Wednesday, March 23, 2011

Simple aquatic insect sampling

Plastic mesh bags like the one above can be filled with leaves and placed in a pond or stream in order to collect aquatic insect larva and nymphs. 

For a number of years now we have been assessing local aquatic insect populations through a rather simple plastic mesh bag method.  The mesh bag, which is slightly larger then a sandwich bag, is filled with leaves, the openings tied shut, and then sunk in a few feet of water.  The bag can be submerged in any type of body of water, whether moving or still, the important part is that you have the ability to retrieve it.  It doesn't have to be deep to produce good results.  Also, make sure you place adequate weight in the bag (a rock will do) so the bag doesn't flow away.  Once the bag is placed on the bottom of a body of water it becomes habitat for immature insects as they colonize it over a period of a few weeks.  After a few weeks the bag can be opened up and the bugs sorted out out from the leaves.

1. Obtain a mesh bag of some type.  Mesh bags that onions come in will work well.
2. Cut the mesh bag down to a more manageable size.  If you end up with multiple openings you can simply tie them shut.
3. Fill the bag with dead leaves.  Be sure not to pack them in too tightly.
4. Place a rock in the bag and tie it shut with string.
5. Submerge the bag in a few feet of water at most.  It is important to have the ability to retrieve it.  If it is too deep you might have to swim to get it.
6. Leave the bag underwater for about two weeks.
7. After about two weeks, retrieve the bags, open them up and carefully sort through the leaves to find small aquatic insects.  When transporting the bags from the body of water to where the leaves will be sorted out be sure to keep the entire pack in water.  If the leaf packs dry out the insects will die.  To avoid this place your leaf pack in a one gallon plastic bag or a bucket that has enough water in it to submerge everything.

These leaves are the contents from one of our leaf packs.  The leaves are taken out of the pack and sorted out carefully in trays or bowls filled with water from where the leaves were submerged.  Using a magnifying  glass, tweezers, spoons, and cheep paint brushed help in sorting through the leaves and looking for insects.

Sorting through the leaves in the bag can be quite interesting.  There should be all sorts of tiny creatures swimming around.  A magnifying glass, tweezers, spoons, and paint brushes help in carefully sorting through the leaves to find insects.  Most of the insects will likely be less then a half inch in length so look closely!

In the ponds around Phoenix we find an abundance of mayfly nymphs, dragon fly nymphs, scuds, and fly larva (diptera or "midges").

Mayfly nymph


Dragon fly nymph

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