Nematodes: Harmful and Beneficial Organisms

I. INTRODUCTION Nematodes are usually separated into two major groups, the Secementea (Phasmidia) and the Adenophora (Aphasmidia), primarily on the basis of the presence or absence of paired posterior sense organs, the phasmids. The Secementea are found mainly in terrestrial environments and about half of the species in this group are animal or plant parasites; very few species of the Adenophorea are parasites (Fig. la-i). Nematodes occupy almost all environments, including the North and South Poles and deserts, and worldwide there are about 20,000 described species belonging to 12 orders of Adenophora and 8 orders of the Secementea (Maggenti, 1981). Approximately 48% of nematode genera are parasites of animals and plants, with the remainder being free living in fresh­ water, marine, and soil environments (Anderson, 1988). Long established grasslands may have nematode populations as great as 200,000,000,000 per hectare. The economic effects of nematodes make the phylum Nematoda one of the most important animal groups, and research has concentrated on the nematode parasites of humans, animals, and crops.