Closing the Book on Green-winged Cicadas

by Carl Strang

The green-winged cicada (Diceroprocta vitripennis) provides a favorite example of why I need to include 22 counties in my survey of Chicago-region singing insects. I checked a final two locations recently, and am satisfied that I have a good sense of where this species occurs.

Black dots indicate counties where green-winged cicadas can be found.

The most important habitat feature appears to be sandy soil. Cicadas spend most of their lives underground, so it is not surprising that soil texture can influence their distribution. The mapped area contains two sandy regions: the dunes edge of Lake Michigan in Indiana and Michigan, and the Kankakee River corridor. The cicadas do not, however, extend throughout those soil areas. They thin out quickly in western Starke and Pulaski Counties, Indiana, and do not reach into the sand soil portions of St. Joseph and Marshall Counties.

Green-winged cicadas had an important lesson for me this year. Previously I had thought of them as a woodland species, having observed them singing in trees, especially in black oak savannas and woodlands. Then I came to this spot in Newton County, Indiana:

A single large cottonwood and two smaller trees towered above clusters of shrubs.

I was surprised to step out of the car, expecting to focus on sand-soil grasshoppers, and hear green-winged cicadas singing. As I approached them I found that they were singing, not from the big trees, but from plants no taller than me, winged sumacs and a small black oak.

This discovery provided a welcome opportunity to photograph live green-winged cicadas.

They were very alert, and required a glacially slow stalk.

Feel free to wave back. No one is watching.

I also found a site in Jasper County where the cicadas were singing from shrubs. They can be fairly loud at close range, but they are small, with bodies not much more than an inch long, so the song quickly attenuates over distance. It seems to carry better when the singing perch is in a tree. The song is a distinctive pulsing rasp:

These early season cicadas sing mornings to mid-afternoons, and largely are done by the end of July.

Advertisements

Literature Review: Cicadas and Trees

by Carl Strang

The phenomenon of the periodical cicadas raises a lot of questions. Some of these questions are ecological. Millions of the insects emerge at once every 13 or 17 years, depending on the area, and their sheer biomass suggests that they must influence the ecology of the forest as a whole. Their nymphs grow by drawing sap from tree roots. The adults damage twigs when they cut them to insert their eggs. When the masses of cicadas die, their decomposition releases nutrients into the soil. Thus, from a tree’s standpoint some of the effects are potentially beneficial, others harmful. This was the background for the paper I am reviewing today (Speer, James H., Keith Clay, Graham Bishop and Michelle Creech. 2010. The effect of periodical cicadas on growth of five tree species in Midwestern deciduous forests. American Midland Naturalist 164:173-186).

Periodical cicadas also occasionally feed on tree sap during their brief time in the adult stage. These are sitting very still, and their beaks are piercing the bark.

This research group used dendrochronology methods in southern Indiana, measuring growth ring widths in several tree species. They first factored out climatic influences, ruling out the effects of relatively good or bad growing seasons. Then they tested the effect of feeding by cicada nymphs, oviposition damage and the nutrient pulse following emergence.

Periodical cicada nymph emerging from the ground. Did the nymphs’ diet of tree sap impact tree growth?

Cicada nymphs start out very small, and grow to be relatively large, so if their numbers are impacting tree growth, the trees should grow progressively slowly through the insects’ 17-year cycle. Speer and associates could find no nymphal feeding effect in any of the 5 tree species they examined.

Periodical cicada egg slits in a redbud twig. Typically the twig end dies as a result of this damage.

Sassafras, pin oak and black oak showed some growth loss from oviposition (egg-laying) damage, but these plants are less preferred for oviposition than sugar maple and white ash. The latter appeared to be hit harder, but growth rings showed no effect.

Dead cicadas litter the ground as an emergence ends. Does the pulse of nutrients from their decomposition benefit the trees?

There was a nutrient pulse effect for black oak, pin oak and sugar maple, but it happened 5 years after emergence. The authors suggest that trees may need time to absorb the nutrients and produce extra wood after the adult cicada mortality event. Alternatively, this effect may result from a pulse of mortality in cicada nymphs of that age.

Black Oak, Maybe

by Carl Strang

Not being a botanist, primarily, I am inclined to miss plants unless I really focus on them. I count on their flowers or some other eye-catching feature to draw my attention. Such was the case last week with a particular oak in the north savanna at Mayslake Forest Preserve.

It doesn’t show so well in the above photo, but the leaves on the tree had turned yellow.

That is in contrast to the generally reddish and brown tones of the dominant white and bur oaks, as well as the bright red of the Hill’s oaks. I made my way to the tree, and found that its leaves were shaped like typical black oak leaves.

The lobes are shallow, unlike those of pin or Hill’s oak, and they don’t show the rhythmic size and shape of red oak lobes. The bark is consistent with black oak, though the tree is only a foot in diameter and so I can’t call that conclusive. The acorn caps are somewhere between those of black and Hill’s oak, though more like the former. For now I am calling it a black oak, though I think there’s a good chance that it’s a black/Hill’s hybrid. I am comforted by the fact that even botanists struggle with the oaks.

Hill’s Oaks Resolved

by Carl Strang

In a post more than a year ago I made a preliminary case for the possibility that a group of young oaks on a savanna ridge top in Mayslake Forest Preserve may be Hill’s oaks. There I discussed leaf and bud shapes, which were consistent with Hill’s, but I needed acorns to conclude the case. This year a couple of the trees have produced acorns.

Furthermore, a new paper came out by Andrew Hipp, one of the co-authors of the publication I discussed in the previous post. The new paper, available on line here, goes into detail to separate Hill’s, black and scarlet oaks (the complete reference is: Hipp, Andrew L. 2010. Hill’s oak: the taxonomy and dynamics of a western Great Lakes endemic. Arnoldia 67(4):2-14). The following account is drawn from that paper.

Hill’s oak, Quercus ellipsoidalis, also known as northern pin oak, was distinguished from scarlet oak (Q. coccinea) in 1899 by Ernest J. Hill, who studied populations in Cook County, Illinois. Hipp and Jaime Weber surveyed the genetics of these trees and concluded that Hill’s oaks and scarlet oaks are distinct, though they are one another’s closest relatives. Except for a few individuals introduced by people from farther south, scarlet oaks do not occur in northeast Illinois (though there may be some in northwest Indiana). Hill’s oaks and scarlet oaks apparently separate according to geology, with the former inhabiting savannas on dry or well drained soils that developed from glacial deposits. Hill’s oaks are distinct from other members of the black oak group, though they sometimes hybridize with black oaks (Q. velutina). Their habitat, buds and leaves distinguish them from all other species except black oaks.

The biggest difference between Hill’s and black oaks is in the acorns. Black oak acorns have caps in which the scales are fuzzy, and loose at the tips, as shown by this example from an enormous black oak growing in Culver, Indiana.

In Hill’s oak the caps are shiny and their scales are tight. This description fits acorns from the ridge top trees at Mayslake.

The inside of the black oak acorn cap is fuzzy, while that of Hill’s is smooth. Photos in Hipp’s paper illustrate this, and the Culver and Mayslake acorn caps separate accordingly. So it seems that this is one question I can shift from my mental file of mysteries unresolved to that of inquiries concluded.

Hill’s Oaks?

by Carl Strang

 

Last fall as I was making my initial acquaintance with Mayslake Forest Preserve, I was passing along the top of the savanna ridge east of Mays Lake when I noticed, in addition to the magnificent bur oaks all around, a cluster of smaller oaks with bristle-tipped leaf lobes.

 

hills-oak-maybe-leaf-b

 

In that dry exposed spot, with leaves shaped like this, I ruled out red oak and pin oak, and assumed they were a cluster of planted Hill’s oaks. Then recently I was talking with Conrad Fialkowski, who has been steward at Mayslake for years, and he told me that no trees had been planted in the savanna. This aroused my interest, as my impression is that you don’t run into Hill’s oaks every day. District Plant Ecologist Scott Kobal said that his records mentioned black oak but not Hill’s oak as being present at Mayslake, but there has been some recent taxonomic work on the latter species, and he kindly sent me a copy of the paper [Hipp, Andrew L., and Jaime A. Weber. 2008. Taxonomy of Hill’s oak (Quercus ellipsoidalis: Fagaceae): evidence from AFLP data. Systematic Botany 33:1-11].

.

Hill’s oak has been regarded as very close to the scarlet oak, Quercus coccinea, and possibly not separate from that species, though the oaks in general sometimes seem more like a hybrid swarm than a group of well behaved species. Hipp and Weber found, in a comparison of fragmented DNA’s of several species, that Hill’s and scarlet oaks are distinct both genetically and in their geographic ranges, to the point that only the former occurs in northeastern Illinois except for a few probably transplanted individuals. The kicker is that the distinction between Hill’s oak and the black oak, Quercus velutina, is not sharp and where the two co-occur they hybridize.

 

Which brings us back to Mayslake. The inventory of mapped trees near the mansion included a large black oak. Beneath that tree I found a fallen branch from a recent storm and got photos of its buds, which have the gray fuzzy coating and angular cross section typical of black oak.

 

black-oak-buds-b

 

The buds of the savanna trees proved harder to photograph.

 

hills-oak-maybe-buds-b

 

They are smaller, but the gray coating is limited to the tips, a characteristic of Hill’s oak. However, the buds are angular like a black oak’s. What I need are acorns, which are distinctive between the species, but the trees all are still small, so I will have to watch and wait.

%d bloggers like this: