CHAPTER 3

AQUATIC HABITATS

 

 AQUATIC HABITATS AND ASSOCIATED COLLECTING TECHNIQUES

One difficulty for some collectors is to visualize the richness of discrete habitats that make up the aquatic realm for aquatic beetles. For this reason, we have inserted a chart of habitat types and thier ecological relationships. This chart should be studed well when you are working an area with a diversity of aquatic habitats..

 

Natural Aquatic Habitats

 

Do not forget the diversity in form and richness of beetle species that live in the water or at the water's edge. This is too easily done when your focus is usually on terrestrial habitats. There are a large number of beetle families with one or more members that are aquatic in either larval or adult stages. The following discussion is broken down into passive collecting or the use of traps specifically for water beetles, and active collecting.

 

 METHODS

 

PASSIVE COLLECTING

Terrestrial Light traps

Aquatic beetles are often attracted to lights. Generally it is believed that water beetles are attracted to shiny surfaces when they are dispersing -- the reflected light probably mimics sunlight or moonlight reflecting off of the surface of a water body. These lights and protocols are the same for terrestrial beetles, so refer to Chap. 4, section 4, for detailed discussion.

 
Terrestrial light trap

Aquatic Light Traps

See following section on Bottle Traps.

 

Bottle traps

Bottle traps can be made from modified minnow traps or from large plastic bottles, 4 liter plastic, wide-mouth bottles with a funnel inserted into the opening or smaller traps from 2 liter soft drink bottles on which the top third of the bottle is cut off, and inverted and inserted into the base of the bottle and glued, or stapled in place. Bottle traps are useful for capturing larger, active swimming beetles such as larger dytiscids and hydrophilids. They may be used without bait or baited with liver or fish. The placement of bottle traps is important and they should be placed in ponds or lakes at the edge of open water with the long axis of the bottle parallel to the margin of the vegetation. "Light sticks" can be used inside of the bottle traps to add a light trap function.

 
Bottle traps

ACTIVE COLLECTING

Aquatic Coleoptera are usually poorly represented in most kinds of quantitative or semi-quantitative samples such as those mentioned under passive collecting above. Reasons for this are probably twofold; a - many species occur in low density (perhaps because they are larger predators), and b - adult beetles and their larvae are frequently in difficult places to sample such as among dense vegetation or debris or within the substrates. Solutions to these problems involve sampling a relatively large area and using techniques that separate beetles from substrates.

 

Net collecting

The best piece of basic equipment is a rugged-framed, triangular or "D"-shaped dip net. The dip net should be designed so that the net bag is hung behind/below the net frame and secured to it by rope or wire, and the net bag should have a canvas skirt protecting the mesh. These features allow the net to be used vigorously in areas with vegetation or abrasive rocks by protecting the bag from snagging and tearing. Most beetles are in shallow water, near the water's edge so that the margins of the habitat should be sampled aggressively. Sweeping the net at the surface or slightly into the substrate will catch a few beetles, but often it is only after several passes that insects start appearing in the net. Typically, the first few passes dislodge the insects and permite capture of many specimens. Roiling the water and substrate particularly with the toe of your boot and then sweeping the area is productive. However, be cautious about too much material plugging the net bag and permitting escape of beetles.

The net contents can be hand searched but even more effective is the use of a separation device. A simple technique is to place the debris from a net sweep onto a 5 to 10 mm mesh screen suspended over a shallow pan. Placing this in the sun will speed the rate at which small larvae and slow-moving adults fall through the screen into the pan. If the pan accumulates water, this water containing insects from the sample can be poured through a sieve to recover the insects. Aquarium nets are good for sieving for small water beetles. Many specimens can be taken by hand-picking the residue from samples using this technique.

Adults of some groups (e.g. Hydraenidae and small Hydrophilidae) do not swim well. When substrates are placed in water and agitated, these beetles rise to the surface where they can be hand picked.

 
Net collecting

Shore Collecting

Splashing water up onto the margins of streams, lakes, and springs will often flush out semiaquatic and terrestrial beetles that occur along the banks. In some habitats depressing soft substrates with the foot allows temporary flooding of the substrate and escape attempts by the beetles. Saturated soils, moss or debris along the water margin often contain a mixture of terrestrial and aquatic Coleoptera. Most aquatic Coleoptera pupate in terrestrial sites near the water's edge. Pupae with larval exuviae suitable for rearing for larval - adult association can be found by carefully turning stones and logs, searching through leaf litter or by scrapping away the loose surface soil with a trowel or pocket knife.

 

Aquatic Plants and Other Benthic Substrates

Brass screens of various mesh size are useful for finding larvae and pupae of Donaciinae (Chrysomelidae) and aquatic weevils. A prospective host plant is dug up and placed on the screen, and the roots flushed with water to remove the soil. Larvae and pupae usually occur attached to the roots or at the juncture of the roots and stem. These screens are also useful for dry or wet sieving of soil, debris or mosses, or used as separation devices as described for sample processing above.

Sometimes it is desirable to take substrate samples back to the lab. If such material is placed in a shallow pan of water in a darkened room with a small light shining on the open-water portion of the pan, larvae will be attracted to the light and leave the substrate. Alternatively these materials can be placed in a Berlese funnel or other funnel-type extractor. Substrate samples can be collected at any time of the year.

 

Hand-Collecting

Adult Scirtidae, Donaciinae (Chrysomelidae), and Curculionidae can be collected by sweeping emergent or spring-side vegetation with a sweep net. Hand-picking of specimens from vegetation, while less efficient, allows an accurate association of specimens with their hosts.

 

Insecticide Sprays

A newly attempted technique for collecting aquatic beetles can be successful in special circumstances. This method is to use a pyrethrum insecticide spray on mosses and other small plants in the splash zone of seeps, small streams, on boulders in a river, and within the splash zone of waterfalls. In smaller habitats a conventional collecting net with a very fine mesh size can be used to catch the insect fallout and in the larger habitats a net is placed between boulders so that it will catch the surface and subsurface drifting beetles. After spraying the moss, wait 5 to 10 minutes and then splash the sprayed area with water while rubbing with your hands. Many small aquatic and semiaquatic beetles and their larvae can be taken in this manner. A natural pyrethrum-based spray is prefered because of the greater human safety and environmental biodegradability of these chemicals. Synthetic pyrethroids, such as permethrin, can also be used, but with caution as their environmental effects may be greater.


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