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New Predators and Parasites of Earthworms in Illinois

Earthworms are important members of the soil community, comprising 20% or more of its living biomass. By fragmenting plant residues and incorporating them into the soil, earthworms act as regulators of decomposition and nutrient cycling. Their burrowing and casting activities affect many physical characteristics of the soil, including macroporosity, aggregate size and stability, water infiltration and drainage, and soil aeration. Earthworms are a part of the diet of hundreds of vertebrate and invertebrate animals, and can also serve as hosts for a variety of parasitic and disease organisms. The influences of predators, pathogens, and parasites on the dynamics of earthworm populations have proven difficult to determine. They have cryptic habits that make them difficult to observe undisturbed, it is difficult to accurately estimate their abundance, and they leave behind little trace when they die.

Phasmarhabditis sp.: a parasitic nematode isolated from nightcrawlers

A variety of parasites and pathogens have been shown to infect earthworms. These include bacteria, fungi, protozoa, rotifers, platyhelminths, mites, parasitic fly larvae, and nematodes. Of the nine families of nematodes that have been recorded from lumbricid earthworms, most use earthworms as intermediate hosts and are mostly innocuous to the worms. During 1997 in Champaign, Illinois, we collected a nightcrawler (Lumbricus terrestris) suffering from a nematode infection that has never before been described. Within days of isolating this earthworm, it died and thousands of nematodes emerged from its body 
(Fig. 1). Subsequently, earthworms infected with the same nematode have been found on two more occasions, most recently in April 1999. In all cases, infected worms died shortly after isolation, and were quickly covered with thousands of nematodes.

In the laboratory, we have been able to culture this nematode by exposing it to healthy earthworms or by growing it on an artificial diet. The nematode forms an infective juvenile stage called a dauer that locates and enters its host. The host appears to be overcome by a bacterial infection, and the nematodes subsequently feed on the bacteria and reproduce. L. terrestris is the only worm we have found infected with this nematode under natural conditions. However, in laboratory experiments nematodes have successfully infected the earthworms Allolobophora chloroticaAporrectodea turgidaAp. trapezoidesEisenia fetida, and Octolasion tyrtaeum.

Figure 1. Phasmarhabditis nematodes devouring an earthworm.

This nematode appears to be a member of the genus Phasmarhabditis, which includes five described species from Europe, Asia, Africa, and Australia in marine, littoral, and terrestrial habitats. The three terrestrial species that have been described are parasitic in snails and slugs. One of these, P. hermaphrodita, has been developed as a biological pesticide for controlling slugs and is now commercially available in several European countries. Our isolate is morphologically indistinguishable from P. neopapillosa, another slug parasite. Unfortunately, the group has not been well studied and it is not certain that P. hermaphrodita and P. neopapillosa are truly two separate species.

Very little is known about the ecology of species of Phasmarhabditis. Because of the possibility of using these nematodes beneficially for managing pest slug populations, it is especially important to understand details about their host ranges and pathogenicity. Our work will add to this knowledge.

Bipalium adventitium: an earthworm predator newly reported in Illinois

Bipalium adventitium (Fig.2) is an exotic land planarian that feeds on earthworms. Thought to be a native of Southeast Asia, B. adventitium was first described in 1943 from specimens collected in California. It was probably introduced to North America accidentally during the 1900s in soil on the roots of horticultural plants. Since its original description from California, new reports of its discovery have been published in Connecticut, Massachusetts, Maryland, New York, Pennsylvania, Tennessee, and Washington. Recently, dozens of specimens of B. adventitium were collected from a residential area in Urbana, Illinois, further expanding its known distribution in North America. Because the planarians appear most commonly in urban and suburban settings, their dispersal appears to be passive, most likely in soil on horticultural plants and turf in the horticultural trade. Subsequent dispersal is active, with planarians eventually being found in some wooded and agricultural habitats.

Figure 2. Bipalium adventitium is an exotic land planarian that feeds on earthworms.

Despite early suggestions that it may also prey on slugs, laboratory studies to date indicate that B. adventitium is primarily an earthworm predator. In all published studies, B. adventitium attacked all species of earthworms presented to it (eight species in all). In laboratory trials with B. adventitiumfrom Illinois, all species of earthworms presented were attacked, including three species of earthworms not previously reported as prey. Earthworms many times the size of the planarians were attacked, and earthworms up to 10 times greater in size rarely survive attacks.

The pattern of colonization of B. adventitium in North America is very similar to that of an ecologically similar New Zealand land planarian,Artioposthia triangulata, that was accidentally introduced into Ireland in the early 1960s. Where this planarian has become established, it has been shown to reduce earthworm populations to below detectable levels, possibly to extinction. Few ecological studies of B. adventitium populations in North America have been conducted thus far, and it is not known what impact its establishment may have on earthworm populations or on the important soil processes that earthworms mediate, such as soil formation, organic matter transformations, and nutrient cycling.

The extent of the distribution of B. adventitium in Illinois is unknown. Home gardeners and nursery workers may see these animals on the soil surface under boards and stones on the soil, or crawling on the soil surface after rains. It is slow moving, up to 2.5 inches long and up to 1/8 of an inch wide when crawling. It is pale brown or tan with a thin dark brown line running down its back. Survey scientists would appreciate receiving reports of any such findings. Contact Ed Zaborski at (217) 265-0330, or by e-mail.

Ed Zaborski, Center for Economic Entomology

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