Several approaches have been taken to extracting earthworms from soil based on their behavioral response to certain stimuli. A number of
Washing and sieving
Electrical extraction Mechanical vibration Trapping Mark-recapture
Known volume of soil cut with spade or corer, broken apart and worms removed by hand
Known volume of soil cut with spade or corer, soaked in dispersant/preservative, and washed through sieve(s) by hand or mechanical device Material from hand sorting or washing/ sieving floated in high-density solution (e.g., MgSOJ
Soil saturated with chemical irritant (e.g., 0.2% formalin) causing earthworms to emerge onto soil surface Soil blocks or cores suspended under heat lamps in water into which earthworms migrate
AC electrical source Stake or rod inserted into soil and vibrated with bow or flat iron Pitfall or baited traps placed in soil and sampled at desired intervals Individuals tagged, released, and population sampled at intervals
Simple, reliable in the field; low cost
Higher recovery of cocoons and small individuals
Separates earthworms from soil and plant debris; cocoons and small individuals collected
Simple; effective on deep-burrowing anecic species
Useful for selective or comparative sampling Simple; useful for selective or comparative sampling Simple; useful for selective or comparative sampling Useful for estimating population density, dispersal, and mortality
Surface castings enumerated and identified Simple
Laborious; may not collect deep-burrowing species, small earthworms, and cocoons Laborious; may not collect deep-burrowing species
Laborious; may not collect deep-burrowing species
Not effective on all species, in all soils or under all conditions
Not effective on all species; inconvenient for field use
Highly variable; not convenient in the field; dangerous Not effective on all species
Not effective on all species
Not a quantitative estimate of population density
Summarized from Lee (1985) and Edwards and Bohlen (1996); reproduced from Hendrix (2000).
chemical irritants have been used, including HgCl2, KMnO4, mustard, and formalin. Aqueous solutions of 0.165-0.550% formalin are most commonly used and have been shown to be effective on L. terrestris when applied in three sequential doses totaling 18 L • m-2; but formalin may be less effective on other species (Satchell, 1969; Callaham and Hendrix, 1997). Chemical extraction with aqueous mustard powder solution has been shown to be as effective as formalin in some cases; this method avoids the use of toxic formaldehyde. Effectiveness varies with earthworm species and activity, soil water content, porosity, and temperature. Comparisons with hand sorting should be done before adopting extraction techniques for quantitative sampling.
Heat extraction is a modification of that used for enchytraeids (discussed in next section). Soil cores or blocks are placed in pans of water and exposed to heat from overhead light bulbs; earthworms are collected from the water after several hours. This technique was more effective than hand sorting or formalin extraction on small earthworms in dense root mats (Satchell, 1969). As with hand sorting, it is not effective on deep-burrowing, anecic species such as L. terrestris.
Mechanical vibration employs a rod or stake driven into the soil, vibration for a few minutes with a bow or flat piece of metal such as an automobile leaf spring, and collection of earthworms that emerge onto the soil surface. Some megascolecid species have been sampled with this technique (Reynolds, 1973; Hendrix et al., 1994), but it is not effective on lumbricids and probably only useful for selective or comparative sampling of certain populations.
Electrical extraction of earthworms involves inserting metal rods into the soil, connecting them to a source of alternating current, and collecting earthworms that come to the soil surface. Different voltages and amperages have been used with varying degrees of success; effectiveness of the technique is highly dependent on soil water content, electrolyte concentration, and temperature. As with mechanical vibration, the soil volume sampled is not known and therefore this method is best suited for qualitative or comparative sampling. However, a commercially available electrical sampler ("octet" device developed by Thielemann ) was evaluated by Schmidt (2001) and found to be highly effective for quantitative sampling of lumbricid species in pastures. Electrical extraction methods are potentially very dangerous and should only be used with extreme caution.
Two earthworm-trapping techniques have been described. Pitfall traps (open-top containers buried level with the soil surface and containing a fixative solution such as picric acid) may be useful for sampling surface-active species in diurnal or seasonal studies. Arrays of traps are installed and sampled at 12-hour, 24-hour, or longer intervals. Baited traps, such as perforated clay pots containing manure or other attrac-
tants and inserted into the soil, may also be useful for collecting certain species. As with other behavioral methods, trapping is probably highly selective and best suited for qualitative or comparative sampling.
Mark, release, and recapture techniques have been widely used to study population dynamics of animals including earthworms. Large numbers of individuals of desired species are collected, marked (e.g., with brands or nontoxic dyes), and released into the population of interest. Sampling over time and distance from the target site, and enumeration of tagged relative to untagged individuals, yields information on dispersal, mortality, and population density. Radioisotope and, more recently, immunofluorescent antibody techniques have been employed in earthworm mark-recapture studies.
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