Extraction of Microarthropods from Samples

There are many modifications of the basic Berlese funnel (Edwards, 1991) and most will yield satisfactory results. Heat is used to desiccate the sample, driving arthropods out and down into a collection fluid. Many designs are called Tullgren funnels, after the originator of the use of electric lights as heat sources. (The original Berlese funnels used steam as a heat source). Larger funnels (Fig. 9.3), used for extracting big samples of litter, can work effectively with small cores as well. Arrays of smaller funnels can handle more samples in a smaller space, and have become the most widely used piece of extraction equipment (Bater, 1996). Soil cores contained in their sleeves are extracted in an inverted position, surface layer down, so that arthropods can escape using natural channels in the soil. The upper portion (bottom) of the core should be moistened with water to improve extraction efficiency (T. R. Seastedt, personal communication). Seventy-percent ethyl alcohol is the usual collection fluid for the extracted arthropods. A ten-percent picric acid solution is preferred by some authors (e.g., Meyer, 1996). Care must be taken to keep mineral soil from falling into the sample (the berleseate), because samples contaminated with soil are hard to sort. To this end, a single layer of cheesecloth may be inserted between sample and funnel.

Litterbags are an alternative method to soil cores for sampling microarthropods. They offer several advantages: Microarthropods

FIGURE 9.3. Schematic diagram of an extractor for soil macrofauna: (a) sample cover, (b) soil sample, (c) sample screen, (d) aluminum funnel, (e) collection container with 70% alcohol or similar collection fluid (from Gorny and Gram, 1993).

using different substrates, such as different litter species, may be detected (Hansen, 2000). Different stages of litter decomposition may be compared, and time sequences described. Litterbags are readily extracted, intact, on large Tullgren funnels.

Flotation is another alternative method for extracting microarthro-pods from soil samples. Using organic solvents or saturated sugar solutions, arthropods may be separated from the soils, washed through a fine mesh screen, and thus recovered (see Fig. 4.9). In comparison with Tullgren extraction, flotation usually yields higher numbers of microarthropods. Some collembolans, such as members of the family Onychiuridae, respond poorly to Tullgren extraction; flotation is the method of choice for sampling these arthropods. The disadvantages of flotation are (1) the method is extremely laborious in comparison to Tull-gren extraction and (2) it is not effective for samples with large amounts of organic matter. Finally, the use of organic solvents probably violates most laboratory health and safety considerations (Griffiths, 1996).

Amore attractive procedure uses saturated sugar solutions instead of organic solvents (Snider and Snider, 1997). The method (Table 9.1) yielded much higher captures of microarthropods than did Tullgren extraction alone, in samples from two hardwood forest sites in upper Michigan. The method was extremely laborious; the authors report that 10 to 12 hours were required to sort a single sample. They note that financial constraints usually preclude labor-intensive procedures such as flotation, even though much more accurate population estimates were obtained.

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