ECTOPARASITIC NEMATODES

This group includes stubby root (Paratrichodorus and Trichodorus), dagger (Xiphinema), needle (Longidorus, Paralongidorus), ring (Criconemella, Macroposthhonia), stunt (Tylenchorhynchus and Merlinius), pin (Paratylenchus), and spiral (Helicotylenchus, Rotylenchus, and Scutellonema) nematodes. These genera share a migratory ectoparasitic habit. They do not enter into host root tissue but feed externally by inserting their mouth stylets into root cells from the root surface. Some species of sheath and spiral nematodes will insert the head region into the outer cell layers of the root during feeding and will retract the head and stylet when feeding is completed. The stubby root nematode fastens its lips to the root surface, pierces the cell, and then enzymatically creates an opening from which it can draw out the contents. Ectoparasites tend to congregate in the soil rhizosphere (soil on and around roots) where they browse along roots and feed for awhile, before moving on to a new feeding site. Some species (e.g. ring nematode) may remain feeding at one site for a prolonged period and are referred to as sedentary ectoparasites. The region of root elongation and root cell division behind the root tip, as well as the root tip itself, are commonly preferred feeding sites. Stubby root and needle nematodes particularly feed on the root tip area. Sites of lateral root junctions also are favorite feeding sites for some of these nematodes. The first molt occurs in the egg. The second stage juvenile hatches from the egg under appropriate moisture and temperature conditions, and moves toward roots for feeding.(Stubby root nematode is an exception with the first stage juvenile hatching from the egg.) As feeding continues, the juvenile molts to the third stage and fourth stage, before molting a fourth time to become the male or female adult. Since each of these stages is vermiform, the nematode can move freely in soil to new feeding sites on the same or new roots, or can locate a sexual partner. Some species reproduce parthogenetically (without male fertilization), such as Paratrichodorus minor , which is the most common of the stubby root species in California, and in which males are rarely found. Eggs are laid singly in the soil around roots. Depending on the species, a single adult plant parasitic female may lay up to 2,800 eggs. After a period of embryogenic development, which may last from a few days to several weeks, the eggs are ready to hatch. Eggs provide an overwintering stage for many nematode species. Thus, a proportion of eggs laid in autumn may not hatch until the soil temperature increases in spring. The fourth stage of certain genera, including Paratylenchus, also shows some resistance to unfavorable environmental conditons.

From: McKenry, M.V. and P. A. Roberts. 1985. Phytonematology Study Guide. University of California, Division of Agriculture and Natural Resources.

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