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
Email: streamdoctor@aol.com
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 streamdoctor@aol.com.
Originally published April 5th, 2004 on Fly Anglers Online by Robert Younghanz.