eat? Well, I have seen areas 5 feet across totally killed.

There is a kind of religious fervor surrounding earthworms, especially among gardeners. Earthworms are these sacred creatures that make all good things happen, lots of worms gives you healthy soil and plants canÂt help but grow stupendously. Without earthworms you canÂt have healthy soil and your plants will probably fail. These notions are all so much baloney. Oh sure, the presence of numerous earthworms may mean you have healthy and fertile soil, however, they didnÂt make it that way. Your soil would still be healthy and fertile without the earthworms. Common earthworms and night crawlers are introduced organisms to North America along with the colonists. So how did the flora and fauna thrive for thousands of years without them? The fact of the matter is, there is nothing beneficial that they do that the natural soil processes, insects, bacteria and fungi donÂt already do. Earthworms dramatically speed up the process, which is certainly not necessary in a balanced soil ecosystem. I believe the bounty would be just as great in my vegetable garden without them, perhaps even more so. They do not increase the lushness of the soil, the release of nutrients from organic matter breaking down will still occur with the bacteria and fungi. Release of nutrients can actually be too rapid, thereby allowing them to leach from the soil with heavy rainfall.

In my moss garden I consider them to be exotic pests. I am a lifelong gardener and I believe that the best place for an earthworm is on the end of a hook when IÂm hungry for a supper of fish.

Speaking of earthworms as exotic pests, I vividly remember my college forest soils professor making the statement to the class that earthworms are the one of the worst pests of forests. This was 40 years ago. The shocked, indignant reaction of nearly all the students was immediate. They had all been told since childhood that earthworms are necessary for a healthy soil. He later took us on a field trip to a forested area to show us a large area that had been infested with earthworms and he reported that each year the area grew in size. The entire leaf mold layer was gone and much of the native forest floor vegetation had died out. The exposed soil surface was loose and increased erosion was taking place. I have since seen documentaries by ecologists saying the same thing. Earthworms are killing native wild flowers in forest areas by destroying the mulch protection of the leaf mold layer.

Now I get to the subject of controlling earthworms. I have tried every method suggested to me on a small scale, even those that would be prohibitively expensive on a large scale like in my moss garden. As mentioned earlier, limiting organic matter did not get the earthworms to move on. Soil drenches like mustard, vinegar and ammonia got a few to come to the surface, though many more simply retreated deeper into their burrows only to come back up a few days later and resumed their damage. For adequate control I obviously needed something that killed on contact and prevented them from retreating downward. The worms I did pick up I threw out on the lawn, a feast the birds appreciated. My garden is already overpopulated with earthworms and I didnÂt want more worms or birds in there. The other disadvantage of these chemicals is that there was noticeable moss burning from some of them.

Next I tried the agri-chemical suppliers for the farmers. Most of these guys were equally shocked at my request, while those that understood that there really could be a problem refused to recommend anything. Since nothing is legally allowed on labels for earthworm control they would not say which would work, they were afraid of the government on their heads. After literally hours on the internet I finally connected with an international turf management forum. I discovered that turf managers for golf courses, cricket fields and croquet lawns have a need for earthworm control. The uneven turf, especially on areas like golf greens caused by heaps of castings is unacceptable. Some of these folks, especially in other countries are using some pretty awful stuff for lack of availability of sensible control methods on the market. Resorting to heavy rolling of the turf to smooth out the casting mounds is an unfortunate necessity for some managers. This in turn compacts the soil, eliminating even more aeration of the soil than the earthworms were giving. Lawn care contractors for homeowners do the same, making money on earthworm damage.

I did learn of a good earthworm control chemical from the turf managers. While itÂs not cleared for that use, it is cleared for use on soil insects and grubs, and is widely sold. So if asked, IÂm using it for grubs, controlling my earthworms is just a beneficial side effect. I applied it as a soil drench in combination with a wetting agent in early autumn, after irrigating the moss garden to draw the earthworms closer to the surface. Earthworm activity ceased immediately. The earthworms didnÂt come to the soil surface, certainly an advantage as that eliminated the risk of secondary poisoning of birds. I did notice that mole activity ceased in a few days; secondary poisoning of them would be a definite plus. Although itÂs also possible that the moles just moved away in hunger because earthworms are a major food item for them.

After 2 weeks a minor amount of earthworm activity could be observed. The chemical is short lived and the earthworms that were too deep for the application to reach were left alive to come to the surface. I stress that the activity was greatly reduced, IÂll watch in spring and if a second application is warranted for complete control IÂll do it then. ItÂs a good characteristic of the chemical to be short lived to avoid ground water contamination should there be a heavy rain. The final major advantage of this chemical is that it did not damage the moss as the other control methods did.

By late fall I noticed tiny bits of moss re-establishing in the earthworm devastated areas. ItÂll probably take 2 years to get good moss coverage, but I finally will have the progress I have been straining to see for 5 years. Do I feel guilty over the dead earthworms? Definitely not, no more than killing aphids on the rose bushes. May they rest in peace.

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botann(z8 SEof Seattle)

You must be from the upper Midwest. I heard there was a problem there with worms and the leaf litter they ate. Seems to be changing the ecology. The worms are thought to have come from fishermen that threw away unused bait worms.

We have native worms here in Washington State that are very big and endangered. Google Giant Palouse worm.

    Bookmark   December 25, 2008 at 2:18PM
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In my area we also have native worms as this area was below the glaciers' reach. But I have heard about the non-native worms aggressively eating leaf litter (and I guess other stuff!) in the areas that the glaciers covered.

    Bookmark   December 25, 2008 at 6:20PM
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Although what you write is compelling I dont agree with all of it. I do understand that trying to grow moss in an area with a high population of earthworms who might not have any other food source would impact your garden, I do not believe an entire forest can be decimated by earthworms eating all of the leaf litter, especially since most of the food of an earthworm is litter which is in the process of becoming decayed not just freshly fallen leaf litter. I would be interested in seeing paperwork on exactly what species of earthworm was the culprit in this devastation.

    Bookmark   January 8, 2009 at 9:47PM
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taz6122(N.W. AR.6b)

Are you sure these aren't flat worms. They are shaped like Nightcrawlers but they are flatter. They will devastate an area very quickly.

    Bookmark   April 18, 2009 at 12:43AM
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ezgrower(z5 IL)

Mazer, below is a link to the Minnesota DNR website with some information about the impact of exotic earthworms in native hardwood forests in the upper midwest. You can find more information--including abstracts for scholarly articles describing research studies, and articles in such magazines as National Geographic and Scientific American--by Googling on: invasive earthworm forest

Similar changes are happening in eastern forests, although it isn't as obvious because them are mostly secondary regrowth and already "altered". Reports of commercial impacts too, like Asian earthworms consuming the soilless mixes out of the pots of perennial plants at nurseries.

The validity of a value-laden label like "good" or "bad" is subjective; if you value the experience of visiting a natural, undisturbed forest rich with floral and faunal diversity, then the changes being wrought by exotic earthworms in native hardwood forests is NOT "a good thing".

Hope this information is helpful.

    Bookmark   May 14, 2009 at 6:02PM
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Non Native worms invading and changing the forest floor ecosystem is a pretty serious problem, and you are correct, not many gardeners have information about this. I garden in a very urban area, not much chance of my red worms escaping and wrecking havoc, but in northern states this is actually very critical, that gardeners and fishermen do not introduce any more non natives.

But , like the introduced honeybee, earthworms now have an important place in agriculture.You might enjoy reading what Darwin has written about worms.

diatomecous ( sorry, bad spelling!) earth is harmful to earthworms.It might encourage them to migrate elsewhere,
and might be less toxic to work with than the chemicals you mention.

    Bookmark   May 18, 2009 at 1:16AM
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ezgrower(z5 IL)

Effectiveness of diatomaceous earth against earthworms, if any, is at best transient. Probably a waste of good money. Most earthworms prefer soils of neutral to slightly acidic pH (6.0-9.0), although some prefer more acidic conditions. As I understand it, mosses prefer growing on firmly packed acidic soils with a pH between 5.0 and 5.5. Perhaps you can make a moss garden less attractive by creating an "island" of more acidic soil, optimum for mosses but less so for earthworms. However, acidiying the soil for the mosses/earthworms may be detrimental to other plants in that spot (e.g., the tree your mosses are growing under).

There are no magic bullets. Gathering earthworms by hand and removing them, poisoning them, etc., is going to be a lot of work/expensive with no result in the long run. If the environment is desirable for the worms, their population will grow to the sites carrying capacity. If you can modify the environment so it is less desirable for earthworms but good for what you are trying to grow, then you have a chance.

If you are in the northeast, the worms you may be struggling with aren't the introduced european lumbricids, but asian earthworms. These are very different beasts. They overwinter in the northeast as eggs that hatch out in the spring, develop to maturity and die in the winter. You can recognize them by having the clitellum (the swollen ring of segments on adults) further forward than on lumbricids, and they are EXTREMELY active -- they can flip right out of your hand if you pick them up. They tend not to burrow into the mineral soil, but burrow through and decompose the organic/litter layer. In Pennsylvania, there are reports of them hurting nursery operations by getting into containers of perennial plants and literally eating up the soilless potting media. No know solution.

    Bookmark   May 18, 2009 at 11:17AM
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ezgrower(z5 IL)

Oops, should proofread better... should read:
"Most earthworms prefer soils of neutral to slightly acidic pH (6.0-7.0)..."

    Bookmark   May 18, 2009 at 2:27PM
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I am wondering if your specifc problem with regard to your moss is due to nematodes and not earthworms, I would want to actually inspect the area you are having issues with.
I read the information on the link about the introduced earth worms and I see no scientific information on the link so far. It is not that I dont think it is not possible, I would just like to see more solid information...

    Bookmark   May 22, 2009 at 7:34PM
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ezgrower(z5 IL)

Mazer, as I suggested in my post, you can find more information--including abstracts for scholarly articles describing research studies, by Googling on: invasive earthworm forest. Peer reviewed scientific journals have expensive subscriptions; if you want to read full peer-reviewed journal articles, you'll need to go you your university library.

    Bookmark   June 1, 2009 at 11:06AM
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hykue(2b Saskatchewan)

As a recently graduated environmental biologist, I have talked to people who were actually working on some of these studies, and they are definitely worried about nightcrawlers specifically. Their range is still expanding, and in biological circles, I got the impression that that was considered dangerous. I never looked into it more at the time, but the impression I got was definitely in line with the "not a good thing" posts above. I believe it was in the same discussion as we talked about snow geese, who are destroying their breeding habitat (which they share with other critters) at an alarming pace. Being associated with snow geese is not a sign of being a benign organism! Don't get me wrong, I like earthworms, but that doesn't mean that they are harmless, just that that's what I learned as a kid.

    Bookmark   June 1, 2009 at 11:39PM
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Thanks to all who posted above - I know it's been years, but it's still helpful if anyone keeps up. I recently moved to CT and have been amazed how efficient the nightcrawlers are at breaking up my efforts to establish a moss garden in my shaded entry. They churn up about 4" of topsoil, decimating the surface the moss needs, even when not eating the moss directly. And they do it again and again. That bothers me most - but equally shocking is how they can turn 8 yards of mulch in the spring on other beds into plain worm casting dirt within a month.

I'm minimizing my moss garden to begin, trying to lower the ph to 5 in that area. Has anyone ever tried that? I will post my results.

    Bookmark   September 18, 2011 at 8:21PM
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