Geocenamus brevidens




Rev 04/03/2023

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Geocenamus brevidens (Allen, 1955) Siddiqi, 1970

Transferred from Merlinius

 Synonyms:      Tylenchorhynchus brevidens Allen, 1955

                        Geocenamus brevidens Brzeski

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Morphology and Anatomy:

  • Female: Body straight to ventrally arcuate, length 0.54-0.69 mm.

  • Annules about 1.1 µm wide near middle, lacking longitudinal striations.  

  • Lateral fields with 6 smooth incisures, about one-third as wide as body, not areolated.  

  • Deirids present near or at level of excretory pore; latter just behind poorly developed hemizonid, situated between nerve ring and basal esophageal bulb.  

  • Lip region broadly rounded, continuous with body contour.  Lip region has 5 or 6 annules; labial framework lightly sclerotized. 

  • Stylet moderately developed; basal knobs rounded, backwardly sloping.  Orifice of dorsal esphageal gland 2 to 5 µm behind spear base.  

  • Median esophageal bulb well developed, oval, with distinct refractive valve in center.  

  • Esophageal base not extending over intestine; esophago-intestinal valve (=cardia) conoid-rounded.  

  • Vulva a transverse slit, about one-third body-width long.  

  • Spermathecae rounded, well formed and with sperms only in impregnated females, which are not common in a population. 

  • Oocytes in one line except for a few in germinal zone of ovary.

  • Intestine not overlapping rectum or extending into tail cavity.  

  • Tail sub-cylindrical, finely annulated, with unstriated, round to flattened terminus.

  • Phasmids minute, usually just behind middle of tail.


  • Male: Uncommon, probably not essential for reproduction. 

  • Length, male: 0.56-0.64 mm

  • Bursa finely crenate, extending to end of tail.  

  • Hypoptygma distinct.  

  • Spicules slightly cephalated and ventrally arcuate, with distinctly notched distal ends.

  • Gubernaculum in ventral view disc-like, 4.5 µm wide, not protrusible.


Reported median body size for this species (Length mm; width micrometers; weight micrograms) - Click:

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U.S., Australia, Europe, and South America.

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Economic Importance:

D-rated pest in California Nematode Pest Rating System.


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Merlinius brevidens is an ectoparasite on root hairs and epidermal cells; average feeding time is 11 min. 

The nematodes appear to be indiscriminate in their choice of feeding sites, and move rapidly from one site to another.

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Grasses, cereals, garlic, alfalfa, potato, peas, pears - Alfalfa in the Davis area and elsewhere in Northern California.

For an extensive host range list for this species, click


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Life Cycle:

Ecophysiological Parameters:

For Ecophysiological Parameters for this species, click If species level data are not available, click for genus level parameters

Male uncommon, probably not essential for reproduction.   

Bridge (1970) observed that a female laid 11 eggs over a period of 120 hours while feeding, and the newly hatched juveniles fed on the same root as their parent. 

Wet soils shorten survival of M. brevidens; nematode can survive for 1 month in wet soil; 9 months in dry.


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Langdon et al. (1961) found that M. brevidens was associated with stunt symptoms in wheat and barley in Oklahoma.  

Varo Alcala et al. (1970) reported that this nematode causes increased cellulose thickenings of the external cell walls of the root hypodermis of wheat and barley in Spain.

Nematode feeding may increase susceptibility of barley and wheat to invasion by Olpidium fungus (Langdon et al., 1961).

Yields of no-till annual spring wheat (Triticum aestivum) in eastern Oregon were inversely correlated with population levels of Merlinius (Geocenamus) brevidens. Soil treatment with Aldicarb improved grain yields (Smiley et al., 2006).

This species occures in more than 70% of grain-growing soils in Australia�s northern grain region, and population densities as high as 70 nematodes/g soil have been recorded, there has been no attempt to determine whether it causes yield losses in Australia (Stirling, 2023).


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Soil fumigation with ethylene dibromide (EDB) decreased the populations of M. brevidens in Californian pear orchards, but there was no evidence to indicate a correlation between the population density of the nematode and pear vigor (Lownsbery et al., 1964).


Host Plant Resistance, Non-hosts and Crop Rotation alternatives:

For plants reported to have some level of resistance to this species, click


Additional Information and Resources

Australasian Plant Pathology Society Factsheets on Plant-parasitic Nematodes (Prepared by Dr. Graham R. Stirling)

(Use your Return Key or click the Index Tab to return to this Nemaplex page)


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CIH Descriptions of Plant-parasitic Nematodes, Set 1, No. 8 (1972)

Smiley RW, Whittaker RG, Gourlie JA, Easley SA. 2006. Geocenamus brevidens associated with reduced yield of no-till annual spring wheat in Oregon. Plant Disease 90:885-890.

Stirling, G.R. 2023. Ectoparasitic plant-parasitic nematodes known to cause crop damage in Australia. Factsheet PSN034,

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Copyright © 1999 by Howard Ferris.
Revised: April 03, 2023.