Nematodes are interesting and remarkable creatures that escape the average eye and mind because of their hidden existence beneath the hide of man and beast and sheltering cloak of mother earth. They make themselves known to us in the itch we scratch or the plant that declines and dies. Yet to really know them we must use a magnifier or microscope, because most nematodes are very small, and to make things more difficult, nearly transparent. The damage inflicted upon man, beast and plants by nematodes is measured in hunger, disease and untold billions of dollars over the years and for this reason, knowledge of their existence is important.
A nematode is an eel-like animal resembling but not closely related to an earthworm . Most nematodes are minute – 1/50 of an inch to 1/4 of an inch long (0.5 to 6.5 mm long) – but some that live in animals attain much greater lengths.
A nematode has:
- Digestive organs and glands
- Reproductive structures - male and female
- Excretory structures
- Tough cuticle (skin)
A nematode does not have:
- Eyes, ears, or a nose
- Blood vessels or blood
- A skeleton
- A liver, gall bladder, or lungs
- Appendages (arms or legs)
Types of Nematodes:
Nematodes belong to the phylum Nematoda. They are broadly grouped according to feeding habits as:
- Animal parasites
- Insect parasites
- Plant parasites
- Microbial feeders
One hundred adult lesion nematodes (plant parasite) could lie side by side inside this hyphen -
About 8,000 lesion nematodes could lie on one thumbnail without overlapping.
WHERE NEMATODES ARE FOUND
Nematodes are everywhere and in almost everything: the top of the highest mountain, the bottom of the lowest valley, the most arid desert, and the most lush farmland. Nematodes are found by the teeming billions in the icy saltiness of the Arctic seas and are found (but not teeming) in hot springs. The elephant, the gnat, the whale, and the minnow – all have one to many kinds of nematodes within their bodies. Nematodes are even found inside other nematodes. Many nematodes are serious pests of man while many others are not harmful. All plants have one or more nematode pests and these plant-parasites are the focus here.
HOW NEMATODES FEED ON PLANTS
Nematodes that feed on plants have mouths equipped with a miniature hypodermic needle-like structure called a stylet. A nematode feeds on a plant by puncturing the wall of a plant cell with this stylet and sucking out the contents of the cell. Suction is induced by contractions of a muscular bulb in the esophagus of the nematode. Most nematodes that feed on plants inhabit the soil outside of the plant root and penetrate the root only with their stylets (ectoparasites). These outside root feeders always have eel-like bodies in order to move easily to new feeding sites and are usually equipped with long stout stylets so they can reach the most favorable feeding areas deep within the roots.
HOW NEMATODE FEEDING AFFECTS PLANTS
- Nematodes cause lesions, discoloration, deformity, and in some cases, complete devastation in the penetration and feeding areas.
- Plants under attack by nematodes lose vigor and become unthrifty.
- The size and quality of fruits and vegetables are reduced.
- Nematodes cause decline, and in extreme cases, death of the plant.
- The puncture wound left by the stylet opens the door for fungal or bacterial invasion, which may do more damage than the nematode.
CAN A NEMATODE FEED ON ANY PLANT?
No. Some, such as the citrus nematode, attack only very few types of plants, while others such as root-knot nematodes, feed on many different kinds of plants.
THE LIFE HISTORY OF A PLANT-PARASITIC NEMATODE
(Lesion nematode on corn)
- A female nematode inside the corn root lays eggs in the root tissue.
- The growing season ends, the corn dies, and inside the roots the eggs live on.
- Winter passes, a new growing season begins and new corn is planted over the rotted roots containing live nematode eggs.
- Soil and water carry substances from the new corn roots to the dormant eggs, hatching them.
- The newly hatched nematodes swim to the corn roots, enter, and proceed to feed and grow to adults.
- The new adults lay more eggs. Several generations of nematodes may appear before the corn is ripe, constantly adding new stylets to drain away the life of the plant.
Removal of crop residues consequently, not only reduces the insect pests that survive in such materials but also cuts down on the nematode population.
HOW NEMATODES MULTIPLY
Each female lays many eggs inside or outside the plant. These eggs have a tough shell which protects the nematode under unfavorable conditions. These eggs may lie dormant in the fallow land, sometimes for several years. Then, when the land is planted to a suitable crop for the nematodes, the eggs hatch and new populations of nematodes build up in the soil. Each succeeding year, if conditions are favorable, many more eggs are present, spreading the nematode population in ever-widening circles.
HOW CAN AN ANIMAL AS SMALL AS A NEMATODE HARM A PLANT?
Imagine a network of small rootlets surrounded on every side by a multitude of hungry hypodermic needles, each stabbing and sucking repeatedly, and by their concerted feeding action, draining away the life blood of the plant. The weakened plant declines while the nematodes fatten and multiply until by magnitude of numbers they are no longer small.
HOW NEMATODES ARRIVE IN NEW AREAS
- Nematodes migrate into new areas on or in trains, planes, motor vehicles and ships. Carried by man in soil, plants or plant products or by his vehicle in mud under the fenders or in the tire treads.
- Windstorms may disperse eggs and encysted nematodes.
- Nematodes get along fine in water and when an area is flooded, they are moved about.
- Animals and insects also play a role in carrying nematodes into new areas.
ARE ALL NEMATODES ENEMIES?
No. Some nematodes feed on plant-destructive nematodes, mites, and insects. Other nematodes consume huge quantities of plant-damaging bacteria and fungi.
HOW ARE SAMPLES TAKEN TO DETERMINE THE PRESENCE OF A NEMATODE PROBLEM?
Amount of soil needed for each sample: About 1 pint (500 ml) removed from close to the roots of the plant or plants suspected of being nematode infested.
Amount of roots needed for each sample: About 1 cupful (100 ml) from larger plants (trees, shrubs, etc.). The entire root system of small plants if the plant is expendable.
Containers: Sealed plastic bags are the best; glass jars with lids will also do the job. The container must enclose the sample. If the soil is exposed to air, much of it will dry out and many nematodes will die.
Example 1: A home owner sees his grass turning brown in large patches. He suspects nematodes to be the cause. Three to five samples are taken. One from the dying or dead area, one from an adjacent healthy area, and at least two on the border of the patch where the dying grass meets the green. Samples should be six to eight inches (15-20 cm) deep.
Example 2: Some orange trees begin to decline and the owner decides to check his soil for the presence of nematodes. One sample is taken per tree, far enough out from the base of the tree to obtain small feeder roots. A hole is dug 1 to 2 feet (0.3 to 0.6 m) deep. About a cup of small feeder roots is taken from different levels in the holes and a pint jar (500 ml) is filled with soil from the sides of the hole. (Some soil should be taken from the top six inches, the middle and the bottom of the hole to fill the jar.)
Example 3: An active garden club member notices some of her prize roses losing their healthy vigorous appearance. Being well informed, she suspects nematodes. If the rose plants are small and she has some to spare, an entire plant is removed along with a pint (500 mL) of soil from about the roots.
HOW ARE NEMATODES REMOVED FROM LEAVES, ROOTS, OR SOIL FOR LABORATORY STUDY?
Leaves: Leaves in water are cut to pieces in a blending machine and then washed through a series of sieves (screens). The coarse sieve separates the leaf fragments from the water. Then fine sieves separate the nematodes. The nematodes are washed from the finest sieve into a glass dish and identified under a microscope.
Roots: Roots are washed to remove adhering soil and then placed in a sealed jar for at least three days at room temperature. After this time, the nematodes begin to leave the roots. Then the roots are flushed with water and the water with the nematodes is poured on a fine sieve which catches the nematodes. They are then washed into a glass dish and identified.
Soil: Soil is placed in a pail which is half filled with water and vigorously stirred up. The water is then poured into another pail leaving the heavy soil sediments behind. (The nematodes are very light and slowly settle to the bottom.) Then the soil is passed through a series of fine sieves that separate the larger soil particles and the trash from the nematodes. The nematodes are then removed from the finest screen and placed in a glass dish for examination.
This article was prepared as an introduction to nematodes, particularly plant-parasitic nematodes. Websites listed on the ONTA homepage may be consulted for more advanced information on plant-parasitic nematodes.
R. P. ESSER, Nematologist
STATE OF FLORIDA, U. S. A.
DEPARTMENT OF AGRICULTURE