Gentle Herbivores…Not!

by Carl Strang

Crickets and katydids are largely vegetarians, but they are not above adding some meat to the diet. I saw an example of this while sweeping for meadow katydid nymphs at Mayslake Forest Preserve early last week.

This female nymph caught a small beetle that the net also had picked up, and systematically munched it down.

Protein, good for a growing katydid. Incidentally, I have learned from bird banders that they dare not leave birds in a mist net for long or the deer will eat them. So much for the Bambi image!

Literature Review: Butterfly Range and Diet

by Carl Strang

This week’s literature review note is about butterflies. Usually we just think of butterflies as delightful, beautiful bits of nature, but those qualities also attract the interest of scientists. The scientists in this case are J. Slove and N. Janz (2011. The Relationship between Diet Breadth and Geographic Range Size in the Butterfly Subfamily Nymphalinae – A Study of Global Scale. PLoS ONE 6(1): e16057. doi:10.1371/journal.pone.0016057). The butterflies they studied are 182 species in the widespread subfamily Nymphalinae. Our local members of this subfamily include such familiar butterflies as the mourning cloak, question mark and red admiral.

Mourning cloaks pass the winter in the adult form, hibernating in a sheltering refuge.

Slove and Janz were interested in seeing if there is a relationship between the diet breadth and the geographic range of these butterflies. They wanted to test a prediction that species which eat more kinds of plants have larger ranges. The diet of interest is not that of the adult butterfly, but rather of the caterpillar.

Mourning cloak caterpillars eat the leaves of trees in several families, so they would be regarded as having a wide diet breadth.

It turns out that the prediction holds. The point is that there are a lot of different kinds of plant-eating insects. Some have broad diets, others have narrow ones. How did this diversity come about? The possibility being considered is that some insects have large geographic ranges, in part because by eating a number of kinds of plants they can spread over the collective ranges of those plants. Over the course of time, circumstances such as climate change (interposing a glacier or desert, for instance), geological events (raising a mountain range or sea, for example) and chance isolations (a few representatives driven to a remote island by a storm, perhaps) divide a wide-ranging species into separate groups that no longer can interbreed. Each group may then specialize on the reduced menu of plants available to them, and over time can evolve into separate species. This is called the oscillation hypothesis, because over a long period of time it predicts an alternation between wide diets and narrow diets within a genetic line.

Caterpillar Art

by Carl Strang

When I visited Shades State Park in Indiana during my Roesel’s katydid trip two weeks ago, I noticed pieces of leaves lying on the ground beneath some of the tulip trees.

The largest fragment gives a close idea of the intact leaf’s shape. The top was chewed out, but the edge would be a straight line connecting the corners.

The semicircular scalloping along the cut edges is the signature of feeding caterpillars. The petioles, or stems of the leaves, were bitten through rather than broken. I was reminded of Bernd Heinrich’s study of such behavior, and his observation that the caterpillars, after feeding, deliberately cut off the leaves. He concluded that the advantage was to get rid of the evidence that a caterpillar was there, eliminating a clue that might draw the eye of a hungry bird. This also might limit the leaf’s ability to inform the tree that caterpillars are feeding, so the tree might not be stimulated to marshal its chemical defenses.

I enjoyed looking at the shapes the caterpillars created, thinking of them as “found art.” Of course, the caterpillars simply are following the dictates of instinct, molded by evolution. They are not purposefully thinking about avoiding predation or creating art works. That’s our job, to understand and appreciate.

Role Playing

by Carl Strang

 

Sometimes we encounter odd tracks that fall outside the regular gait patterns. This is a sign that something interesting was going on, but what? Here’s an example.

 

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A cottontail was moving over snow. Its gait is the typical slow gallop, with two complete sets of footprints showing here. But in between those sets is something odd.

 

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There is a clear front footprint in the upper right corner. Connecting it to the complete set is a drag mark with toes showing. The drag mark points back toward the right front foot. It appears that the rabbit lifted that foot only, brought it forward, paused with it in the air (toes dragging and marking the point of the pause), then moved it forward some more and planted it.

 

This is a great example of the use of role playing. If you get down and position your feet and hands to approximate the arrangement in the photo, then do what the tracks tell us the cottontail did, you will find something interesting. If you have the discipline to do that without reading further, now is the time to try it. In the meantime, here’s a photo of a cottontail to provide some space.

 

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If you try moving as the rabbit did, you will find your head and shoulders cranking around to the left. The position of your mouth when you plant your right hand will be roughly where your nose was sniffing during the pause you made along the way. The arrow marks the spot in the next photo.

 

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Our cottontail noticed a tasty looking green twig, sniffed it to make sure it was good, then planted its right front foot while it bit off the twig. The arrow points to the bite mark where the twig was removed.

 

Incidentally, this event happened years ago at Willowbrook. I used it as a conservation lesson. The rabbit ate only the one twig from that little shrub, leaving the rest. I could take kids to that shrub after working them through this tracking example and show them the shrub, which thanks to the cottontail’s forbearance had grown many additional twigs for future rabbits.

Poison Ivy as Browse

by Carl Strang

 

Each month brings its own features of interest, its own associations. I have already covered how skunks go on walkabout in February. Another peculiarity of February in northeast Illinois is the sudden preference deer and cottontails have for poison ivy as a food item. Poison ivy was last conspicuous to our eyes in the fall, when it displayed its bright autumn coloration.

 

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Since then it has been its dormant winter self, recognizable by its vine growth form (older vines connected to tree trunks by masses of dark brown or black hairlike projections) and the curved, yellow-brown, finger-shaped buds, visible on two of the three twig ends in the following photo.

 

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But it’s the third twig end that is of interest here. It has been browsed by a deer. If we had seen the deer taking this bite, it would have been standing to the right of the photo and facing left.

 

Through the winter, deer and cottontails change their diets, but in a given month both are eating much the same thing. They are mainly browsers in winter, nipping off live twig ends for the nutritious inner bark and buds. Early in the season they favor members of the rose family like raspberries and roses. In the middle part of winter they shift to a long list of favorites. Rabbits also eat the bark from parts of shrubs they can reach, and take advantage when live branches are broken off in winter storms, as the example from last month illustrated.

 

In February, northeast Illinois deer and rabbits favor poison ivy. Whether this is because that plant undergoes some chemical change that renders it palatable at that time, or whether that is when the more preferred plants have been depleted, or whether it is for some other reason, I do not know and haven’t the laboratory to test such hypotheses. But I suspect that deer and rabbits on the preserves would have a much harder time surviving late winter if it weren’t for poison ivy.

 

Distinguishing whether a cottontail or a deer is the browsing animal in a given case is fairly straightforward. Rabbits, in addition to being shorter and having a more limited reach, have incisor teeth on both jaws. When one nips off a poison ivy or other twig end, the bite is a clean cut at an angle. Deer have incisors only on the lower jaw. A deer bites part way through the twig, pinching against the hard upper jaw, then twists and pulls to remove the bite from the plant. The lower jaw side is cleanly cut, but the upper jaw side is torn.

Brush Removal

by Carl Strang

 

One aspect of Mayslake Forest Preserve that struck me when I moved my office there in November was the impressive restoration work that was taking place (here I am talking about restoration of biological communities rather than the architectural restoration of the mansion, which also is an impressive ongoing process). I think of restoration as the removal of human influences that have disturbed ecological processes and are limiting biodiversity (the variety of life in a place).

 

The dedication of a group of volunteers deserves a major portion of the credit for this progress at Mayslake, and I will feature them in one or more future posts. For now I want to focus on recent support that their effort has received from Forest Preserve District staff.

 

A large part of restoration work is the removal of invasive plants that we have, deliberately or inadvertently, brought to our continent from other parts of the world. Liberated from the consumers and other limits that kept them in check in their native lands, these newcomers have an unfair advantage in their competition with North American plants. So, the task of people doing restoration is to serve as herbivores, evening the playing field by removing some of the invasives.

 

Earlier in January, a crew came in with a brush chomping machine and cleared out an area of buckthorn and honeysuckle that had filled the space beneath the oak trees between Mays Lake and the friary in the southeast corner of the preserve.

 

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I was excited to see this, because it was clear that with those impressive trees in place, the invasive shrubs had been the major obstacle preventing that slope from becoming as beautiful as the previously restored savanna I have shown in past posts.

 

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Then a crew came to burn the scattered brush piles the volunteers had accumulated over the past year in their painstaking hand-cutting of shrubs in woodlands elsewhere in the preserve. Wholesale clearing can’t be done in places where slopes are too steep or where the proportion of native plants is high. There, hand cutting is necessary. The brush pile fires and the smell of their smoke provided a welcome punctuation of my lunchtime walks through the preserve on those days.

 

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I wasn’t the only one noticing those fires, as I will explain tomorrow.

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.

Pachyschelus purpureus

by Carl Strang

 

In recent days I have been describing component communities in two DuPage County forests, and the rules they seem to follow. Today I want to describe an exception to those rules. This is a tiny, wedge-shaped, metallic looking beetle for which I have seen no name in English. Its scientific name is Pachyschelus purpureus.

 

Pachyschelus on Geranium maculatum leaf

Pachyschelus on Geranium maculatum leaf

 

In Maple Grove and Meacham Grove Forest Preserves, I have seen this beetle twice each year (I have seen it in spring at Fullersburg, too). Early in the season they congregate on wild geranium leaves. They scrape little areas off the top surfaces of leaves as they feed, leaving distinctive brown spots (visible in the photo). They reappear in August, but this time feed in the same way on bitternut hickory leaves. Either they go into summer dormancy, or (more likely) have two generations each year, alternating in diet with each appearance.

 

This is a really odd pattern. As I have mentioned, insects that feed on herbaceous plants like the geranium usually must be specialized in their diet because they can deal with only one kind of chemical defense, and each non-woody plant protects itself with a different kind of chemical. This limitation is less true for insects that feed on woody plants like the hickory, because many trees use the same defense, concentrating chemicals called tannins in their leaves. When the leaves are chewed, the tannins grab onto proteins in the leaf tissue, making it difficult for the chewer to digest those proteins. However, the many consumers of tree leaves get around this one way or another, and having done so with one kind of tree often can do so with others.

 

What I described in the previous paragraph summarizes the collective work of many scientists who have looked at hundreds of specific cases. They study insect diets, plant chemistry, draw specific conclusions and then look for general patterns. And for the most part, what I have seen in my study of leaf-eating insects at Maple Grove and Meacham Grove is entirely consistent with established generalizations.

 

But then there is Pachyschelus purpureus. Not only does it eat both an herb and a tree, but as far as I can tell sticks to one kind of herb and one kind of tree only. What it does as a larva I have not found, but I wouldn’t be entirely surprised if there is still more complexity and rule-breaking to be found there. I find it satisfying that so much of the ecology of these forests can be described with discovered rules and patterns. But I also find it satisfying that there are exceptions like Pachyschelus to keep things from being too simple.

Component Communities: Enchanter’s Nightshade

by Carl Strang

 

Today I return to my studies of leaf-eating insects at Maple Grove and Meacham Grove Forest Preserves. In an earlier post I went over my continuing study of the trailing strawberry bush, still recovering from its 1980’s devastation by the ermine moth at Meacham Grove.  Then I reviewed the leaf miners of black/sugar maples, which have been showing a number of interesting patterns in these forests.

 

Now I want to expand the story further by looking at other plants and their consumers I studied in the 1980’s. Today’s focus is the enchanter’s nightshade.

 

Enchanter's nightshade; bird scat mimicking moth resting on leaf

Enchanter's nightshade; bird scat mimicking moth resting on leaf

 

 

 

This plant is a common annual in DuPage County woodlands. It produces arrays of tiny white flowers in mid-summer, then little burs that spread the seeds by sticking to passing animals.

 

Circaea (the genus name refers to Circe, the sorceress of Greek mythology) has a number of consumers. Minute white dots on the leaves are signs that lacebugs or other sucking insects remove the contents of some of the leaves’ cells. Brown areas appear to be caused by a fungus. There is a leaf miner, which I have not identified, and a leaf beetle that I collected but was unable to match to known species. Finally there are two moths: Cerastis tenebrifera, the reddish speckled dart, a member of the huge family Noctuidae, and an inchworm or geometrid moth, Trichodezia albovittata, the white-striped black.

 

The last is interesting in that it is brightly marked and flies in the daytime, so that you might think it a small butterfly.

 

Trichodezia mounted specimen

Trichodezia mounted specimen

 

 

 

The green caterpillars stay on the plant all day, holding their bodies out stiffly when resting so that they mimic their own damage, resembling leaf petioles from which the blades have been chewed away.

 

Trichodezia larva

Trichodezia larva

 

 

 

Cerastis caterpillars feed at night, hiding in the leaf litter during the day. They are the earliest major consumers, active in June, while Trichodezia and the leaf beetle are feeding in July. The leaf beetles fly to the plants at dusk and feed at night. They chew elongate holes between the leaf veins, in contrast to the caterpillars’ chewing in from the edges.

 

There were tantalizing hints of interactions among some of these species in the 1980’s, which have me thinking about going back and looking at this system again. Leaves with brown spots appeared to be avoided by caterpillars and by sucking insects, suggesting that these herbivores were avoiding the fungi or chemical changes induced by them. In a year when Trichodezia were abundant, they and the leaf beetles seemed to be avoiding leaves with one another’s damage.

 

I did not find any of these chewing insects on other plants. This kind of specialization produces little communities centering around each species of plant, leading the ecologist R.B. Root to coin the term “component community.” Collectively these plants and their consumers form a “compound community.” Certainly my observations support this understanding of forests. The component community structure leads to an ecosystem with impressive biodiversity (i.e., variety of living things).

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.

 

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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.

 

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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.

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