Diet Composition and Significance of Earthworms as Food of Harvestmen (Arachnida: Opiliones)

Article excerpt

ABSTRACT.-We gathered dietary data for several harvestman species in a soybean field and adjacent hedgerow habitats to assess the extent of their polyphagy. A total of 1032 harvestmen, predominantly Leiobunum spp., were observed during almost 50 search hours over two seasons. More harvestmen and increased foraging activity were observed in the hedgerow than in the soybean field, and in both habitats harvestmen were more active at night. Earthworms (Oligochaeta: Lumbricidae) were the prey items most frequently recorded as being consumed by all observed harvestman species in both habitats (47.1%), while fruit and other plant material constituted 18.1% of the records. Feeding trials conducted with female harvestmen, Hadrobunus maculosus (Wood), revealed that these arachnids were not capable of subduing live earthworms, suggesting that consumption of these in the field might be limited to scavenging. Implications of harvestman foraging for local food web dynamics are discussed.

INTRODUCTION

Harvestmen (Arachnids: Opiliones) are a ubiquitous yet largely neglected group of arachnids. These facultative predators form a significant component of arthropod communities in woodlands (Todd, 1949; Walker, 1957; Adams, 1984) and agricultural fields (Ashby, 1974; Brust et al., 1986). Several studies further indicate that harvestmen, primarily Phalangium opilio (L.), may be significant biocontrol agents of homopteran (Dixon and McKinlay, 1989), coleopteran (Drummond et al., 1990) and lepidopteran (Ashby, 1974) pests in crop communities. Despite their ubiquity and suggested importance in agricultural systems, detailed knowledge of feeding ecology of harvestmen is lacking. It is generally assumed that harvestmen are predators of smaller invertebrates and opportunistic scavengers of dead animal and plant matter (Bishop, 1949; Todd, 1949, 1950; Phillipson, 1960a; Ashby,1974). Nevertheless, dietary data for these arachnids are either anecdotal (O'Brien, 1947; Bishop, 1949; Bristowe, 1949; Sankey, 1949; Todd, 1950) or come from studies designed to evaluate consumption of a specific primarily pest species (Ashby, 1974; Drummond et al., 1990). In order to place these arachnids in a wider ecological context and to assess their potential for reducing pest populations it is crucial to accumulate additional quantitative information on harvestman foraging in a variety of natural and crop communities.

Our objective was to collect data on diet of harvestmen as part of an ongoing project describing generalist predators in agroecosystems. We gathered evidence for the predatory role of harvestmen in a soybean field and its adjacent hedgerow habitat. Preliminary observations found that earthworms were the most frequently observed items in the harvestmen's diet in both habitats. Many earthworms fed upon by harvestmen appeared freshly killed. Bristowe (1949) reported consumption of small live earthworms by harvestmen. Taken together, these findings raised a basic question: Were harvestmen primarily scavenging or also capable of killing earthworms% These two modes of energy acquisition have fundamentally different consequences for the resource. Whereas predators may impact population dynamics of their prey populations, scavenging does not immediately influence the rate of resource renewal in the system. Considering the ecological importance of earthworms in soil formation and conservation processes (Darwin, 1881; Edwards et al., 1995), frequent predation of these oligochaetes may compromise harvestmen's beneficial role in crop communities. To address this question we conducted a feeding experiment to test harvestmen's ability to subdue live earthworms.

MATERIALS AND METHODS

This study was conducted at the Ecological Research Center of Miami University, Oxford, Ohio. Prey selection and feeding behavior of harvestmen were observed in a 0.7 ha soybean field and adjacent hedgerow habitats. Soybean field observations were conducted in a 0. …