Garlic Mustard Study Update

by Carl Strang


Since treating the garlic mustard plots , I have returned weekly to check on them. As of April 16, the seedlings have barely progressed beyond the cotyledon stage.




Untreated (control) plants continue to add leaves and expand in size, though they remain rosettes.




Some plants have survived the treatments, either because they were so tiny that I missed them or because pinching them off did not in fact kill them.




Later I will examine those survivors in the pinch treatments to see which of these possibilities was the case. A few new plants have appeared each week since the treatment. As of April 16, a total of 16 plants had appeared in the 9 squares where I had pulled plants, and 76 plants had appeared in the 9 pinch treatment squares. Where I had counted 418 seedlings in pinch treatments and 450 in pull treatment squares at the start, by April 16 these numbers were 838 and 944, respectively. At the end of the study period I will apply statistical tests to determine whether these differences are due to chance or to real differences between the treatments. I also will return next year, to see if more seedlings emerge in pull treatment squares than in the others.


Garlic Mustard Removal Study

by Carl Strang


One of the challenges facing people trying to restore biodiversity to native woodlands is invasive plants. Earlier  I outlined the general problem in the context of shrubs. The herbaceous plant causing the most trouble in our woodlands is garlic mustard. Garlic mustard is a biennial. Seeds sprout in spring, grow into rosettes of leaves that survive the winter, then the plants grow up, flower and produce seeds in their second spring. At this point in the season the rosettes look like this.




When I was at Willowbrook I undertook to remove garlic mustard from the fenced area that contains the outdoor animal exhibit. When I was transferred to Fullersburg I did the same in the Wildflower Trail area. Both were high quality areas in terms of the native plants that were present, but garlic mustard was expanding, and suppressing the native wildflowers. I started pulling out the garlic mustard each spring, and was gratified by the quick recovery by diverse native species. Then I began trying fall and winter pulling, and got good results with that, too.


My annual review of the scientific literature last December turned up a study* describing success in controlling garlic mustard by clipping the plants rather than uprooting them. I decided to try a study of my own, that would compare results of uprooting plants versus breaking them off, early and late in the season. Yes, a study had been done. But replication is important in science. I wanted to see for myself that new plants can’t grow up from the decapitated roots. Also, the earliest the other researchers had clipped their plants was late April. I wanted to try it earlier. My move to Mayslake was timely, because the restoration program is relatively advanced there, and I can focus on research rather than rescue.


I have set up 3 study plots, each 3 by 3 meters. I used large nails to mark the corners.




When the time came to treat the plots, I temporarily outlined the plot with bright orange string.




Each of the 9 square meters in each plot gets one of 3 treatments: uprooting, breaking off below the lowest leaves in mid-March, and a control that will be left until the plants are about to flower. At that point I will cut them off below the lowest leaves. This last treatment follows a practice recommended by some experienced restoration specialists, who discourage uprooting because it may stimulate germination of garlic mustard seeds in the soil. I used a random number generator (easy to find on the Internet) to determine which squares got each treatment.


Here is an experimental square from which plants were removed.




And here is a control square with the plants still in place.




I counted the plants in each square as I treated them. In the three study plots combined the total number of pulled plants was 1395, pinched off total 1617, and control plants 1176. The overall average density was 155 plants per square meter. Now, I wait and see what will happen. I’ll provide an update later.


(It should be obvious, but I’ll state it anyway, that I could do this kind of manipulation on public land as a forest preserve district employee, but still had to get clearance to do so.)


*Here is the reference for the study I mentioned: Pardini, Eleanor A., Brittany J. Teller, and Tiffany M. Knight. 2008. Consequences of density dependence for management of a stage-structured invasive plant (Alliaria petiolata). Am. Midl. Nat. 160:310-322.

Gypsy Moth Egg Masses

by Carl Strang


The beautiful oak savanna at Mayslake is facing a challenge.




See the oval, pale yellow blobs on the underside of the oak branch? Those are egg masses of the gypsy moth. Here is a photo of one being produced.




The egg masses are fuzzy, around an inch or a little bigger in diameter, and can be attached to any solid object on the ground or above it. I can’t think of anything you might see in our area that could be confused with them.


The gypsy moth is not native to North America. It escaped from an ill advised breeding program in New England, and its westward spread has reached Illinois. The caterpillars eat a variety of tree leaves, but oaks have the misfortune of opening their leaves late in the spring, just as the tiny caterpillars emerge from the eggs. The expanding leaves have not built up their chemical defenses, and so oaks are perhaps the trees most susceptible to gypsy moth attack. I say “attack,” but that is a human judgment. The caterpillars are being caterpillars. They are new to the area, so their diseases and parasites haven’t caught up with them, yet. Gypsy moth populations typically build within a few years to levels where they totally defoliate trees.


As I mentioned in an earlier post about sycamore anthracnose, a single defoliation episode won’t kill a healthy tree. However, a tree already weakened by another stress, or a tree defoliated repeatedly by gypsy moths, could die. A certain percentage of the forest’s trees will be lost. Ultimately the diseases and parasites will catch up, and the gypsy moth will be cut back to a less harmful density. But they are with us for good. All these predictions are based on what has happened everywhere east of us. The forest preserve district and other agencies are doing everything that can be done to minimize the damage, but we will lose some trees.


There is some debate about what homeowners can or should do to protect their trees. Remember that I am speaking for myself here, but my bias is toward limiting the damage by manual means. There are several steps that can be taken. At this point in the season, the thing that can be done is to look for and destroy egg masses in your yard. Look not only on trees but on any objects, including the house, outdoor furniture, firewood stacks, etc., where an egg mass might have been deposited. Scrape them into a container, fill the container with soapy water, leave it for a few days, then dump the killed egg masses in the trash. Advise your neighbors to do the same. I’ll provide other steps in the appropriate seasons. We’re in for a few unpleasant years, but there will be an end to it. Mayslake should not have major defoliation this year, based on the number of egg masses I am seeing, but now it’s just a matter of time.

%d bloggers like this: