Nematode Pests of Horticultural Crops and their Management (1+1)

Moulting/ Ecdysis

Moulting/ Ecdysis:

• Moulting is the most remarkable feature of post embryonic development. Periods of growth are separated by moults in which the cuticle is shed.
• Nematodes undergo four moults to attain adulthood. At each moult the old cuticle is shed off and is replaced by the new one.
• Moulting in nematodes is controlled by certain enzymes and hormones.
• Once the moulting is to be initiated, the nematode becomes sluggish but its hypodermis becomes thickened and metabolically active with enlargement of its nucleus and nucleoli as it is ready to secrete a new cuticle beneath the old one.
• Once the new cuticle is formed, the old cuticle is shed off.
• While the internal cuticle (cuticular lining of stoma, oesophageal lumen, vagina, rectum and cloaca) is reabsorbed enzymatically, it is epicuticle that is shed off in most of the tylenchids and aphelenchids.
• The cuticle may be shed off either in one piece or its anterior portion is shed off like a cap through which nematode wriggles out.
• Generally the old cuticle starts loosening first in the region of excretory pore followed by dissolution of exocuticle. The cuticular lining of excretory duct, oesophageal lumen and stylet are also renewed at each moult.
• During moulting, the embryo grows in terms of increase in size of the cells and not in number of cells. Plant parasitic nematodes increase 3-10 times in length.

Feeding:

• Being obligate parasites the juvenile on hatching must find a suitable host for feeding.
• Initially the juvenile moves randomly in the soil in search of a suitable host but once it reaches near the vicinity of host plant roots, amphids located in its cephalic region become active.
• The amphidial glands secrete some cuticular receptors which bind with the specific biomolecules (chemo-tactic factors) exuded by the plant roots in the rhizosphere. These coupled biomolecules then diffuse in to the amphidial sensillum which passes the signal to the nervous system.
• Nervous system directs the nematode movement towards the roots of host plant.
• Once in the vicinity of roots, the cephalic papillae which are tacto-receptors help in selection of proper feeding site.
• The gentle probing of the stylet plays a supportive role in selection of feeding site which is preferably the growing tips of the feeder roots.
• Once the feeding site is selected, the juvenile inserts its needle like stylet into the host cell. The enzymatic secretions from dorsal oesophageal glands start flowing through the lumen of the stylet into the cytoplasm of the host cell where extra corporeal digestion takes place.
• At this stage the highly muscular median bulb starts pulsating at a very fast rate. As the muscles contract, the lumen of the median bulb is dilated, thus creating a negative pressure that sucks the semi-digested cell cytoplasm into the nematode stoma.
• As the food passes through the median bulb, the secretions from sub-ventral oesophageal glands mix with the food, which is pushed into the intestine for further digestion, assimilation and absorption.
• Oesophago-intestinal valve (cardia) located at the junction of oesophagus and intestine ensures the unidirectional flow of food.
• As the cell sap of the host cell is exhausted, nematode withdraws its stylet and moves to a new site for further feeding.
• The sedentary endoparasites and semi-endoparasites which do not move after settling at one feeding site ensure uninterrupted flow of nutrients for themselves by inducing certain adaptive cellular changes in the host tissue.

Life Cycle:

• After feeding is over, second and third moults generally occur in quick succession giving rise to third and fourth stage juveniles (J3 andJ4 respectively).
• Generally the juvenile forms resemble their adults except in size and development of reproductive system.
• The size of the nematode increases at every successive moult.
• After fourth and final moult, the adult male or female are formed. Though the precursors of reproductive system start showing in J2, completely developed reproductive system with vulval opening (in female) and spicules, gubernaculum and bursa (in male) appears only in adults.
• The duration of life cycle differs from species to species and is largely influenced by environmental factors like temperature, moisture, soil type, aeration and availability of host plant.
• The normal duration of egg to egg life cycle of most of the plant parasitic tylenchids under sub tropical and tropical conditions is 25-30 days.
• Aphelenchids have a short life cycle of 7-10 days.
• Some dorylaimids complete one life cycle in 1-2 years.

Figure 7.2 Life cycle of a typical plant parasitic nematode Courtesy:

Reproduction:

• Nematodes are basically dioecious, (separate males and females) and amphimictic (sexually reproducing) which need to find their mates for survival.
• The sex attractants (pheromones) released by the females are perceived by the phasmids of the males for mating.
• During mating, the protrusible spicules are inserted into the vagina of females.
• Bursa, the wing like cuticular extension in the tail region of male nematodes supports it to clasp the female during mating.
• Sperms pass through the cloaca into the female genital tract and are stored there in the spermatheca.
• As the oocytes pass through the spermatheca, the sperms fertilize them.
• Additional layers of the eggshell are formed in the uterus before the eggs are laid by females.
• Reproduction by Parthenogenesis (Gk. Parthenos=virgin+ genesis=come in to being) is often observed in nematodes species in which males are either rare or absent. In such cases young ones develop from unfertilized eggs. Parthenogenesis is commonly observed in Meloidogyne, Heterodera etc.
• Hermaphrodites (Herma= male Aphrodite= female) in which gonads are capable of producing both male and female gametes for self fertilization are extremely rare in nematodes.

Oviposition

The oviposition behavior in nematodes varies depending upon their mode of parasitism.

• Most of the soil dwelling ecto-parasites lay eggs singly in soil itself.
• Migratory endo-parasites generally lay the eggs singly inside the plant tissue.
• Sedentary endo-parasites lay their eggs clumped together in a gelatinous substance (egg mass). While rectal glands secrete gelatinous matrix in root knot nematode, in citrus nematode it is the secretion of excretory system.
• In cyst nematodes the eggs are retained within the body of mature female.
• The fecundity (number of eggs laid by a female)also differs in different nematodes. Generally it ranges between 10-30 for ectoparasites and migratory endoparasites and 50-500 in sedentary semi-endoparasites/ endoparasites.