Essentially, nematology is the scientific study of nematodes (roundworms), the phylum Nematoda. These are small worms that range between 1mm and 40 cm depending on type and habitat. Given that they are found in many environments across the globe, nematodes have been shown to be some of the most abundant multicellular organisms on earth.
As such, they can be found in environments where they exist as free-living organisms or parasitic in other environments. Some nematodes have been shown to be beneficial and therefore used in various industries (e.g. agriculture).
* In nematology, nematologists focus on such aspects as taxonomy, morphology, phylogeny, as well as biology in their respective environments.
In general, nematology is divided into three main branches that include; plant nematology, animal parasitology as well as free-living nematology. This section, however, will concentrate on plant nematology and agricultural significance of nematodes.
A majority of nematodes are free-living. They can be found in soil where they feed on fungi, algae, and bacteria among other material in their environment. However, there are many others that exist as parasites of plants while a few have been shown to be beneficial in agriculture.
According to studies, plant-parasitic nematodes affect plants in different ways. For instance, whereas some of the parasites have a short-term interaction with plants in agriculture, others heavily rely on plants for their life cycle. Different nematodes may feed on different parts of the plant from which they obtain the nutrients they need for growth and development.
Regardless of the relationship between the plants and the nematodes, all nematodes use a specialized structure known as a stylet to feed on plants. Here, the size and shape of the stylet is used to classify them (plant nematodes).
* A majority of plant parasitic endoparasites are soil-borne and thus tend to damage plant roots (e.g. members of the order Rhabditida).
Ectoparasites include nematodes with long stylets. These nematodes are able to use their long stylets to feed on plants without the need to penetrate any part of the plant. Given that they are able to feed from outside the plant, ectoparasites can feed on different hosts in their environment. Xiphonema is a good example of an ectoparasite.
* Some ectoparasites have been shown to be capable of manipulating the size of plant cells in order to obtain more nutrients.
* Some ectoparasites (e.g. members of the class Enoplea) can spread viruses in plants that cause significant damage.
These nematodes partially penetrate given parts of the plant at given stages of their life cycle. The Rotylenchulus reniformis is a good example of a semi-endoparasite. Rather than fully penetrating the plant, only the head of the organism penetrates a given part. This creates a permanent source of food.
Typically, the nematode remains attached to the feeding source and is therefore not as mobile as ectoparasites. While the nematode benefits from a source of nutrition, they are exposed to various harmful environmental conditions and predators.
* Some of the nematodes (e.g. Tylenchulus semipenetrans) only live as semi-endoparasites during a given stage of their life cycle. As juveniles, however, they live as ectoparasites.
These nematodes migrate through root tissues of plants causing significant damage. This behavior significantly affects plant roots ultimately causing the plant to die.
Some of the species (e.g. Pratylenchus) have been shown to lay eggs in the root tissue which causes extensive damage as the new generation starts feeding on the roots. As migratory endoparasites move from one point to another in the soil, they also find new roots to invade and feed on.
Sedentary endoparasites have been shown to be some of the most destructive nematodes. Because they have shorter stylets, these nematodes (e.g. Globodera) penetrate the plant through the root and make their way to vascular cells of the plant where they obtain nutrients.
In the vascular cells, these nematodes they stimulate the formation of larger feeder cells that serve as permanent sources of nutrients. As they grow and develop, they also lay eggs and give rise to offspring that obtain nutrients from the plant.
Foliar nematodes (e.g. members of the genus Aphelenchoides) penetrate leaves of foliar plants through the stomata where they feed and lay eggs.
These activities have been associated with chlorosis and necrosis that ultimately causes the leaves to die off. As they migrate from one leaf to another, they cause extensive damage which also allows eggs to be dispersed as dead leaves are blown around in the environment.
Stem and bulb nematodes are parasites that affect such plants as potatoes and onion. They affect the upper and lower parts of plants. For the most part, the fourth stage of the juvenile has been shown to the most infectious stage.
Following entry into the plant through the roots, stem and bulb nematodes (e.g. Ditylenchus species) migrate to the upper area of the plant as they derive nutrients from the different parts. This behavior causes significant damage to onions and potatoes.
Like ectoparasites, seed gall nematodes also feed on plants from the outside. Using their long stylets, seed gall nematodes feed on the leaves.
They have also been shown to penetrate the plant through the floral part and migrate to feed on the seeds. This causes damage and consequently death of the seed(s). As such, these nematodes are responsible for seed gall (blackened seeds).
Following the death of the plant, juvenile nematodes have been shown to be capable of halting their development. This allows them to transform and survive unfavorable conditions.
In dry environments, these nematodes are capable of surviving in this stage for many years. Once conditions improve, they revert to the active stage and continue affecting new plants.
Some of the nematodes that affect food crops include:
· Tylenchulus semipenetrans - Infect between 50 and 90 percent of regions that produce citrus across the globe
· Rhadinaphelenchus cocophilus - Case red ring disease in coconuts
· Heterodera zeae - Common in India and parts of America, Egypt and Pakistan where it affects corn
· Root-knot nematode - Cause root disease in cotton
· Globodera rostochiensis - Affect potatoes
· Aphelenchoides besseyi) and Ditylenchus angustus - affect rice
In agriculture, the use of chemicals for pest control has been associated with several disadvantages. Apart from health concerns, some pests have been shown to gradually develop resistance.
It has become important to find new strategies for controlling pests in a safe and more effective manner. While many species are plant parasites, the feeding habits of some species have been shown to be beneficial in agriculture. As such, they can be used as biological agents to control pest and weed.
In agriculture, some of the beneficial/important nematodes are grouped as follows:
Important EPNs nematodes include members of the families Steinernematidae and Heterorhabditidae that have been shown to kill soil-based insect pests within 48 hours.
EPNs are used against the following agricultural pests:
P. hermaphrodita is an example of slug parasitic nematodes. As a parasite of various slug species (e.g. members of the families Arionidae and Milacidae etc), the nematode controls slugs by releasing bacteria inside the host that kills them in about 21 days.
One of the biggest advantages of using P. hermaphrodita is that it's highly effective during wet weather conditions. Given that slugs are also highly active during these conditions; the nematode can be used to destroy them thus protecting crops.
Some of the other pros of using the nematode include:
As the name suggests, these nematodes tend to infect insects. They can be used to control crop pests (insects) due to their host preference.
The term Entomophilic nematodes has also been used to refer to slug parasitic nematodes, Entomopathogenic nematodes and predatory nematodes that parasitize insects. The mermithid nematodes are good examples of Entomophilic nematodes. In their environment, they have been shown to affect arthropods.
As a discipline, nematology is further divided into several sub-disciplines including animal nematology (largely focused on parasitology), free-living nematology, plant nematology, and entomopathogenic nematology. Therefore, students have an opportunity to specialize in any of the sub-disciplines of interest to them.
In these sub-disciplines, students also have an opportunity to further specialize in specific fields. For instance, in plant nematology, students have the opportunity to train in and become zoologists, parasitologists, animal scientists and entomologists among others.
With nematodes being some of the most abundant organisms on earth, then, there are many career choices for students that are likely to increase with the ever-changing environmental factors.
D. Dastogeer. (2019). Plant Nematology Lecture.
David G. McNamara, John M. Webster, and K. Bengt Eriksson. (2008). An Anecdotal History of Nematology.
Keith Graham Davies. (2006). Review of Plant Nematology. ResearchGate.