Two tree crickets that look and sound identical: Forbes's and Black-horned Tree Crickets
in NE Ohio's overlap zone.
Forbes's Tree Cricket singing
Black-horned Tree Cricket singing
Black-horned Tree Crickets (Oecanthus nigricornis) are an expected tree cricket species for NE Ohio, and I initially assumed that all the tree crickets that look and sound like Black-horneds here in NE Ohio were exactly that. My assumption had to change rapidly when tree cricket researcher Nancy Collins observed that a recording I’d made of a Black-horned Tree Cricket in one of my blog posts was actually a Forbes’s Tree Cricket. Forbes’s Tree Cricket? Oecanthus forbesi? Isn’t that a more western species?
Not necessarily. In NE Ohio, I’m learning that we are in an overlap zone where both species are found. I’m currently documenting where each species is present and those locations where both can be heard.
It’s quite a challenge. They look identical and live in the same kinds of habitats, so visual cues aren’t helpful, and their exact ranges aren’t known.
The only way to tell them apart is to measure the number of sound pulses per second in their songs. A pulse of sound is generated when a singing cricket closes his wings, engaging the file on one with the scraper on the other. (“Closing” doesn’t mean “lowering – the wings are still raised.) The number of pulses is measured against the temperature at which he is singing in order to get the “pulses per second.”
Because crickets and katydids sing higher and faster when it’s warm and slower and lower in cooler temperatures, the number of pulses per second will be affected by temperature. For example, there will be more pulses per second at 78 degrees than at 68. The number of pulses per second could indicate Forbes’s at one temperature, but Black-horned at another, so the combination of temperature and number of pulses per second is essential.
Black-horned Tree Cricket at Novak Sanctuary in Aurora, Portage County, His song is 47 wing strokes per second at 72.5 degrees F, which puts him right on the graph line for Black-horned. The section of the sonogram I've highlighted in white is one second, I counted the green lines within that one-second period, as they represent the wing strokes.
Forbes's Tree Cricket from Bath Nature Preserve in Summit County, He sounds rather old because he is: it was 11-1-16 when I made this recording. His song is 59 wing strokes per second at 73 degrees F - considerably more than the 47 wing strokes per second of the Black-horned.
Sunlight will also affect the temperature at which a cricket is singing, so the temperature cannot be measured in the sun. My most accurate readings will be after dark or when the singer is in the shade. Overcast days seem to be accurate as well.
The temperature has to be taken as close to the cricket as possible, so getting right next to him, or at least the leaf he’s on, is my goal. Crickets do not necessarily welcome such disturbances, however, so their annoyance adds to the challenge.
The questions I hope to answer – at least to whatever extent I can - include:
Who lives where, and what’s the actual overlap zone?
Do the two species coexist, or does one replace the other?
If this is changing over time, what’s the base line as of now?
I would like to be able to determine if the Forbes’s is moving east, or has been here all along, but that may not be possible at this point.
Female Forbes's Tree Cricket
Female Black-horned ovipositing
Forbes’s Tree Crickets had been documented as far east as the Sandusky Bay area on Lake Erie’s central Ohio shore and also in the Columbus area. My records are the first for them here in NE Ohio. My recordings, not my photos, tell me who they are. I record the pulse-counting/temperature data for all the Black-horned/Forbes’s I record at a specific location, then enter the data in a graph for that particular location.
Dr. Thomas Walker, Entomology professor emeritus at the University of Florida, created a graph on which a Forbes’s or Black-horned’s song can be placed by pulses per second at a given temperature. The upper line would be the Forbes’s, as they sing at more pulses per second at a given temperature. The Black-horned Tree Crickets would fall along the bottom line, as their songs have fewer pulses per second at the same temperature. In my graphs, the upper line is orange and the lower line is blue.
Novak Sanctuary near Aurora in Portage County has Black-horned Tree Crickets
Lake Erie Bluffs on this night had Forbes's Crickets. On this and other evenings and afternoons, I have only found Forbes's at this NE Lake County lake shore location
At Bath Nature Preserve in northern Summit County, both species are present.
I believe I’ve learning how to distinguish between the two species when both are singing in the same habitat. When I hear them together, I can detect a difference that is likely based on tone quality, as the overtones generated by each species’s song are somewhat different.
There is also a subtle difference because one species has more wing strokes per second than the other when they’re at the same temperature. I've had success on several occasions by recording two crickets at the same parks that sounded different but looked the same and did indeed have different pulse rates. I plan to continue practicing in the coming field season, but I don’t know that I’d be able to identify one species by itself – only when I can compare them to each other.
I have not found any consistent visual difference between them. In this region, Forbes’s typically have dark heads, though some are occasionally somewhat lighter. They are never as light as the Forbes’s that Nancy Collins has photographed from more western states such as Wisconsin.
Both species are absolutely abundant in goldenrod, and goldenrod is absolutely abundant in NE Ohio meadows. You WILL encounter these crickets. But which species?
Forbes's Tree Cricket in goldenrod and blackberry
Black-horned Tree Cricket on blackberry
You won’t know. If you’re in Lorain and Medina Counties, they will very likely be Forbes’s. Forbes’s seem to be common near the lake shore. But inland? Unless you want to do a controlled count of wing strokes per second at a recorded temperature, it’s only a guess. Both species are present. Portage County seems to have the highest percentage of Black-horned as of 2017.
Perhaps we need a compound name like “Black-horned/Forbes’s for our region!
As time permits, I will update a Google Map with the species I’ve found at various locations. Keep in mind that this is very much a work in progress. If you would like to see my Google Map for these two species, you can find the Black-horned/Forbes's overlap zone map here.
You can find my Listening in Nature blog post on sorting out these two species at http://listeninginnature.blogspot.com/2017/01/identify-yourself.html