When possible, specimens used to identify species of Steinernema should be reared in vivo (Galleriamellonella or another appropriate host), and adults of the first generation should be dissected from the cadavers. Infective juveniles collected for a week after their frst emergence from cadavers usually meet the criteria of the original descriptions but the body lengths of those collected after that period tend to be significantly shorter. All data may be obtained from either live or fixed specimens. Species of Steinernema can be identified with the following key. But, while using a key, one should keep in mind that keys are only used to narrow decision, the identity should be verified by comparing its morphometrics, morphological characters with the data from original descriptions. Ratios and abbreviations used in the following key are: D% = EP/ES x 100 (EP = distance from anterior end to excretory pore; ES = esophagus length); E% = EP/T x 100 (EP = distance from anterior end to excretory pore; T = tail length); IJ = infective juvenile; SW = spicule length divided by anal body width.

 Key to species of the genus Steinernema

 1-   Infective juvenile (IJ) with a double horn-like structure on labial region .......................... 2
        IJ without a double horn-like structure on labial region ..................................................3
 2-  Body length of IJ 769 (648-873) um, spicule length 65 (53-70) um, SW=2.22
         (2.18-2.26) ......................................................................................... S. bicornutum
     Body length of IJ 706 (691-800) um, spicule length 71 (54-90) um SW=1.4
         (1.0-2.0) ..........................................................................................S. ceratophorum
3-   Body length IJ >800 um  ...............................................................................................4
      Body length of IJ <800 um  .........................................................................................13
4-   Average length of IJ greater than 1,000 um ................................................................... 5
      Average length of IJ less than 1,000 um .........................................................................9
5-   Tail of IJ 67-88 um..................................................................  ................................... 6
     Tail of IJ >90 um .......................................................................................................... 8

6-   D% about 93 (88-102), spicule length 84 (81-91) um, spicule tip ball-shaped
       ...........................................................................................................     S. arenarium
      D% about 70, spicule length about 77 um, spicule tip not ball-shaped .............................7
7-   EP about 82 (76-89) um, E% about 94 (87-100), c about 12.6.........................S. caudatum
      EP about 102 (87-110) um, E% about 131 (122-138), c about 14.7...................... S. glaseri
      EP about 106 (101-114) um, E% about 160, c about 19.2.............................   S. cubanum

8-   E% averaging 101 (88-108); in male D% about 77.…….............…....… S. puertoricense
      E% averaging 85 (range not known); in male D% about 62............……  S. longicaudum
9-   In IJ, EP extremely short, 18 (14-22) um, E%=23; in male, D% averaging 19 (13-25)
       .............................................................................................................S. neocurtillae
      In IJ, EP=53 um or more, E% about 69-110; in male, D%>50.. ..................................10
10-   Male tail with mucron  ..............................................................................................11
        Male tail without mucron ................................................................ ......................... 12
11-   Spicule head as long as wide, spicule length 49 (42-53) um, body
        length averaging 951 (797-1102) um ......................................................... S. kraussei
        Spicule head 1.5-2.0 times as long as wide, spicule length 70 (65-77) um, body
        length averaging 849 (736-950) um  ...........................................S. feltiae (=bibionis)

12-   Body length averaging 980 (820-1110) um; spicule length 71 (65-73) um
         ............................................................................................................ S. oregonense
        Body length averaging 932 (876-982) um; spicule length 83 (73) um
        .......................................................................................................................S. karrii
13-   Average length of IJ >600 (622-693) um ................................................................. 14
        Average length of IJ <600  (510-589) um  ................................................................17

14-   IJ tail length 77 (71-95), E%=76 (63-86), c=9.3 (7.6-11.1); in male D%=55 (49- 61)
       .............................................................................................................S. monticolum
        IJ tail length averaging 62 (maximum 74), E%>94, c>10; in male D%>60................. 15

15-   Spine-like structure inside IJ tail tip; mucron present, small.............................. S. affine
         No spine-like structure inside IJ tail tip; mucron absent  16

16-   Spicule length about 93 (80-106) um; E% about 96 (89-108) ............. S. intermedium
        Spicule length about 67 (63-75) um; E% about 105 (93-111)…..............….S. riobrave

17-   Average body length of IJ  about 510 um or less.......................................................  18
       Average body length of IJ much longer........................................................................ 21
18-  Average body length 446 (398-495) um, tail length 36 (31-41) um
       …………………………………………………...............…..…….……S. siamkayai
      Average body length about 510 um or more, tail length 50 um……...................……….19
19-   First generation male without mucron; spicule length 69 (58-75) um, SW=1.56
       (1.44-1.57); in IJ, E% averaging 88................................................................. S. ritteri
       First generation male with mucron; spicule length 47 (42-52) um, SW = 0.94
       (0.91-1.05); in IJ, E% averaging 72 .................................................................S. rarum

20-   Male tail without mucron ..........................................................................................21
        Male tail with mucron ...............................................................................................22

21-  Body length of IJ averaging 589 (424-662) um, spicule length/ spicule width  about 3.9
      (3.6-4.3)……………….................……..............................……….…….S. kushidai

      Body length of IJ averaging 541 (496-579) um, spicule length/spicule width about 5.1
      (4.7-5.8) ………………………….............................………………….…S. abbasi

22-   Spicule length 83 (72-92) um, spicule shaft prominent, SW ratio averaging 2.52
          (2.04-2.80), female with large, double-flapped epiptygma..................S. scapterisci
        Spicule shorter, female without epiptygma ..............................................................23

23-   Body length about 558 um, spicule length 66 (58-77) um, spicule shaft very short ,
        SW ratio averaging 1.72 (1.40-2.00)………………....………….…S. carpocapsae
        Body length 530 um, spicule length 77(71-84) um, spicule shaft long, SW ratio
        averaging 2.0 ………………………………………………………………S. tami
 

      To identify species of Steinernema and Heterorhabditis, the following  should be considered:
    a) IJ morphometrics usually are insufficient for species identification, and male and female characteristics must be used.
    b) IJ produced on artificial media (laboratory reared or commercial products) are shorter (rarely longer) than those produced in vivo, and usually     do not meet the criteria of the original description. Males and females collected 4 or 5 days after the host dies, and IJ collected for one week after they first appear from cadavers, usually meet original species descriptions.