Behavioral Techniques

Several approaches have been taken to extracting earthworms from soil based on their behavioral response to certain stimuli. A number of

Method

Description

Advantages

Disadvantages

Passive

Hand sorting

Washing and sieving

Flotation

Behavioral

Chemical extraction

Heat extraction

Electrical extraction Mechanical vibration Trapping Mark-recapture

Indirect

Cast counting

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

Metal rods inserted into soil and connected to

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

Effective on dense root mats

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

Laborious

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 [1986]) 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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