Rev 02/13/2023

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           Gracilacus Raski, 1962


The name is derived from the Latin gracilis (slender) and acus (pin).

The genus Gracilacus Raski, 1962 was distinguished from Paratylenchus by the following characters:

  1. female stylet greater than 48 um.
  2. swollen females in some species,
  3. most juveniles with elongate stylets
  4. excretory pore in region of metacorpus although opposite isthmus in some species.

 Gracilacus is distinguished from the genus Cacopaurus Thorne, 1943 by:

  1. the elongation of the body posterior to the vulva, compared to the very short and blunt tail in Cacopaurus.
  2. the finely annulated cuticle without ornamentation.

(Raski, 1976)

Raski (1962) established the genus Gracilacus essentially to differentiate those Paratylenchus species with a stylet longer than 48 μm. Several nematologists have considered Gracilacus as a valid genus, others have not agreed and have synonymized Gracilacus with Paratylenchus (Geraert, 1965; Siddiqi, 2000). Recent studies on the ITS rRNA gene showed that phylogenetic relationships of putative species included in Gracilacus are not resolved, and the validity of this genus is in question (Esmaeli et al., 2016).

Authorities remain split on the validity of Gracilacus. Some retain it as a valid genus (e.g. Andrassy, 2007) while others liist the species formerly assigned to this taxon as Paratylenchus (e.g. Nguyen et al., 2004, Ghaderi et al., 2014) (Ghaderi et al., 2016).,

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Morphology and Anatomy:


Female:  Small, under 0.5 mm, vermiform or swollen in prevulval region.

Labial framework weakly sclerotized.

Stylet long (41 to 199 um - approximately 1.5 to 3 times that of Paratylenchus) and flexible.

Excretory pore from level of base of stylet to level of nerve ring.

Annuli smooth, more rarely (three species) with small tubercles.

Females monovarial.


Juvenile: Resembling female.  

Stylet generally present, well developed.

Male: When males are present they lack a stylet or the stylet is much reduced.


Caudal alae absent or comprised of thickened evaginations of cuticle.

[Ref: Raski (1962), Raski & Luc (1987), and H. Ferris.]  

Body size range for the species of this genus in the database - Click:

Key to species of Gracilacus (slightly modified from from Raski, 1962)


1.  Lateral field 2 or 3 lines

     Lateral field 4 lines

go to 2

go to 5

2.  Lateral field 2 lines, stylet 110-119 um

     Lateral field 3 lines, stylet 54-88 um

G. elegans

go to 3

3.  Female stylet 87-88 um, small vulva flap

    Female stylet <69 um, no vulva flap

G. idalima

go to 4

4.  Female stylet 54-62 um, tail broad

     Female stylet 61-69 um, tail slender

G. aculenta

G. acicula

5.  Spermatheca absent, males unknown

     Spermatheca present

go to 6

go to 7

6.  Stylet average length 89 um (83-94 um), swollen to obese females common

    Stylet average length 77 um (70-85 um), females slender

G. intermedia


G. mira

7.  Female stylet 85-119 um

    Female stylet 48-71 um

go to 8

go to 10

8.  Lips offset from body contour

    Lips rounded

G. peratica

go to 9

9.  Tail elongate and conoid, males unknown

    Tail short, blunt, males common

G. anceps

G. epacris

10.  Female ave. length 0.48 mm, male with stylet

    Female ave length 0.39 mm, male without stylet or male unknown

G. goodeyi

go to 11

11.  Tail claw-like, female ave. length 0.34 mm

    Tail conocal with rounded tip

G. audriellus

go to 12

12.  Female stylet average length 53 um (48-61 um), male without stylet

   Female stylet average length 67 um (63-71 um), male unknown

G. sarissus

go to 13

13.  Female ave. length 0.28 mm (0.24-0.31 mm), excretory pore near base of retracted stylet

    Female ave. length 0.39 mm (0.34-0.42 mm), excretory pore near nerve ring

G. steineri

G. marylandica

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Economic Importance:

D-rated pest in California Nematode Pest Rating System.

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First described by Allen and Jensen from black walnut in Tulare County, California..  

Based on stylet length, probably feeding on deeper cell layers than Paratylenchus.      

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Walnut, grape, oak, laurel.   

For an extensive host range list for this genus, click
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Life Cycle:

For Ecophysiological Parameters for this genus, click 

Fourth stage juveniles survive for long periods; 42 months in plastic bags, 80 days in saturated soil. Cuticle of J4 less permeable than in adults (Kondo and Ishibashi, 1979). 

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Andrassy, I. 2007. Free-living Nematodes O Hungary. Vol 2. Hungarian Natural Histroy Museum.

Brown G.L. 1959.  Three new species of the genus Paratylenchus from Canada (Nematoda: Criconematidae). Proc. Helminth Soc. Wahington 26:1-8.

Esmaeili, M., Ramin Heydari, Pablo Castillo, Mozhgan Ziaie Bidhendiand Juan E. Palomares-Rius. 2015. Molecular characterisation of two known species of Paratylenchus Micoletzky, 1922 from Iran with notes on the validity of Paratylenchus audriellus Brown, 1959. Nematology 18:591-624. .

Geraert, E. (1965). The genus Paratylenchus. Nematologica 11, 301-334.

Ghaderi, R., Kashi, L., Karegar, A. 2014. Contribution to the study of the genus Paratylenchus Micoletzky, 1922 sensu lato (Nematoda, Tylenvnchuldae) Zootaxa 3841:151-187.

Ghaderi, R., Geraert, E., Karegar, A. 2016. TheTylenchulidae of the World. Academia Press, Ghent, Belgium

Raski, D.J. and Luc, M. 1987. A reapraisal of Tylenchina (Nemata) 10. The superfamily Criconematidea Taylor, 1936., Rev. Nematol. 10: 409-444..

Raski, D.J. 1976. Revision of the Genus Paratylenchus Micoletzky, 1922 and Descriptions of New Species. Part III of Three parts--Gracilacus. J. Nematology 8:97-115.

Kondo, E and N. Ishibashi. 1979. Ultrastructural changes associated with development of pin nematode, Gracilacus sp., with special reference to its survival.  Appl. Ent. Zool. 14:1-11.

H. Ferris.

Raski, D.J. 1962.  Paratylenchidae n.fam. with description of five new species of Gracilacus n.g. and an emendation of Cacopaurus Thorne, 1943, Paratylenchus Micoletzky, 1922 and Criconematidae Thorne, 1943.  Proc. Helminthol. Soc. Washington 29:189-207.

Siddiqi. M.R. (2000). Tylenchida parasites of plants and insects, 2nd edition.Wallingford, UK, CABI Publishing.


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Copyright © 1999 by Howard Ferris.
Revised: February 13, 2023.