The Stream Doctor

April 5th, 2004

Email YOUR Questions directly to the Stream Doctor. This is your opportunity to get an experts professional opinion on anything stream related.

Q. We all know that trout eat insects, but what do insects eat? I would be interested to know in general terms, what caddis eat. Thanks, Aaron

A. Well, Aaron, insects in general, including caddis, have a fairly broad range of food resources. Rather than just give you a list of foods, it might be more instructive to add a bit to help you understand.

Basically, there are 3 major sources of food available to stream insects; these are:

    (1) Coarse Particulate Organic Matter (CPOM) which includes all pieces of organic matter greater than 1 mm in diameter,

    (2) Fine Particulate Organic Matter (FPOM) which includes all pieces of organic matter less than 1 mm in diameter, and

    (3) instream plants including algae and flowering plants. CPOM includes such things as leaves, twigs, even logs - anything that reaches the streambed from either terrestrial or aquatic origin. This material is eventually broken down and colonized by bacteria and fungi, thus becoming a nutritious source of food for insects. FPOM includes the finer pieces of organic matter originating from the breakdown of CPOM, fecal pellets from insects, decomposition of algae, etc. The most important sources are FPOM and algae, especially the algae covering the surface of rocks and other solid objects, making them slick to the touch.

OK, those are the food sources and the answer to your basic question. Of perhaps more interest is the way that insects obtain their food - we call these functional feeding groups, and there are basically four of these. First are the shredders; these are insects that feed on CPOM and begin the breakdown of this material to FPOM. Common insects found in this group are the salmon fly (Pteronarcys californica) and many species of larger caddisflies that build their cases of plant material. The second group is the scrapers or grazers. These insects obtain their food by scraping algae, mainly diatoms, from rocks and other hard surfaces. Examples include many caddis such as Glossosoma and Dicosmoecus, the October caddis. The third group is known as collectors, and is broken down into two subgroups: the filtering collectors and gathering collectors. Filtering collectors feed on FPOM carried in the water column and do this by a wide and fascinating variety of means, including filtering nets, long hairs on the legs, and modified mouth-parts. Typical filtering caddisflies include the genera Brachycentrus and Hydropsyche. Gathering collectors pick up FPOM where it has fallen to the streambed; common genera include Baetis, Ephemerella, and many others. The fourth functional feeding group is the predators. These organisms eat other organisms. Common caddisfly predators are in the genus Rhyacophila, and most stoneflies are in this group.

There are some minor functional feeding groups that ecologists have used for specialized insects. These include gougers, insects which gouge their food from the surface of submerged logs; piercers, insects with mouthparts adapted to pierce and suck out the juices from aquatic plants; and miners, which burrow in sediments.

Well, you asked for "general terms" but got a bit more than this. I hope it is of interest.

If you have a question, please feel free to contact me.
~ C. E. (Bert) Cushing, aka Streamdoctor
105 W. Cherokee Dr.
Estes Park, CO 80517
Phone: 970-577-1584

The 'Stream Doctor' is a retired professional stream ecologist and author, now living in the West and spending way too much time fly-fishing. You are invited to submit questions relating to anything stream related directly to him for use in this Q & A Feature at

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