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The Nematodes. Introduction and General Characteristic. Phylum Nematoda greek: Thread nem-ah-to-dah.
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The Nematodes Introduction and General Characteristic
Nematodes are the most numerous multicellular animals on earth. A handful of soil will contain thousands of the microscopic worms, many of them parasites of insects, plants or animals. Free-living species are abundant, including nematodes that feed on bacteria, fungi, and other nematodes, yet the vast majority of species encountered are poorly understood biologically. There are nearly 20,000 described species classified in the phylum Nemata .
Nematodes are structurally simple organisms. Adult nematodes are comprised of approximately 1,000 somatic cells, and potentially hundreds of cells associated with the reproductive system . Nematodes have been characterized as a tube within a tube ; referring to the alimentary canal which extends from the mouth on the anterior end, to the anus located near the tail. Nematodes possess digestive, nervous, excretory, and reproductive systems, but lack a discrete circulatory or respiratory system. In size they range from 0.3 mm to over 8 meters.
Paired lateral sensory organs on the head, derived from cilia and opening to the outside through a small pore • Ubitquitous, unsegmented,acoelomate and pseudocoelomate worms. They are the most abundant multicellular animals alive today.
Body covering and Body cavities • Typical nematode ranges from 1-2mm long and shows no external segmentation • Cuticle- covers the body of a nematode • Peritoneum a covering which envelopes the nematode’s organ • Pseudocoel fluid, when present, serves as medium of circulation
A cuticle is shed and resecreted 4 times during development from juvenile to the reproductively mature adult
Unlike the situation in most arthropods, nematodes continue to increase in size between molts and even after molts. But unlike most other animals nematodes grow mainly by increasing the size of individual cells rather than by increasing the number of cells.
Musculature, internal pressure and locomotion • The body walls of nematodes contains no circular muscle • Internal hydrostatic pressures as high as 225mm Hg can be measured inside some nematodes. • Alternate contractions of longitudinal muscles on the dorsal and ventral surfaces of the body
The body of a nematode is design not well for free-swimming existence rather for free living in soil • Muscle contraction is under control by a simple nervous system consisting of an anterior brain and four major longitudinal nerve cords
Organ system and behavior • Nematodes have a linear digestive system with a mouth at anterior ends leading through muscular pharynx, intestines and thence out of the body through anus.
Food digested in the gut is not distributed by any specialized vascular system, and neither is there a respiratory system for the uptake or distribution of oxygen. Rather, nutrients and waste are distributed in the body cavity, whose contents are regulated by an excretory canal along each side of the body.
Reproduction and Development • The most studied, available, and versatile of all entomopathogenic nematodes. Important attributes include ease of mass production and ability to formulate in a partially dried state that provides several months of room-temperature shelf-life. Particularly effective against lepidopterous larvae, including various webworms, cutworms, armyworms, girdlers, and wood-borers. This species is a classic sit-and-wait or "ambush" forager, standing on its tail in an upright position near the soil surface and attaching to passing hosts. Consequently, S. carpocapsae tends to be most effective when applied against highly mobile surface-adapted insects. Highly responsive to carbon dioxide once a host has been contacted, the spiracles are a key portal of host entry. It is most effective at temperatures ranging from 22 to 28°C
Nematodes exist almost everywhere, including soil, plant and animal surfaces and interiors, decomposing life forms, and fresh and salt water and their subtending sediments. Nematodes can be grouped according to what they eat. The different groups are fungal feeders, bacterial feeders, predators, animal parasites, algal feeders,omnivores, and plant parasites.
Parasitic Nematodes • Root-lesion nematodes (Pratylenchus) rip a hole in the sides of root cells and crawl inside. They move through the root, piercing, sucking, and leaving behind a trail of both cell-killing metabolites and eggs. Cell death results in brown lesions on the roots. Lesions begin on one side, but may encircle a root and thereby girdle it. The overall effect is a weak, shallow root system with a lot of dead areas. Root-knot nematodes (Meloidogyne) enter roots as juveniles, select a feeding site of three to eight cells, and swell up in their chosen spot as they progress towards adulthood. They introduce hormone-like substances into the plant cells, causing the plant to swell in the area producing galls, or root knots. Males regain their slender profiles and leave the root at adulthood, but the fattened adult females remain inside. They exude eggs into the soil, and after the juveniles hatch, the cycle begins again. Roots infested with root-knot nematodes usually have visible galls and may exhibit excessive branching. Parasitized plants may be weak and stunted. Root systems may be deformed, and underground organs such as potato tubers and carrot taproots may be damaged and unmarketable.
Pin nematodes (Paratylenchus) pierce root cells from the soil outside of the plant. Low numbers may appear inconsequential, but in high enough numbers, pin nematodes can damage crops. Ring nematodes (Criconemella) behave similarly. Little is known about ring nematode damage on many crop plants. • Stubby-root nematodes (Trichodorus and Paratrichodorus) are also external soil-dwelling parasites. Besides piercing root cells and sucking out the contents, they can vector viruses such as the Tobacco Rattle Virus, which causes corky ringspot disease in potato tubers. Other virus-vectoring plant-parasitic nematodes are Xiphinema, daggar nematode, which can vector Tomato Ringspot Virus, a disease of many fruit crops, and Longidorus ("mint nematode").
NematodeDevelopment • Because the symbiotic bacterium kills insects so quickly, there is no intimate host-parasite relationship as is characteristic for other insect-parasitic nematodes. Consequently, entomopathogenic nematodes are lethal to an extraordinarily broad range of insect pests in the laboratory. Field host range is considerably more restricted, with some species being quite narrow in host specificity. When considered as a group of nearly 30 species, however, entomopathogenic nematodes are useful against a large number of insect pests, many of which are listed in the table below. As field research progresses and improved insect-nematode matches are made, this list is certain to expand. Regrettably, nematodes have yet to be found which are effective against several of the most important soil insects, including wireworms, grape phylloxera, fire ants, or corn rootworms.
Beneficial Nematodes • Infective juvenile nematodes migrate through the soil in search of insect hosts. They enter the pest insect through the mouth, spiracles or other body openings. Once inside the pest insect, the nematodes release an associated bacteria. This bacteria rapidly multiplies and kills the host. The nematodes feed on the bacteria and decomposed host tissue. They continue development and reproduce within the host until the food supply has been exhausted and population density necessitates their emergence. Third stage juveniles then form a protective cuticle which helps protect them from the soil environment, becoming what is referred to as an, "infective juvenile". After entering into the soil, infective juveniles actively begin searching for a new host.
Insect parasitic nematodes are effective against a wide range of known species of pest insects. Once inside the host, death may occur within 24-48 hours. As a long term control agent, nematodes continue to reproduce and seek out new pest hosts for extended periods of time providing soil temperature, moisture and host availability are optimal. In field crop applications, nematodes will infect and kill cutworms, grubs, root worms and other soil dwelling pests. For greenhouse pest control efforts, nematodes are particularly effective against fungus gnats and aid in the control of pupating thrips. Orchard pest control is enhanced against codling moth, lepidopteran pests that bore into tree bark and pests that are found in the soil during various stages of their life cycle. Parasitic nematodes invade and destroy white grubs, Japanese beetles and other harmful pests commonly found in lawns and turf. It is important to identify exactly what pest insect is being targeted. We can then advise you of the proper species of nematode for release
