Transcript The Nematodes

The Nematodes
Introduction and General
Characteristic
•Phylum Nematoda
greek: Thread
nem-ah-to-dah
• 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 roomtemperature 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 virusvectoring plant-parasitic nematodes are Xiphinema, daggar
nematode, which can vector Tomato Ringspot Virus, a
disease of many fruit crops, and Longidorus ("mint
nematode").
Nematode Development
• 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