Thursday, October 18, 2012

Assigning Tolerance Values and the Common Netspinner Problem

(Above, a common netspinner (Hydropsychidae), genus Hydropsyche, species not yet determined: found this morning at Buck Mt. Creek.)

As I've increasingly focussed on species ID for common netspinners over the summer, I've become more acutely aware of a problem that could develop in the process of monitoring the health of our streams in the state of Virginia.  At the moment, VA assigns tolerance values by family: the TV given to all common netspinners we find -- across the board, irregardless of genus or species -- is 6.0.  But Virginia plans to change at some point in the future to a system of assigning tolerance values  by genus.

In the case of common netspinners this is an important change that has to be made: the common value of 6.0 is really misleading and doesn't give us reliable data to use.  To date, I've found five genera of common netspinners in local streams, and a total of ten different species.  The tolerance values assigned to these taxa in North Carolina differ in significant ways.  Have a look.

Ceratopsyche bronta                    2.3
Ceratopsyche morosa                   2.3
Ceratopsyche slossonae                0.0      
Ceratopsyche sparna                    2.5

Cheumatopsyche                         6.6

Diplectrona modesta                   2.3

Hydropsyche betteni                    7.9
Hydropsyche rossi                       4.8
Hydropsyche venularis                5.1

Macrostemum                             3.4

Now, it will be possible, it seems to me, to assign a single tolerance value to each of these different genera, something like -- for example -- 2.0 for Ceratopsyche; 6.6 for Cheumatopsyche (this is the value used in NC for all Cheumatopsyche larvae); 2.3 for Diplectrona (D. modesta is the only known species at the moment); maybe 6.0 would still be okay for Hydropsyche; and Macrostemum can stay at 3.4 since there are only two species, and they're difficult to distinguish.  (These are only "for examples" -- these are not recommendations.)

Good.  Now here's the problem: can community volunteers distinguish the common netspinner genera, or, in the future will they have to start preserving every netspinner they find for laboratory ID?  (Not something anyone wants to do.)

Some thoughts.

1) Diplectrona and Macrostemum netspinners can, in fact, be distinguished from the rest without a whole lot of work.   Diplectrona modesta larvae can be distinguished by color: they all look like the larva in the photo below -- brown/tan body, brown nota, and an orange/bronze head (at least that's true of every larva that I've seen so far).  Not only that, they are normally found in only one type of stream -- small, mountain streams, clean and cold -- and even then not in big numbers.  (The kind of stream that might be used as a "reference" stream.)

2) Macrostemum larve are also easy to pick out by sight.  They're bigger than most of the netspinner clan, they're fat, they're an odd shade of green, and they've got that oddly sloped head.

3) Cheumatopsyche present more of a problem.  It is true that every C. larva I've seen has been the same color: lime green body with brown nota and head.

But the only safe way to distinguish Cheumatopsyche from Hydropsyche and Ceratopsyche is to look for two distinct physical features: 1) The two large sclerites below the prosternal plate on Hydropscyhe and Certopsyche are either tiny or absent on Cheumatopsyche, and 2) on Cheumatopsyche larvae there is a notch at the front of the head (the frontoclypeal apotome).

I'm not sure that any of us will pick up those features using a loupe: these larvae will have to go to the lab for ID.

And now for the biggest problem of all: Ceratopsyche and Hydropsyche.  These genera MUST BE DISTINGUISHED -- look at the difference in tolerance values!!!  Of the species assigned TV's in the North Carolina list, the average TV for Ceratopsyche is 1.1; the average for Hydropsyche is 4.3.   But, I'm not sure that anyone -- let alone amateur volunteers -- can tell them apart at the stream without looking closely at the abdominal segments, a very time consuming practice.  Hydropsyche larvae have "scale" hairs on at least some abdominal segments; Ceratopsyche do not.  You can see the difference in the photos below.

Hydropsyche venularis: all of the tiny black dots on the abdominal segments are "scale" hairs.

Ceratopsyche bronta: no little black dots.

Hydropsyche betteni: little black dots.

Ceratopsyche sparna: no little black dots.


So there you have it.  Given the wide range of tolerance values among common netspinner genera, we cannot continue to use a single value of 6.0 for every netspinner we find.  That yields results that are not at all valid.  But when we move to tolerance values by genus, the systems of sampling currently used in the state will require a major adjustment.  I trust that a way will be found to include community volunteers in the work that we do on our streams.  But this is an issue that will not be easy to solve.

1 comment:

  1. This is a very good point. I hope you're sharing your findings with the folks who could use this information about tolerance values.