Trailing Strawberry Bush 2011

by Carl Strang

One of my autumn rituals is to visit Meacham Grove and Maple Grove forest preserves to continue a study I began in the 1980’s, of two plants and their herbivores. One of these, which occurs only at Meacham Grove, is the trailing strawberry bush.

Trailing strawberry bush is a low, sprawling member of genus Euonymus.

Once an abundant understory plant, this species was reduced to a minor component of the forest community by the colonial caterpillars of the ermine moth, which defoliated the plants repeatedly in the 1980’s. Though I have not seen signs of that moth in years, the trailing strawberry bush has not grown much, in part because of browsing by deer, and in part because of scorching by controlled burns. No new fruits have been produced since 2000.

This year, herbivory again was minimal though a couple study patches had been browsed a little by deer. There was no burn last year, and two of the 16 study patches showed some growth, but 5 were smaller, probably because of overtopping by other plants. One of these apparently is gone as I could find no trace beneath the Virginia creeper and other plants. Mean and median measures of patch size were close to last year’s, but these are very small (median ground coverage by patches is 0.1 square meter).

There is no question that the overall floristic quality of this forest has improved thanks to the burning and other management measures, but such good work has its casualties as well. The trailing strawberry bush is not endangered there yet, but at best it is holding on.

Literature Review: Ermine Moth Origins

by Carl Strang

One set of journals I am able to follow continuously is published on-line by the Public Library of Science (PLoS). A paper in PLoS ONE attracted my attention this past year because it related to one of my own studies. Here’s the reference:

Turner H, Lieshout N, Van Ginkel WE, Menken SBJ (2010) Molecular Phylogeny of the Small Ermine Moth Genus Yponomeuta (Lepidoptera, Yponomeutidae) in the Palaearctic. PLoS ONE 5(3): e9933. doi:10.1371/journal.pone.0009933

Each year I follow the story of the trailing strawberry bush at Meacham Grove Forest Preserve (most recent chapter here). That plant’s main consumer historically at Meacham has been the ermine moth Yponomeuta multipunctella.

This paper by Turner and company gave me some context. They looked at all the members of genus Yponomeuta worldwide, and concluded that the genus first evolved in far eastern Asia, originally feeding on leaves of plants in family Celastraceae. As they diversified and expanded west they spread to other plant host families, but some reverted to Celastraceae (which includes Euonymus obovatus, the trailing strawberry bush).

It turns out that Y. multipunctella is the only North American moth in that genus. Science is about story, and connecting stories and giving them context is part of the satisfaction I draw from science.

Literature Review: Food Web Stability

by Carl Strang

Though my annual scientific literature review focuses on the current year’s publications, sometimes I have to backtrack because I learn of a significant paper I missed in a previous year. My most recent time in the Northern Illinois University library included the search for such a reference. I learned of it through a review or news article in Science, which I count on to keep me informed about significant papers in the journal Nature. I don’t have the time to follow both.

Neutel, Anje-Margriet, et al. 2007. Reconciling complexity with stability in naturally assembling food webs. Nature 449: 599-602.

Random models of communities predict that complexity will lead to instability. If such models were correct, there would be fewer species in wild communities than we observe. This study looked at soil communities in which increasing primary productivity correlated with increasing biodiversity. Critical to stability were interactions involving omnivores and diet switching. If a significant predator became too abundant, threatening food web stability, its numbers were reduced when its own predators switched their diet to concentrate on it. An example involved bacteria, a bacteria-feeding nematode, and another nematode that could feed on either of the others.

Diet switching is a common behavior in animals. Gulls, like the glaucous gulls in the photo, have a very broad diet. In my graduate study of these birds in Alaska, I found them flexibly switching among such diverse foods as fish, marine invertebrates, small rodents, bird eggs, young birds, carrion and berries as these different foods became available in different seasons and different places. Gulls have predators of their own, as I observed on Adak Island.

The young eagle caught the glaucous-winged gull in flight, but shortly after I took the photo the youngster was rewarded for its effort by the adult eagle driving it away from its catch. Neutel et al. point to diet switching as a mechanism for maintaining biodiversity. I also have seen an example of what happens when systems lack such switching. In earlier posts I have described my study of the trailing strawberry bush and ermine moth at Meacham Grove Forest Preserve. 

The ermine moth caterpillars have only one food, the trailing strawberry bush, in this forest, and apparently their own specialist parasites lag behind them. There is no capacity for switching, and the result has been boom-and-bust population dynamics.

Trailing Strawberry Bush 2009

by Carl Strang

A continuing study that I began in the 1980’s regards a low forest shrub, the trailing strawberry bush (Euonymus obovatus), at Meacham Grove Forest Preserve. Last year I outlined the history of this study. The plant’s nemesis, colonial web-spinning caterpillars of a tiny ermine moth, have been absent from the scene since 2002, and did not return in 2009. The photo below shows a caterpillar-free sprig this past June.

EUOB 7JE09b

September is when I make my annual check of Euonymus patches at Meacham. Leaf consumption by herbivores was minimal in 2009, less than 10% in 14 of 16 surviving patches. The other two patches lost around 10%. Though the plants were affected by a controlled burn in 2007, the net effect for them appears to have been positive as competitors were hurt more than were Euonymus. Of the 16 patches, 14 showed growth in 2009, one was the same size as last year, and one was smaller. The median product of patch length x width is 5.5 m2, an increase from last year’s value of 1 m2. Since these patches are rather sprawling, containing a lot of empty space, a better measure is the rough coverage if the scattered elements of the patch all were brought together. In 2009 the range was 0.01 – 2 m2, median 0.25 m2. Even 2 m2 apparently did not provide enough photosynthetic power for fruit production. I have not seen fruit at Meacham since 2002.

Euonymus obovatus fruit b

However, if growth continues I expect to find the beautiful fruits of these plants returning in the next few years.

Tortricids

by Carl Strang

Last winter over an intermittent series of posts I summarized some of my research on leaf-eating insects in DuPage County forests. Most of that work was in the 1980’s, but I have continued a couple of studies to the present day. One of those is following leaf miners of sugar/black maple leaves . In my study forests I found that the maples host a long list of leaf consumers. Each year the parade of them begins with the tortricids. Here is the adult stage of one of those moths, Choristoneura rosaceana.

Tortricid adult b

The wingspan of this mounted specimen is three-quarters of an inch. Its small larva looks like this:

Tortricid caterpillar b

The caterpillar bites through the major veins of the maple leaf at the base, so that the leaf wilts.

Tortricid collapsed leaf b

This presumably cuts off the leaf’s ability to produce defensive chemicals, and also provides a shelter that protects the caterpillar from birds, which focus on more easily gleaned prey. When the caterpillars become abundant, their numbers get knocked back by parasitic wasps. I found that the wasps’ eggs are readily visible in the parasitized caterpillar.

Tortricid parasitized b

This year I decided to go back to my study areas and see what the tortricids are up to. The bottom line answer is, not much. There were a few, but in my random samples of 20 maple saplings per preserve I found only one tortricid caterpillar, at Maple Grove. There were none at Meacham Grove, though I did see a few on saplings outside the sample. This compares to the peak year of 1982, when 63 percent of maple saplings at Maple Grove and 88 percent at Meacham Grove hosted at least one tortricid caterpillar. I plan to continue taking this measurement in coming years.

Incidentally, while sampling Meacham Grove I checked out the trailing strawberry bush (Euonymus obovatus) plants to see if they have ermine moth caterpillars this year (I reviewed this study last winter). The plants all were clean and green.

EUOB 7JE09b

At one point I looked down and saw this.

Campaea perlata 1

Sticking out beyond the edges of the enchanter’s nightshade leaf were moth wingtips. I tried holding the camera underneath and taking a photo without looking through the viewfinder.

Campaea perlata 2b

The pair of moths apparently had mated the previous night and were waiting out the day. Having acquired my contingency photo, I carefully inverted the leaf. The male moth took off, but I was able to get a clearer shot of this beautiful pale green, leaf-mimicking Campaea perlata female.

Campaea perlata 3b

On the way out of Meacham Grove I got the opportunity to photograph this mourning cloak caterpillar.

Mourning cloak caterpillar b

All in all, this was an enjoyable return.

Some Herbivore Generalizations

by Carl Strang

 

Today I want to conclude this little series on leaf-eating insects by sharing a few generalizations and observations.

 

First, when I looked across all the insects and plants of the Maple Grove and Meacham Grove Forest Preserves in the 1980’s, I found that the more abundant a plant’s leaves were in the understories of these forests, the more kinds of damage they had. I take damage types to be an indication of herbivore types, so more abundant resources are exploited by more kinds of consumers.

 

Second, the amount of leaf surface a plant loses to herbivores is related to the length of time its leaves are out. Plants with leaves open for the entire growing season accumulate more loss than plants which are only out in spring, for example. An interesting demonstration of this pattern is that two kinds of plants, Virginia waterleaf and false rue anemone, send up new sets of leaves late in the season, too late for insects to focus on them. It seems likely that this allows the leaves to gather energy, free of consumers and in a time when the canopy is opening up again with leaf fall.

 

False rue anemone, Isopyrum biternatum

False rue anemone, Isopyrum biternatum

 

Third, the different insect species show few indications that they are interacting. Their abundance is kept low enough that they don’t impact one another most years. Also, they often feed on different plant parts, even on different parts of leaves. This may be a result of past competition selecting for this specialization, but nothing in my study addresses that possibility.

 

Finally, there are many patterns in the herbivore population ups and downs. Few are abundant every year. Most have years of high abundance and years of low abundance. One extreme was the ermine moth , which built up over several years to the point where it killed about 90% of its host, then collapsed and could not be found for years at a time. Another example is the day-flying moth Trichodezia, which in some years was seldom to be seen, and in other years up to 5 might be spotted in an hour by someone walking through the forest.

 

I will continue to follow at least the Euonymus and the maple leaf miners, and may go back to gather more information on other component communities of these forests.

Trailing Strawberry Bush and Ermine Moth

by Carl Strang

 

Trailing strawberry bush is a beautiful, uncommon woody plant in DuPage County forests where there has been relative long term protection from fire. It is a native member of the genus Euonymus, E. obovatus to be exact, a close relative of the popular winged euonymus (problematic invader in native woodlands) and wahoo (a better choice, as it’s native) of landscape design. Despite its diminutive stature, E. obovatus is recognized as a euonymus by its green stems, opposite leaves and distinctive flowers and fruit. Healthy stems can stretch up to a foot above the ground, but it mainly grows horizontally in colonies that can spread over more than 50 square meters, though usually much less than that.

 

euonymus-obovatus-fruit-b

 

Caterpillars of the ermine moth Yponomeuta multipunctella are specialist consumers of euonymus leaves. They are small, the moths perhaps half an inch long, but the caterpillars live in colonies, collectively spinning webs among the foliage, and together can defoliate patches of the euonymus. I began studying the interaction between this moth and plant in 1983 at DuPage County’s Meacham Grove Forest Preserve.

 

yponomeuta-larvae-b

 

When I first collected data in 1983, the ermine moth population was rapidly increasing in the many and often large Euonymus colonies. Defoliation in subsequent years, with a peak in 1985, resulted in decimation of the Euonymus and population collapse of Yponomeuta, which nearly were gone by 1987. After a hiatus I returned to the study in 1998, and found that a slow recovery by Euonymus had begun. In 2000, with Euonymus still well below its 1983 abundance, Yponomeuta reappeared in significant numbers, and sustained them in 2001, but not nearly at the levels I saw in the 1980s. In 2002 they were practically gone again. I have not seen them or signs of their presence since that year.

 yponomeuta-moth-b

Nevertheless, Euonymus recovery has been slow at best, in part because it was decimated so drastically by the ermine moth caterpillars. The median product of patch length and width in 1983 was 9.5 square meters, in 2008 was only 1 square meter. In recent years there has been some browsing by whitetail deer and possibly eastern cottontails.  A managed burn of the leaf litter in the understory of my study area last year appears to have had a mixed impact on the plants. Some trailing strawberry bush patches clearly were hurt, but others showed good growth this year, possibly because they were hurt less by the fire than were their competitors.

 

Euonymus obovatus has produced no fruit in study colonies since 2002, and there has been some attrition of the reduced patches in recent years. Larger patches continue to hold their own, though, with some continuing to show slow growth.

%d bloggers like this: