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The Pervasiveness of Invasiveness From the September 2003 issue of "The Buckeye", the official magazine of the Ohio Nursery & Landscape Association. Jim: Lets begin a series on invasive species, Joe. Joe: Agreed. Lets start with something to really sink our chewing mouthparts into! Jim: Which invasive species do you think we should feature? Joe: Emerald ash borer (Agrilus planipennis), a truly frightening new threat. Jim: So, not surprisingly, Joe, you would choose an insect for an example. I think something historical would be better to start with by way of background. How, about the Dutch elm disease pathogen (Ophiostoma ulmi) which was first reported in the U.S. in the late 1920s. We agreed to start at the beginning. Joe: And of course, Jim, with your pathological bent, you have chosen a plant pathogen. Why, you... Larry: ...Boys, let me settle this. Invasive species go back a long way, longer then your two choices, in fact far earlier if you think of it in the long term. Many plants, pests and pathogens, and even you two West Virginians, have invaded Ohio. At any rate, lets start a few years earlier than the invasive Dutch elm disease pathogen, which by way of fairness to Joe, also involved an invasive vector, the European elm bark beetle. Jim and Joe: You are a true gentleman, Larry. Your students at ATI are indeed fortunate. Proceed with your invasive species example. Larry: My choice this month is garlic mustard (Alliaria petiolata), not exactly an ancient invader of Ohio but earlier than your two choices. According to references in the M.S. thesis of Mark Frey (OSU, 2002) garlic mustard was brought over from its native Europe, probably for herbal or nutritive purposes, with a first report in the US. on Long Island in 1868 and in Ohio in 1899. It seems to have exploded in recent years in woodlands and parks for unknown reasons. Joe: Why is it so successful? Larry: Several reasons, according to Mark Frey’s thesis advisor, OSU weed ecologist John Cardina. Massive seed production in a stand of garlic mustard is one with high seed viability and survival in the soil for five years or more. Also, garlic mustard reportedly produces allelopathic chemicals that may change soil microflora to its advantage over other species. Additionally, it thrives in soils with high earthworm activity, doing well where thatch has decomposed. Garlic mustard also has the great advantage of not being beloved by browsing deer. In addition, when weed-pullers try to remove it they often fail to do it early enough. Even if plants have only flowered but not yet gone to seed when pulled, this weed has the ability to continue to produce viable seed on the decapitated refuse. So, as Mark Frey points out in his thesis, if garlic mustard is pulled once flowering, do not let the plants lie unless you are willing to bag them in plastic. Jim: So, is hand pulling the best mode of control? Larry: No. Mark’s thesis presented a lot of insight into a more effective control of garlic mustard. This weed is a biennial, with a two-year life cycle, and this life cycle is crucial in a number of ways. Garlic mustard germinates in the late winter and early spring, and in that first year develops a rosette of scallop-shaped leaves, often mistaken as ground ivy or some unknown, but inconsequential herbaceous plant. As is the case with biennials, this first year growth does not flower and produce seed. This first year rosette of leaves on garlic mustard, though, does not really go dormant at the end of most plant’s growing season and remains green pretty much throughout the subsequent fall and winter. And therein lies its vulnerability and our opportunity. Glyphosate herbicide (Roundup, e.g.), applied during cold fall and winter temperatures to the green rosette of garlic mustard leaves will control this invasive weed. The late fall and winter is a great time to do it, because... Joe: Non-targeted plants, which do not have green leaves and are dormant, are not affected by the glyphosate. Larry: Exactly. Waiting for the second year, when the plant shoots upward and develops its set of wedge-shaped leaves and tiny flowers, in very early spring along with spring beauties (about the same time as Claytonia virginiana) is not the best time for control. As mentioned, the long narrow silique fruits can develop viable seed unless you pull the plants before flowering is too far afield. Glyphosate applications for one year may not be all she wrote, even if you get good coverage of the rosettes of first-year leaves, since seeds can remain viable in the soil for at least five years, but it is a start, and because of the knowledge of the life cycle of this invasive weed, a useful approach. Jim: Well, I must say, Larry, your example is indeed a good one. Joe: A great prelude, as it were, for my insect example, the more recent emerald ash borer story. With this invasive insect, the plot is rapidly sickening. Thought to have been introduced from Asia into southeastern Michigan five years ago or so, it was first identified there and in the U.S. only last year (2002) in July. Dan Herms recently took us to the southeastern Michigan sites in August and the developing devastation in southern Michigan is spreading and serious. So serious that it is no longer legal to import, export or sell ashes in the state of Michigan. Jim: The problem occurs on both stressed and non-stressed ashes? Joe: Yes, and on green ash (Fraxinus pennsylvanica), white ash (F. americana) and black ash (F. nigra). Jim: Rapid death of the ashes? Joe: Yes, these bright green buprestid beetles, in the same genus (Agrilus) as bronze birch borer but feeding only on Fraxinus species, lay eggs in May and June in the bark crevices of ash trees. The larvae that hatch feed on phloem and outer sapwood tissue, developing extensive S-shaped galleries, resulting in disruption of the vascular tissue of the tree, resulting in girdling of branches and tree death , typically within 1-2 years. The larvae overwinter in the sapwood, emerge from the tree in spring, leaving characteristic D-shaped exit holes, pupate in April and early May, and adults start the cycle anew. Adults feed on foliage, leaving jagged leaf edges where they have fed with their chewing mouth parts. Many ashes affected by emerald ash borer develop extensive epicormic branches and, as the trunk dies, you will see a dense cluster of root sprouts at the base of the tree. Jim: So this emerald ash borer is going to be a big deal? Joe: As I said, it already is in Michigan. What part of the EAB quarantine for 13 counties in Southeast Michigan and a one-year ban on sale and movement of all ash nursery stock into and within the lower peninsula of Michigan don’t you understand? Jim: I hear you. And emerald ash borer in Ohio? Joe: Several known incursions, with regulatory tree eradications the result. Jim: So, now for my history lesson of Dutch elm disease first being identified by Dutch pathologists in Europe in 1921 and then first being identified in the U.S. (in Cleveland) in 1930 from elm logs being railroaded across the country and then reversing the trip and becoming a major problem in England in the 1960s. It is a long story... Joe: Lest we go too rapidly from borers to boring, editor Jennifer Gray reminds us that our time is up. I do hope that the story of emerald ash borer in the U.S. will not be that sort of tale as Dutch elm disease or chestnut blight or countless other horror stories. Larry: The pervasiveness of invasiveness over the years, example which we all have to learn from and lie with, from kudzu... Joe: ...To gypsy moth... Jim: ...From bacterial fireblight... Larry: ...To Japanese knotweed.... Joe and Jim: Will be the topic in Point CounterPoint for the next several months. Jennifer Gray: And a new upcoming feature of the Ohio
Nursery and Landscape Association’s consumer website BuckeyeGardening.com! Larry Steward, Joe Boggs
& Jim Chatfield |