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Stream Bugs of Montgomery County

Montgomery County is home to hundreds of species of aquatic bugs (Benthic Macroinvertebrates). These aquatic bugs process nutrients and energy, powering the stream ecosystem! Because they are crucial to the stream, they are excellent indicators of stream health.


What is a Benthic Macroinvertebrate or Stream Bug?

  • A small organism that is visible to the naked eye

  • Does not have a backbone

  • Lives on the bottom of streams for at least part of their lives

  • They include aquatic insects, crayfish, mussels, worms and leeches, snails, sponges, and flatworms.


Why Call them Benthic Macroinvertebrates?

Benthic = bottom dwelling

Macro = visible without a microscope

Invertebrates = lack a backbone 


Why are Stream Bugs Important?

Stream bugs are an important member of aquatic communities.  Many stream bugs eat aquatic plants, algae, and terrestrial plants that fall into the water. Those bugs form the base of the food chain and are called Shredders.

Stream bugs are great indicators of stream health, because they:

  • Live in the water for all or most of their life

  • Stay in areas suitable for their survival

  • Are easy to collect

  • Differ in their tolerance to amount and types of pollution

  • Are relatively easy to identify in a laboratory

  • Often live for more than one year

  • Have limited mobility


Stream Bug Groups

Montgomery County categorizes stream bugs into groups based on their ability to withstand pollution. Each genus has a unique tolerance value that helps DEP assess the health of our streams.


Common Stream Bug Groups (Orders)




Mayfly Nymphs (Ephemeroptera)

Image of a stonefly



Stonefly Nymphs (Plecoptera)

Image of Caddisfly



Caddisfly Larvae (Trichoptera)

Image of Dragonfly Nymph



Dragonfly and Damselfly Nymphs (Odonata)

Image of Dobsonfly



Dobsonfly Larvae (Megaloptera)

Image of Riffle Beetles





Water Pennies and Riffle Beetles (Coleoptera)

Image of Blackfly



Midge and Black Fly Larvae (Diptera)





Non-Insect Groups

Image of Gammarus




Image of Corbicula



Freshwater Mussels and Clams




Image of Leech




Image of Snail





Want to learn more about benthic macroinvertebrates? Check out the DNR Factsheet (PDF, 1.40MB)


Monitoring for Stream Bugs

DEP collects samples of benthic macroinvertebrates in order to gauge the health of the stream.  Using their hands, stream rakes and other tools, DEP staff members shake loose the stream bugs from the rocks they cling to within the stream and catch them downstream in nets.  The different stream bugs are then identified in the lab.  For a full, detailed explanation of this process, watch the video below.  



What Data Does DEP Collect?

DEP collects a sample of benthic macroinvertebrates from the stream and returns to the laboratory for further sub-sampling. Individuals from the sub-sample are identified (usually to the genus level) and enumerated. From these data, metrics are calculated, scored, and then summed to obtain a final Benthic IBI score.

There are a total of eight metrics comprising measures of biological structure and function; each metric is scored either as a one, three, or five. The highest possible final score is 40 (5 x 8). To determine the overall stream condition, benthic IBIs are averaged with fish IBIs.


Benthic Macroinvertebrates IBI Metrics

Taxa richness (Total number of taxa)

Biotic index

Ratio of scrapers (Scrapers divided by (scrapers + filter feeding collectors))

Proportion of Hydropsyche sp. & Cheumatopsyche sp.

Proportion of dominant taxa

Total number of EPT* taxa

Proportion of EPT* individuals

Proportion of shredders


* EPT = taxa that are either mayflies (Ephemoptera), stoneflies (Plecoptera), or caddisflies (Trichoptera); aquatic insects that spend all of their juvenile or larval life stages instream.

To determine a narrative based on a final metric score, the following breaks are used. These breaks are determined by plotting reference stream benthic conditions.


Benthic Macroinvertebrates IBI Narratives
Score Narrative
> or = 36 Excellent  
26-35 Good  
17-25 Fair  
< 17 Poor  



Available Benthic Data

Montgomery County has tabular raw benthic data and benthic narrative summaries from 1994-present for most monitoring sites. Also available are GIS coverages (or maps) showing benthic conditions. Maps can be made to order depending on the request.

Benthics Data Table
Field Name Description
STATION Two letter stream code + two letter stream reach code + order (1-4) + reach number (01-99).
DATE The date the sample was taken.
TAXA Benthic macroinvertebrates are identified to family.
[#GRIDS] Number of grids sampled during subsampling routine to estimate benthic population sampled.
[#INDVS] Number of macroinvertebrate individuals sampled from subsampling routine.
EXCLUDE This is a taxa that is automatically excluded from further use in calculating metrics for IBI.
REPLICATE This is a repeat of another sample.
Benthics Narrative Table
Field Name Description
STATION The station field is a nine character code that identifies the station name. The stations are a combination of the two letter code for the watershed+the two letter code for the subwatershed+ the single digit stream order code+ the sequential reach number.
DATE The date the station was sampled.
SUMMARYSCORE The final IBI summary score (1-5).
NARRATIVE Descriptive word to describe the condition of the stream in relation to reference streams. Narratives are either Excellent (>4.5), Good (3.3-4.5), Fair (2.2-3.2), or Poor (<2.2).











Interested in other DEP monitoring data?  Visit our Monitoring webpage for a list of the different monitoring data sets the County collects.