Heterodera medicaginis (Alfalfa cyst nematode) is a plant pathogenic nematode which is cited as an invasive species

Invasive species

"Invasive species", or invasive exotics, is a nomenclature term and categorization phrase used for flora and fauna, and for specific restoration-preservation processes in native habitats, with several definitions....

. It is closely allied to Heterodera daverti, H. glycines and H. sonchophila in the H. schachtii-group. The only known host plant of this obligate parasite is the important crop alfalfa or lucerne, Medicago sativa.

Description

The female alfalfa cyst nematode is white and shaped like a lemon with a body length of 0.4 to 1.2 mm and a width of 0.2 to 0.9 mm. The male is vermiform

Worm

The term worm refers to an obsolete taxon used by Carolus Linnaeus and Jean-Baptiste Lamarck for all non-arthropod invertebrate animals, and stems from the Old English word wyrm. Currently it is used to describe many different distantly-related animals that typically have a long cylindrical...

 with a body length of 1.2 to 1.4 mm and a width of about 0.028 mm. The eggs

Egg (biology)

An egg is an organic vessel in which an embryo first begins to develop. In most birds, reptiles, insects, molluscs, fish, and monotremes, an egg is the zygote, resulting from fertilization of the ovum, which is expelled from the body and permitted to develop outside the body until the developing...

 are reniform

Leaf shape

In botany, leaf shape is characterised with the following terms :* Acicular : Slender and pointed, needle-like* Acuminate : Tapering to a long point...

, measuring 0.12 by 0.05 mm and the first two instar

Instar

An instar is a developmental stage of arthropods, such as insects, between each molt , until sexual maturity is reached. Arthropods must shed the exoskeleton in order to grow or assume a new form. Differences between instars can often be seen in altered body proportions, colors, patterns, or...

 larva

Larva

A larva is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle...

e are vermiform. The second instar larvae are mobile in damp soil and search down to a depth of 30 centimetres for suitable roots to invade. Here they develop further and become sedentary, burying their heads in cells and feeding on the cell sap. The central part of their bodies swells, and after moulting they develop into bottle-shaped third instar larvae and after moulting again, lemon-shaped fourth instar larvae. The swollen larvae break out of the root cortex. At this stage they differentiate into males and females. The males become mobile and migrate through the soil searching for females with which to mate. The females remain attached to the root tissues by their heads, continue feeding and start laying eggs. These are held within the bodies of the females which later die and turn into brown cysts with hard cuticles. These eventually become detached from the roots. The eggs develop into larvae inside the cyst and some second instar larvae emerge into the soil to start a new cycle of development. Most of the larvae lie in a state of anabiosis

Dormancy

Dormancy is a period in an organism's life cycle when growth, development, and physical activity are temporarily stopped. This minimizes metabolic activity and therefore helps an organism to conserve energy. Dormancy tends to be closely associated with environmental conditions...

within the cysts where they can remain dormant for several years and still be viable.

Economic significance

The alfalfa cyst nematode causes a decrease in yield of herbage and seeds. The main criterion is the population density of larvae in spring. A 5-10% threshold of harmfulness occurs when there are 330 to 890 larvae per 100 cubic centimetres of soil. Control measures preventing yield losses include crop rotation and the use of tolerant varieties. Other factors affecting the harm done include the date of sowing, the density of seeding, the use of cover crops and the weather.