Males, first generation: Body curved posteriorly when relaxed. First generation male larger than second generation male. Cuticle smooth under light microscopy, but with fine transverse striae under SEM. Lateral field and phasmid inconspicuous. Head truncated to slightly round, continuous with the body. Six lips fused, tips distinct, with one labial papilla each. Four cephalic papillae. Amphids small, posterior to lateral labial papillae. Stoma small with inconspicuous sclerotized walls. Procorpus cylindrical metacorpus slightly swollen, non-valvated; indistinct isthmus followed by pyriform basal bulb containing reduced valve. Nerve-ring usually surrounding isthmus or anterior part of basal bulb. Excretory pore opening circular, located in posterior 1/3 of metacorpus. Testis single, reflexed. Spicules paired  Fig. 1 , symmetrical, curved, with orange-brown in color. Manubrium romboid. Lamina with rostrum and 2 internal ribs. Velum present. Gubernaculum large and slender, 3/4 length of spicules, arcuate, distally forked in ventral or dorsal view. Tail conoid and mucronated. . There are 23 genital papillae arranged as follows: 6 precloacal subventral pairs, one precloacal lateral pair; one single precloacal papilla; one pair postcloacal subventral; one pair postcloacal subdorsal, 2 subdorsal pairs near tail tip.

Males, second generation: Similar to first generation males but smaller in size. Second generation males with mucron longer than first generation

Measurements: (all measurements are in micrometers)

Male, first generation (n = 20). Total length 1,035-1,278 (1,135). Greatest width 107-159 (139.5). Stoma length 3-4 (3.5), width 3-5 (4). Distance from anterior end to excretory pore 47.5-67 (5 7), to nerve ring 81-101 (91), to esophagus base 123-141 (134). Testis reflexion 296-355 (341). Tail length 22-32 (27.5). Mucron length 2.5-3.5 (3). Body width at cloaca 37-54 (45). Spicule length 75-80 (77.5), width 7.5-9 (8). Gubernaculum length 47-65 (53.5), width 4-7 (5). SW (spicule length divided by body width at cloaca) 1.4-2.2 (1.7). GS (gubernaculum length divided by spicule length) 0.6-0.8 (0.7). D% (ratio D) 0.35-0.49 (0.42). E% (ratio E) 1.66-2.57 (2.07).

Male, second generation (n = 20). Total length 716-952 (830). Greatest width 47-75 (57). Stoma length 7-10 (8.5), width 5-8 (6). Distance from anterior end to excretory pore 54-68 (58), to nerve-ring 80-91 (85), to esophagus base 108.5-121 (112.5). Testis reflexion 90-120 (110). Tail length 19-23 (22). Mucron length 2-4 (3). Body width at cloaca 25-31 (29). Spicule length 55-73 (62), width 5-6.5 (5.5). Gubernaculum length 37-55 (41.5), width 3-4.5 (3.5). SW 0.97-1.17 (1.08). GS 0.66-0.75 (0.69). D% 49-56 (5 1). E% 26-29 (27).

Females, first generation: Cuticle, lip region, stoma and esophageal region as in male. Body C-shaped when heat-killed. First generation females larger than second generation females. Ovaries opposed, reflexed in dorsal position; oviduct well developed; glandular spermatheca and uterus in ventral position. Vagina short, with muscular walls. Vulva located near mid-body. Vulval lips protruding,  double-flapped epiptygma present . Tail conoid, blunt, with mucron  without post-anal swelling.

Females, second generation:
Similar to first generation females but smaller. Post-anal swelling present .

Measurements:

Female, first generation (n = 20). Total length 3,161-5,172 (3,937). Greatest width 170-280 (198). Stoma length 6-10 (8), width 7.5-12 (9.5). Distance from anterior end to excretory pore 50-84 (66), to nerve-ring 116-147 (133), to esophagus base 152-199 (177). V% (distance from anterior end to vulva as percentage of length) 48-53 (51). Tail length 22-37.5 (31). Mucron length 4.5-11 (7). Body width at anus 48-53(51).

Female, second generation (n = 20). Total length 1,410-2,560 (1,836). Greatest width 83-128 (105). Stoma length 6-8 (7), width 7-10 (8). Distance from anterior end to excretory pore 65-70.5 (68), to nerve ring 113-130 (123), to esophagus base 141-169 (151). V% 53-58 (56). Tail length 28-52 (41). Mucron length 3-6 (4.5). Body width at anus 30-49 (38).
 
 

Third-stage infective juvenile:Body slender, tapering regularly from base of esophagus to anterior end and from anus to terminus. Cuticle with transverse striae; lateral field distinct with 6 or 8 longitudinal ridges in mid-body region. Labial region smooth; mouth closed. Esophagus long, narrow. Nerve ring located at level of isthmus. Excretory pore located in anterior 1/3 of esophagus. Basal bulb valvate. Cardia present. Anterior portion of intestine with dorsally displaced pouch containing symbiotic bacterium. Lumen of intestine narrow; rectum long; anus distinct. Genital primordium evident. Tail conoid with pointed terminus.

Measurement:

Infective juveniles (third-stage juvenile (n = 20). Length 398-495 (446). Greatest width 18-24 (21). Distance from anterior end to excretory pore 29-38 (35), to nerve-ring 68-80 (72), to esophagus base 80107 (94.5). Tail length 31-41 (35.5). Body width at anus 9-15.5 (11.5). Ratio a 19-23(21). Ratio b 4.06.1 (4.7). Ratio c 10.3-14.8 (11.3). D% 31-43 (37). E% 85-112 (96).

Type-host: Unknown in nature, from bait-insect Galleria mellonella (L.) in sandy-clay loam soil under sweet tamarind Tamarindus indicus L.

Type locality: Lohmsak district (16.4 degree N, 101.2 degree E), Petchabun Province, Thailand.

Type-specimens: Holotype (male), allotype (female first generation), paratypes and infective juveniles are deposited in the University of California Nematode Collection, Davis, California, USA and paratypes in the Division of Entomology and Zoology, Department of Agriculture, Bangkok. Thailand.

Etymology: The specific epithet derives from Siam, the old designation of the country of origin and kayai after Harry K. Kaya, a leading scientist in insect pathology and biocontrol.

Diagnosis

Steinernema siamkayai is characterized by the following combination of morphological features: third-stage infective juvenile with a body length 445 (398-495) um, tail short 35.5 (31-41) um, and the lateral field with 6-8 longitudinal ridges; tail of both first and second generation adults with a cuticular mucron; males with long spicules, 77.5 and 73 um in first and second generation respectively; gubernaculum large and slender, about 3/4 of the length of the spicule; 23 genital papillae; first generation females with protruding vulval lips; second generation females without protruding vulval lips; first and second generation females with a double flapped epiptygma; first and second generation females tail conoid with a cuticular mucro of 7 and 4.5 um, respectively; first generation females without post-anal swelling; and second generation females with a post-anal swelling. The RFLP banding pattern is unique when compared to other Steinernema spp..

The life cycle of Steinernema siamkayai  is similar to other Steinernema spp.
 

Relationship

Steinernema siamkayai  is most similar to S. rarum (De Doucet) S. carpocapsae (Weiser) and S. monticolum Stock, Choo & Kaya in the general morphology of the infective juveniles, males and females, but it can be separated from these species by a combination of morphological, DNA and hybridization techniques. The average length of the infective juveniles of S. siamkayai  (446 um) is the shortest of all described Steinernema spp. The third-stage infective juveniles of S. siamkayai differ from S. carpocapsae by the tail length (31-41 um vs 46-61 um), by the D% (31-43 vs 25-28) and E% (85-112 vs 54-66) . The infective juveniles of S. siamkayi can be separated from S. rarum by the tail length (31-41 vs 44-56 um); value of ratio c (10.3-14.8 vs 8.7-11) and E% (85-112 vs 63-80). The juveniles of S. siamkayai can be separated from S. monticolum by the location of the excretory pore (29-38 urn vs 54-62 um); the tail length (31-41 vs 44-50 um); the value of D% 31-43 vs 44-50 and E% (85-112 vs. 63-85).

Males of the new species can be separated from S. carpocapsae, S. rarum and S. monticolum by the shape and size of the spicules and gubernaculum.

Reference

Stock, S. P. , V. Somsook & A Reid 1998. Steinernemasiamkayai n. sp. (Rhabditida: Steinernematidae), an entomopathogenic nematode from Thailand. Systematic Parasitology 41:105-113.