Flagging in the Heat

by Carl Strang

On Saturday I completed my survey of 2020 periodical cicada reproduction in the Chicago suburbs. I focused on 18 suburban cities where I had observed wave chorusing, mapping out walking routes where I counted the flags that indicate where female cicadas deposited eggs in tree and shrub twigs. Altogether I walked 33.4 miles on hot afternoons, but the main physical challenge was the stiff neck I developed from constantly looking up into the trees. I will need to consult with periodical cicada specialists to be sure, but it seems to me that the numbers support a persistent, parallel splinter regional population that will continue to appear 4 years ahead of each major emergence.

Here I will share two sets of data. First, the highest numbers of flags in single trees.

Map of Chicago suburbs I surveyed for periodical cicada reproduction. Numbers indicate the highest count of flags in a single tree. Flags are withered twig ends that result from female cicadas inserting their eggs and cutting off the twig’s vascular system.

The high count of 33 was in Brookfield. I was walking residential streets, so my counts are transects rather than area surveys. The next map gives average numbers of flags per 100 meters. I used that number as the divisor because I often observed 1-2 wave choruses per block while driving earlier in the season, and 100m represents a middle-of-the-road block length.

Average counts of flags per 100m in Chicago suburb survey routes.

The highest counts of 10.5 and 9.1 are in LaGrange and Western Springs, respectively. These counts are conservative: I usually could not see all sides of a tree, canopies blocked part of the view, and to an unknown but certain degree flags had fallen to the ground and were removed by homeowners (this takes away some eggs, but there are others in the twig above the break).

Flags fallen to the ground at Madison Meadow Park, Lombard. In a residential yard these likely would have been removed before I could count them.

This fallen flag carried some eggs, leaving others still on the tree. Sugar maples were a popular choice by female periodical cicadas.

In addition to these transects in the cities, there were isolated parks and sites worth considering. At Pioneer Park Forest Preserve in Naperville, a single isolated non-wave chorus tree produced 7 flags spread between two trees. A similar case at Cook County’s Bemis Woods produced 18 flags. A park in Downers Grove had 37 flags. Other forest preserves had enough flags to suggest that their local off-year populations will continue: 22 at Lyman Woods, 13 at Greene Valley, and though I was disappointed by the size and timespan of the wave chorus at Wood Dale Grove, there were well over 30 flags there.

Tree with flags at Pioneer Park

Periodical Cicada Update

by Carl Strang

The 2020 periodical cicada emergence in the Chicago region has reached its peak and is winding down. It has been a mad and sometimes confusing scramble to collect data, but an overall complex picture has emerged. First, a wide-angle look:

Collected observations of periodical cicadas in 2020. White dots indicate towns or parks in which at least one or a few cicadas were documented. Yellow dots mark countable numbers, i.e. from one spot you could hear multiple cicadas singing. Orange dots mark small choruses (though sometimes occurring over large areas), in which the cicada songs were blended to the point where individuals no longer could be picked out, but the choruses were not organized. Red dots mark areas with full choruses, formed into periodic waves of song, loud and with both species audible.

These assessments of numbers are conservative. Many of the white dots are taken from iNaturalist submissions based on a photo of one insect. It is possible that in many cases the peak numbers were above that level, some possibly all the way to full wave chorusing. Also, some of the orange dot communities were assessed before the peak time and may well have developed full wave chorusing later. I visited nearly all the red dot communities myself, walking or driving to get some sense of the area involved. Several to many square blocks were typical here, and often I observed mating pairs and females laying eggs or saw lines of eggs in twigs. The wave chorusing lasted up to two weeks in these places.

Cassin’s 17-year cicada laying eggs in Mokena, Will County

Periodical cicada eggs in winged euonymus twig, Woodridge, DuPage County

Areas with wave chorusing generally were imbedded within a matrix of lower level numbers. Overall, the Chicago suburbs saw the emergence of enormous numbers of periodical cicadas. Apart from the specific observations of mating insects, egg-laying females and eggs in twigs, there is reason to believe that this is an on-going, reproducing population separate from the main regional emergence that next will occur in 2024. That reason is that these suburban neighborhoods have relatively few predators. The reason periodical cicadas have been so successful is understood to be that they overwhelm the many species of consumers that eat them. Suburban residential communities cover huge areas and are ecologically depauperate in that they are reduced to mowed lawns and trees, with some shrubs and herbaceous plantings that typically are non-native and therefore support few insects. That results in low numbers and diversity of predators compared to what the remnant forests harbor.

Example of habitat in River Forest, Cook County, a wave chorus area

I simply did not observe predators beyond the rare crow, territorial robins and usual densities of other species typical of these neighborhoods. I suspect that not only has this population split off from the main emergence, it has been building in numbers with each generation (I use the singular here because I am referring mainly to the Cassin’s 17-year cicada. Though Linnaeus’s 17-year cicadas were present in all the red dot areas, their numbers are more difficult to assess, and the only evidence I have of their reproducing is a single submitted photo of a female laying eggs).

Linnaeus’s 17-year cicada laying eggs in southeastern DuPage County. Photo by Leslie Bertram, used with permission.

I do not want to give the impression that predation wasn’t occurring. There were plenty of signs like this. The numbers removed simply were not making a dent in the mass choruses.

Now I want to focus on DuPage County, because I have the background of a detailed study of the cicadas’ history and their 2007 major emergence there.

This is my working map of the periodical cicada emergence of 2007, with superimposed 2020 observations. Pink areas mark the extents of the 2007 emergence areas. Colored dots represent the same levels of 2020 cicada numbers as in the regional map above.

If you find this image bewildering, I welcome you to the club. The emergence pattern was so spotty and localized that I cannot comfortably mark out large uniform areas. Dots are expanded either along my survey driving routes, or to cover the size of areas more expansively explored. To get a sense of the biological significance, try to tune out the white and yellow dots, as those locations had few enough cicadas that predators would have had no trouble wiping them out.

Again the red areas had so many cicadas that I had no trouble finding examples of mating or egg laying. Red areas adjacent to one another could be combined, as the habitat among them was uniform and continuous. This is particularly true of the cluster of red areas in the southeastern quarter of the county where the cities of Downers Grove, Clarendon Hills, Westmont and Hinsdale blend their similar residential neighborhoods. This area in fact continues east across the Tri-State highway corridor into Cook County’s adjacent city of Western Springs.

I am especially interested in the areas with loud, extensive wave chorusing that are outside the 2007 emergence regions. I will be referring to my detailed notes when I return to these locations in 2024.

The big emergences were in residential areas. Forests were a different story. For the most part, forested areas did not rise above the countable numbers of cicadas. I admit to being disappointed by what I observed in Wood Dale Grove and Salt Creek Park Forest Preserves and their connecting and surrounding residential areas. Both species emerged in both preserves, but never got above the countable level at Salt Creek Park. In part of the forest at Wood Dale Grove there was an area of a few acres in which wave chorusing developed, but it was in only part of the forest and lasted at most 3 days before collapsing to countable numbers again. I draw consolation from this result because it supports my suspicion that forests harbor numbers and diversity of predators that contrast with the expanses of residential communities that form the Chicago suburbs. Though the residential areas surrounding Wood Dale Grove and Salt Creek Park resemble those that had large and sustained cicada choruses, it would be easy for predators to spill out of the preserves and consume cicadas emerging in the surrounds. Cicada specialists may be interested to know that I did check Raccoon Grove Forest Preserve in Will County. I heard only two singing Cassin’s males there.

I have one more step this year, and that is to return to many of these places and look for egg damage to trees and shrubs. This will better assess reproductive success than the several but scattered observations mentioned above. Then it will be a wait of four years until the big show, and I wonder if there will be any peculiarities in the spots where there were mass emergences this year.

Cicada Emergence Stages

by Carl Strang

The 2020 emergence of periodical cicadas in northeast Illinois is under way now. It has taken a while to get going and is running about two weeks behind the last main emergence in 2007. People have sent or posted reports from 28 towns or preserves so far in Cook, DuPage, Lake and Will Counties. I have begun to visit places where significant numbers of the insects have been reported.

The pattern is reminiscent of what we observed in 2007. A few individuals come out of the ground at first, then a few days later there may be a night that brings out many. It takes them a few days of recovery before the males begin to sing, few or countable numbers of Cassin’s 17-year cicadas in most places. So far only some of the more open residential areas have reached this stage. Forests lag behind because the trees retard the warming of the soil which triggers the cicadas’ emergence.

This Cassin’s 17-year cicada was the first I found in Wood Dale Grove Forest Preserve, the June 4 date well after they had begun appearing in open residential areas.

Where there are good numbers of cicadas, small choruses may develop in which there are too many Cassin’s to count. Such has been the case in 5 communities so far. The ultimate step, which to date I have found in only one spot in southwest Brookfield, has numbers of Cassin’s 17-year cicadas chorusing so loudly that it can be nearly painful to stand under them. Their chorusing produces waves of loud then softer sound on a regular rhythm, and they are joined by the second species, Linnaeus’s 17-year cicada, whose songs add a high-pitched overtone to the Cassins’ buzzing quality. When visiting that place in Brookfield I saw many cicadas on the ground, many nymphal skins, many holes in the soil, and a pair of mating Cassin’s. I saw a crow eating a cicada, but there may be too few predators in these residential neighborhoods to have much impact on this off-year emergence.

Many emergence holes and empty nymphal skins marked the high density area in Brookfield.

Mating pair of Cassin’s 17-year cicadas in Brookfield on June 5

I made the following sound recording in the same area. The surrounding Cassin’s choruses wash out the focal one somewhat, but you may be able to pick out the 6 waves of high volume spread evenly here (5-6 seconds between peaks in this 32-second recording). Also, listen for a few Linnaeus’s “pharaoh” calls along the way. They are not coordinated with the Cassins’ waves.

A visual rendition of the recording

Periodical Cicada Tunnels

by Carl Strang

On Monday I went to Wood Dale Grove and Salt Creek Park Forest Preserves in DuPage County and looked for periodical cicada tunnels. As mentioned in the previous post, these are dug to the surface by nymphs, often weeks before they emerge to molt into their adult form.

My choice of those two preserves was not random. In the 2007 major emergence there were practically no periodical cicadas in the adjacent cities of Addison and Wood Dale, but residents reported that there were plenty in 2003. I had found no cicadas in either of those preserves when I checked them in 2007. Is it possible that those cicadas had completely shifted their timing to form a separate population? If so, this would be scientifically significant. I started finding cicada tunnels soon after entering the forest at Wood Dale Grove on Monday. I was elated at what I saw when I lifted a piece of old concrete.

Three cicada tunnels were in that one small area.

In 40 minutes of searching I found 10 tunnels. A couple were in chimneys, similar to those some crayfish produce.

Cicada chimney

How did I know these were not crayfish tunnels? Size tells the tale. A periodical cicada tunnel is half an inch in diameter, around the size of my forefinger. Crayfish tunnels are at least as big around as a quarter. Nightcrawler tunnels are much smaller, less than a pencil’s diameter.

Most of the tunnels I saw were sheltered, like those under the concrete, under bark or an overhanging fallen tree stem. Those in unsheltered bare soil had sharp edges, indicating they had been dug in the few days since a very heavy rainstorm passed through. Were it not for that storm, I believe I would have seen many more, as the rain washed soil into the tops of the tunnels.

At Salt Creek Park it was much the same story. In low areas resembling those at Wood Dale Grove I found tunnels at the same rate, one sharp-edged one about every four minutes. I did not find any in the upland woods, which may prove to be significant. Cassin’s 17-year cicadas are associated more with lowland woods, Linnaeus’s 17-year cicadas with uplands.

All of this has me very much looking forward to what I will find in a few weeks in this area. If there is indeed a major emergence in these two preserves, which are fairly close together, what is the extent of the emergence area? Are both species involved, or just the one? Can I document mating and egg laying? And, of course, all of this observing must be done under the social distancing, mask-wearing safety protocols dictated by the covid epidemic. Stay tuned.

Periodical Cicadas’ 2020 Emergence

by Carl Strang

In the previous post I outlined the history of DuPage County’s periodical cicadas. Part of that story remains to be told, and this next month will reveal much.

Newly emerged periodical cicadas

Huge numbers of our two species of periodical cicadas emerge every 17 years. The last major emergence was in the spring of 2007, so the next one will be in 2024. At least as far back as 1969, however, there has been a significant appearance of the cicadas four years early. This has happened every cycle since, in 1986 and in 2003. Will 2020 bring another instance?

This phenomenon first was noted by Henry S. Dybas, a biologist at the Field Museum of Natural History in Chicago. He invited observers to report locations where the cicadas were emerging in 1969. Illinois extension entomologist Phil Nixon followed suit in the next two cycles. These generally were not quantitative records, however, and no one noticed whether the cicadas were reproducing.

Towns where periodical cicadas were reported to Phil Nixon in 2003

As the map shows, most of the locations were in eastern DuPage County, adjacent Cook County, and extending southeast from there to the southern Cook border. I want to repeat this mapping in 2020, with two important modifications: a quantitative aspect, and an effort to document egg-laying. The question that needs to be answered is whether these early emergences are independent of one another. In other words, have they simply represented mistakes by a few individuals each time, emerging after 13 years of development rather than the usual 17? Alternatively, do these cicadas represent a new, splinter population of cicadas whose ancestors emerged four years early at some point, but ever since have reverted to the usual 17-year lifespan and are reproducing each time?

I want to compile site observations with maximum numbers of cicadas observed at each place: counted numbers where they are few, or massed numbers where there are too many to count. Each location also could have one or both species, readily separated by their different-sounding songs. Places with only a few, countable cicadas are worth mapping but have little biological significance, as the insects are munched on by every predator larger than themselves and few if any will survive to mate and lay eggs. The best possibility for an ongoing, separate population will be in places, if there are any, where massed numbers locally overwhelm the predators so that significant reproduction can happen. This is where observations of mating and especially egg laying are important.

Cassin’s 17-year cicadas mating. The area between the red compound eye and the base of the wing is all black in this species. In the slightly larger Linnaeus’s 17-year cicada there is an orange line connecting the eye and the wing.

This female is laying eggs. You can just see the ovipositor that has levered out from her abdomen and is piercing the twig.

The cicadas produce visible rows of slits on the undersides of twigs when laying eggs. This often kills the twig, so that leaves beyond the oviposition site turn brown.

I welcome shared observations to help me in this process. Already in April the nymphs have begun to tunnel to the surface. I suspect this allows them to monitor conditions aboveground so that they can know when best to emerge. Cicadas should begin singing in mid- to late May and continue through much of June, peaking in the first half of June. Location information to report is town plus nearest intersection of streets (or name of park or preserve if relevant). The Cassin’s 17-year cicada song is a rapidly clicking buzz that rises and falls, as with the single individual in this recording:

The Linnaeus’s 17-year cicada song is more of a tone rather than a buzz, starting high and descending, often rendered “pharaoh.” The next recording is of massed songs of Linnaeus’s periodical cicada. I don’t have a good recording of a single cicada singing:

Finally, here is what massed songs of both periodical cicada species together sound like:

Sound recordings and photos will be helpful, especially photos of ovipositing individuals and of twigs with egg damage. Naturally I will be making as many observations as possible myself and will follow up reports of massed cicadas. You can either send me information by e-mail to wildlifer@aol.com, or post your observations on the Insects and Spiders of Illinois Facebook page.

Landscape Ecology of Singing Insects 3: Changes Over Time

by Carl Strang

Some of the formalism developed by landscape ecologists clarifies the current status of our singing insects and their prospects for survival. From the standpoint of a given species, the landscape is viewed as patches of habitat imbedded within a matrix of non-habitat. If that matrix is hostile to the species, the habitat edge is a “hard boundary,” and unless the species is good at dispersing long distances, they will be confined to their habitat island. To different degrees that extreme may not be met if there are steppingstones or corridors that can act as acceptable temporary refuges for dispersing individuals. I suspect that such is the case with pine tree crickets, for example. These habitat specialists are common in conifer groves across the region, many of which seem too isolated for such a small, specialized insect to cross the intervening distances. Nancy Collins, a Wisconsin specialist on the subfamily, has noticed that pine tree cricket nymphs can live, at least temporarily, in non-conifer, herbaceous habitats. Edges of conifer groves thus are not hard boundaries for pine tree crickets. In other cases, river corridors and highway rights-of-way can serve as travel lanes. Thus, the little-known dispersal abilities of our various species are key to understanding their status.

Pine tree cricket

In some of the crickets and katydids which normally are short-winged and flightless, long-winged morphs occasionally appear. Roesel’s katydid, mentioned in the previous post, essentially is never long-winged in its native Europe, but commonly is so here, where the species is expanding its range. There is a tradeoff in play for such species. Long-winged variants are good dispersers, but their fecundity is reduced.

Roesel’s katydid, long-winged variant

Dispersal also can be facilitated by humans. I have seen examples, with Japanese burrowing crickets and jumping bush crickets, of individuals and small groups showing up far ahead of the front of their range expansion, in places where landscape materials are stored or sold.

Mate finding motivates some movement by singing insects. The females in general must travel to meet up with singing males. The males themselves also may need to change their position. In DuPage County, where swamp cicadas are relatively few and scattered, I have observed males frequently changing position between songs by tens of meters at a time. An Iowa study (Shaw, Bitzer and North 1982) found that sword-bearing coneheads shifted position an average of 6.2m between nights, but otherwise remained associated with their group of other males.

Swamp cicada

The singing insects whose habitat needs are met by landscape alterations for agriculture and residential areas are the abundant, widespread ones. They are easily picked out by the large number of sites marked on their maps in my singing insects guide. Dispersal is relatively easy for them as there are large habitat blocks well connected by amenable corridors. Even they can suffer local extinction as land is cleared of vegetation for buildings or roadways. As new vegetation grows into such places, the weedy species are quick to re-establish themselves, but this underlines the dynamism of the habitat patch mosaic.

Of greater interest, and greater concern, are those species whose needs are not met by human-created habitats. Here the habitat patches are only a small percentage of the landscape, and elements of patch size, patch isolation, and insect dispersal ability become critical to understanding. Theory suggests that when a habitat falls below 10-20% of the landscape, dispersal ability is expected to drop dramatically in the absence of viable corridors or steppingstones; such is certainly the case for many of our habitat specialists. A few species anecdotally are good dispersers. I have reports, or have seen myself, instances of slender meadow katydids and long-tailed meadow katydids showing up in locations remote from their respective habitats. These individuals were unusual among the small meadow katydids in being long-winged morphs; most cannot fly.

Slender meadow katydid

Patch isolation in some cases is such that successful dispersal is impossible. Distances among the few surviving sphagnum bogs, for example, are too great to be crossed by sphagnum ground crickets. Prairie cicadas can fly, but apparently are disinclined to do so far enough to matter. At one of their sites, the West Chicago Prairie in DuPage County, they remain confined to one area of mixed grasses and forbs. They have not crossed the 350 meters of grasses, small shrubs and wetlands that separate them from a similar area within the same preserve. So far, their populations have persisted in remnant prairies ranging from 1 to a few acres. Can such small populations survive long term?

Prairie cicada

That question raises another concept from landscape ecology: extinction debt. This is the idea that a species’ habitat may have been so reduced that it still may be present but is doomed to fade away to extinction because its numbers are inadequate to maintain reproductive viability. Such may or may not be the case for prairie cicadas. I suspect this is what happened to northern wood crickets (NWC) in the region. NWC were known from two forested areas in northern Indiana at the beginning of the 20th Century (Blatchley 1903). One of these was in Marshall County. Years ago I used the original survey notes and county soil map to reconstruct the presettlement landscape of the township in question.

Presettlement map of Union Township, Marshall County, Indiana. Mesic forest was the large green area east of Lake Maxinkuckee.

Blatchley found NWC in the area of mesic forest which, in 1834, was very large. By the end of the 19th century agricultural clearing would have been well under way, and today only a tiny portion of that forest remains, the rest having been converted to pasture and crops. The same is true of forested areas in Lake County, Indiana, the other area where NWC lived in the late 1800’s. Though fragments of the forests where Blatchley found NWC remain, the crickets are gone, and I have checked all the other relatively large forests in the region without finding them. This raises the disturbing question: how many other species presently in the Chicago region are in a state of extinction debt? Patch size needed to maintain a species is dependent upon the characteristics of the species, its population dynamics, and patch quality. These are unknowns for all the uncommon species.

Our two species are Linnaeus’s 17-year cicada, on the left, and Cassin’s 17-year cicada, on the right

I will close this section with a case study on the periodical cicadas in DuPage County. In pre-settlement times the county was, from the cicadas’ perspective, a matrix of prairies and wetlands with 18 forested areas of various sizes scattered throughout. Historical maps and aerial photos allowed me to trace changes in those forests from the early 19th Century to the present day. I measured the forest sizes, noting their smallest (bottleneck) extents and how those were reflected in the presence of periodical cicada choruses in the 2007 emergence. There was a clear threshold of local extinction: forests which had remained above 61ha (hectares) still had cicadas, those which had fallen below 52ha did not, even when they had grown larger subsequently. Old newspaper accounts placed cicadas in at least some of these. Three forests which fell between those sizes appeared to have been affected by isolation, a remote one lacking cicadas, and two near persisting populations having them. The human history peculiar to the county is important here. DuPage County is immediately west of Chicago. The western half of the county quickly became agricultural, with forests cut back to make room for fields and pastures. Forests lacking cicadas in 2007 were mainly in the west. The eastern half developed residential commuter communities, with forests being protected and expanded as people planted trees around their homes. The largest area with cicada choruses in 2007 was lobular in shape, the various lobes following the routes of commuter-serving railways and the towns that expanded along them, connecting several of the pre-settlement forest locations. When the cicada choruses reached their peak in the first half of June, numbers of the insects suddenly appeared in flight, crossing over highways and other hostile environments. Subsequently, small groups showed up in places remote from the concentration areas. Whether these will result in significant expansion of the species in the county remains to be seen, but this observation supports the notion that competition and population pressure produce responses by the cicadas. They have the advantages of stronger flight capabilities and better vision than other singing insect groups.

A Pause in the Action

by Carl A. Strang

In the early part of the season, from April to early July, my research focus is on those species of singing insects which matured from overwintering nymphs, plus some small early-season cicadas. This is a minority of species, as most of the crickets, katydids, and singing grasshoppers mature after the middle of July, having wintered as relatively secure eggs and needing time to grow up.

I was able to close the book on northern wood crickets last month, and the story here is a sad one. This forest-dwelling member of the field cricket group had been reported from two northern Indiana sites by W.S. Blatchley in 1903. As far as I know, no one has sought them since then in the northern part of the state. Last year I determined that they no longer occur where Blatchley found them. This year I checked the largest other eight forests in the Indiana portion of my study region. If they ever were there, they are gone now. I suspect that forest fragmentation for agriculture and other purposes is responsible for the loss. Blatchley’s detailed descriptions leave no doubt that he knew how to recognize the species.

This northern wood cricket is from the northernmost site where I know they still occur, Eagle Creek Park in Indianapolis.

I was able to close the book on another southern species, the spring trig, in June.

This tiny, early-season cricket is common in southern Indiana.

I have found a few scattered groups of spring trigs in southernmost Fulton and Jasper Counties in Indiana. A thorough search failed to turn them up in neighboring Pulaski and Newton Counties. I may check again in a few years, on the possibility that the species is expanding northward.

One positive result was finding sulfur-winged grasshoppers in the East Main Street Prairie of Cary, Illinois. This adds McHenry to the counties where I have found the species. They probably occur in every county in my region but are common only on sandy or gravelly soils such as Cary’s kame-like hills. I have learned of another candidate site which may add Fulton County, Indiana, next year.

Sulfur-winged grasshoppers are characterized by bright yellow hind wings, which they rattle in flight to produce their song.

Prairie cicadas started a little late this year. I was pleased to find that management efforts to remove brush from the West Chicago Prairie Forest Preserve near my home appears to have paid off in both rebound of diverse prairie vegetation and an increase in the cicada numbers.

Prairie cicadas, are tiny, around an inch long.

Failure to perform such restoration work has a cost. Once known to occur in Kankakee County, prairie cicadas apparently are gone from there, the prairies having been degraded by brush, teasel and other invasive plants.

A final story is that of the periodical cicadas. In each cycle since 1973, the main appearance of 17-year cicadas in Chicago’s western suburbs has been preceded by a significant, 4-year-early emergence. This happened in 1969, 1986, and 2003. I suspect that in a small part of this area, all the cicadas now have switched to the early time. If you have done the math, you realize that it may happen again next year. One predictor to watch for are what I call oops cicadas, a few individuals who jump the gun by a year, or miss the main emergence and come out a year late. As expected, this has been happening this spring. I have heard 3 individuals myself in two cities and seen photos of the insects from 3 more. Counting and mapping them will be a highlight of next year’s early field season.

I predict that some areas will have good numbers of 17-year cicadas next year.

Literature Review: Periodical Cicadas

by Carl Strang

This week’s literature focus is on a single paper, which looked at a significant aspect of periodical cicada biology.

The northern periodical cicada species, Linnaeus’s 17-year cicada on the left, Cassin’s 17-year cicada on the right

The northern periodical cicada species, Linnaeus’s 17-year cicada on the left, Cassin’s 17-year cicada on the right

Karban, Richard. 2014. Transient habitats limit development time for periodical cicadas. Ecology 95:3-8. He studied septendecim and cassini (our two local species of Magicicada) in New York state. There are several hypotheses explaining why their development times are so long: Pleistocene historical influences (long life span buffered annual climate variation in glacial refuges), predator satiation (some early maturing individuals wait for slower ones to catch up, and long life spans facilitate this), low nutrition forces long development, and increased fecundity (17-year species have been shown to be more fecund than the more southern 13-year versions). Here he examined the possibility that habitat quality changes rapidly enough to put an upper limit on such advantages of long lifespans. Though past studies pointed to possible advantages of edge trees, here he compared weights of newly eclosed adults from edge vs. forest interiors, finding the former to be only slightly (4.9%) heavier in septendecim but no difference in cassini. He took density of emerging nymphs as an indication of habitat quality. Changes in study sites were significant between emergences, enough to limit any advantage of longer life. He commented on the Raccoon Grove study site in Will County, once one of the highest-density populations known, mentioning that they plummeted over just a couple sequential emergences, first because of Dutch elm disease killing host trees. Karban and Yang visited that site in 2007, hearing one chorus but finding no emergence holes or nymphal skins.

Literature Review: Arthropod Evolution

by Carl Strang

If you’re a bug nerd you’ll enjoy the following notes on research from 2013. Especially significant were studies of butterflies and moths, and an eye-opening paper on periodical cicadas. This concludes my literature review until next winter.

Butterflies and moths had their origin in the Triassic Period according to recent studies, though the first ones were more like caddis flies than like this red-spotted purple.

Butterflies and moths had their origin in the Triassic Period according to recent studies, though the first ones were more like caddis flies than like this red-spotted purple.

Zhang, W, et al. 2013. New fossil Lepidoptera (Insecta: Amphiesmenoptera) from the Middle Jurassic Jiulongshan Formation of northeastern China. PLoS ONE 8(11): e79500. doi:10.1371/journal.pone.0079500  They found 15 species of early moths representing at least 3 families in Chinese deposits, and details of wing venation led to the conclusion that the Lepidoptera (moths and butterflies) diverged from the Trichoptera (caddis flies) by the early Jurassic Period.

Wahlberg, N, CW Wheat, C Peña 2013. Timing and patterns in the taxonomic diversification of Lepidoptera (butterflies and moths). PLoS ONE 8(11): e80875. doi:10.1371/journal.pone.0080875  They estimated timings of major episodes of speciation in the major groups of butterflies and moths. Their results point to a Triassic origin of Lepidoptera, around 215 million years ago. The timing of diversification episodes at least in some cases corresponds to times when plants were diversifying, and also after the end-Cretaceous mass extinction. Coevolution of lepidoptera with their larval food plants appears to be an important theme. They give origin ages for major Lepidoptera groups (in millions of years ago): Gracillarioidea 120, Yponomeutoidea 117, Glechioidea 106 (these first three are small moths, many of them leaf miners), Papilionoidea 104 (butterflies), Pyraloidea (including many local pyralid moths) 93, Bombycoidea (including sphinx moths) 84, Geometroidea (including inchworm moths) 83, Noctuoidea (the enormous owlet moth group) 82, Tortricoidea (including leaf-folding caterpillars) 68. All these groups are represented by local species.

The Chicago region’s 17-year periodical cicadas: Magicicada septendecim, left, and M. cassini.

The Chicago region’s 17-year periodical cicadas: Magicicada septendecim, left, and M. cassini.

Sota, Teiji, Satoshi Yamamoto, John R. Cooley, Kathy B.R. Hill, Chris Simon, and Jin Yoshimura. 2013. Independent divergence of 13- and 17-y life cycles among three periodical cicada lineages. Proc. Nat. Acad. Sci. 110:6919-6924. They sequenced a number of genes from nuclear and mitochondrial DNA from all known species and broods, and estimated divergence times based on general research that has been done on insect mitochondria. There are three species groups (referred to as Decim, Cassini, and Decula), each of which contains northern 17-year species and southern 13-year species. In any location, the species in the different groups emerge at the same time. The results clearly separated the three groups, and tied together the species within each group (e.g., 13-year Decim are more closely related to 17-year Decim than to 13-year Cassini). Furthermore, each species group is divided into eastern, central and western genetic clusters (this pattern has been documented in other organisms as well; for the most part, Illinois cicadas are in western clusters, Indiana ones in central clusters). Each cluster contains both 13- and 17-year species, “suggesting that life cycle divergence occurred independently in the three regions.” Analyses estimated that the western Cassini divergence of 13-year and 17-year species took place 23,000 years ago, 10,000 years for Decim. Population sizes for both Decim and Cassini groups appear to have been small during the last glacial period, but expanded greatly starting 10,000 years ago. The sequence appears to have been allopatric speciation of the 3 ancestral species, with the species later becoming sympatric and independently splitting into 13- and 17-year cicadas. “Surprisingly, however, the divergence of 13- and 17-y cicadas was asynchronous among the species groups and occurred repeatedly even within a species group.” The implication is “that the three Magicicada groups shared multiple refugia during the last glacial maximum.” The 13-/17-year splits occurred after the last glacial maximum, within the last 23,000 years, “suggesting that the life cycle divergence in Magicicada is closely associated with global climatic fluctuations and shorter growing seasons in the north versus the south.” However, the species groups themselves separated in the Pliocene, and their shared long lives suggest that this did not originate because of glacial climate influences. This shifting between 13- and 17-year life cycles suggests a common genetic basis among the species, and indicates a somewhat plastic nature of this trait. The coordination among species at a given location seems best explained by the selective advantage of low numbers of an invading species into the range of another, surviving best when sheltered by the established species’ numbers.

Zhao, Z, et al. 2013. The mitochondrial genome of Elodia flavipalpis Aldrich (Diptera: Tachinidae) and the evolutionary timescale of tachinid flies. PLoS ONE 8(4): e61814. doi:10.1371/journal.pone.0061814  Their genomic study traced the evolutionary relationships of the parasitic fly family Tachinidae, and molecular clock analysis calibrated to the fossil record points to the middle Eocene as the time of the family’s origin.

Brewer, MS, and JE Bond. 2013. Ordinal-level phylogenomics of the arthropod class Diplopoda (millipedes) based on an analysis of 221 nuclear protein-coding loci generated using next-generation sequence analyses. PLoS ONE 8(11): e79935. doi:10.1371/journal.pone.0079935  They place the ancestral millipedes at 510mya (million years ago), with major groupings established by 200mya.

Lucky, A, MD Trautwein, BS Guénard, MD Weiser, RR Dunn. 2013. Tracing the rise of ants – out of the ground. PLoS ONE 8(12): e84012. doi:10.1371/journal.pone.0084012     A phylogenetic analysis points to soil rather than leaf litter as the nesting habitat for the earliest ant species.

Green-winged Cicada Update

by Carl Strang

I have made a correction to yesterday’s post on green-winged cicadas. I always check my posts when I get to work, to make sure nothing is lost in the translation to another computer. In the process of checking links, I went to the Cassin’s 17-year cicada portion of the Michigan cicada website. There I saw something I hadn’t noticed before, mention of “Court II” and “Court III” signals. The Court III signal is produced by a male cicada as he connects with a female for mating.

Cassin’s 17-year cicadas, mating pair

Cassin’s 17-year cicadas, mating pair

When I played the Court III signal recording, it proved to be what I had attributed to green-winged cicadas in 2007. That both corrected my impression of Diceroprocta in DuPage County, and removed any confusion about my observations in 2013.

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