Life Cycle Strategies

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Migratory ectoparasites are motile nematodes, which feed on the external surface cells (epidermis) of roots (Fig. 1). This is the most common type of life cycle strategy among all plant-parasitic nematodes and is represented in mint by pin, ring, mint and stubby-root nematodes. Eggs are laid only in the soil and develop until a first stage juvenile nematode is present. This nematode molts for the first time while still within the egg and it is the second stage juvenile which emerges from the egg into the soil. Moving through the soil, this nematode finds a root and inserts its stylet into one of the epidermal cells on the root surface. After feeding for some period of time, the nematode withdraws its stylet and moves on to feed in a new location. When the nematode has grown to a certain size, it molts a second time to become a third stage juvenile and later a third time to become a fourth stage juvenile. After a fourth and final molt, the nematode becomes an adult. By convention, a juvenile stage is denoted by the capital letter J and the number of the stage. Thus, a second stage juvenile is referred to as a J2. Generally, all stages of migratory ectoparasites from J2 to adult are motile and actively feed on plant roots.

Figure 1. Life cycle of pin nematode, a typical migratory ectoparasite.

Migratory endoparasites (Fig. 2) are motile nematodes, which may feed on external surfaces of roots but generally burrow into the root to feed on internal root cells (cortex). This tunneling creates considerable damage as the nematodes migrate from one feeding location to another. Root-lesion nematodes found in mint are migratory endoparasites. While some eggs are laid in the soil, most are laid inside roots. Eggs may hatch inside the root or remain until the root decays and the eggs are released into the soil. As the nematode develops in the egg, it molts to change from a first stage juvenile to a second stage juvenile (J2) which then hatches from the egg. The nematode grows and molts three more times to become a mature male or female. Males are very rare in some species, and females of these species do not need to be fertilized by males to lay eggs. All stages of migratory endoparasites are infective to roots, feeding and migrating within the cortex. Nematodes may leave the root to infect other roots at any time during the growing season. These nematodes overwinter in roots or soil as eggs, juveniles or adults. During periods of drought, lesion nematodes may remain quiescent until moisture increases and plants resume growth.

Figure 2. Life cycle of root-lesion nematode, a typical migratory endoparasite.

Sedentary endoparasites (Fig. 3) are nematodes, which invade root tissues soon after hatching and then establish a permanent, stationary feeding location. Females do not move from that site for the rest of their life. Northern root-knot nematodes is a typical sedentary endoparasite which may be found in mint. Eggs overwinter in the soil and in live or decaying root tissues. As the nematode develops in the egg, it molts to change from a first stage juvenile to a second stage juvenile (J2) which then hatches from the egg. Unlike other life cycle strategies, the J2 is the only infective stage and burrows into the root, usually at or near the root tip. The J2 migrates through the cortex of the root until it selects an appropriate feeding site, usually just behind the root tip. The nematode begins feeding and becomes stationary. During feeding, the nematode releases enzymes and plant growth hormones into the root This causes changes in the root's physiology, and "giant cells" form around the nematode’s head. Generally, 5-7 giant cells develop and the nematode moves its head slightly to feed on these different cells. The nematode begins to grow in thickness, becoming "sausage-shaped," and molts again to become a third stage juvenile. After further development, the nematode molts a third time to become a fourth stage juvenile, which may be distinguished as male or female. Adult males become "worm-shaped" again, developing within the fourth stage cuticle and resembling a large juvenile within an egg. The developing male molts, emerges from the fourth stage cuticle and leaves the root to search for a female. After molting a fourth time to become adults, females continue to swell and become "pear-shaped" with their posterior end just beneath the root surface. Although root tissues enlarge to form a gall or "root-knot" around the nematode, the root often ruptures as the female enlarges faster than the gall growing around it. In addition to causing substantial root damage, this rupture allows the male access to the female for fertilization and permits the female to lay eggs into the soil. Males are more common in northern root-knot than for many other species of root-knot nematode in which the female does not need to be fertilized by the male to lay eggs. In some species males do not exist. Over 300 eggs are laid into an egg sac which may be inside or outside the root tissues depending on the position of the female. The egg sac consists of a jelly-like substance that protects the eggs from environmental stress. Eggs may hatch within a few days or remain until the following spring.

Figure 3. Life cycle of northern root-knot nematode, a typical sedentary endoparasite.