Steinernema riobrave

H. E. Cabanillas, G. O. Poinar, J. R. Raulston 1994


Horn-like structures on labial region of exsheathed infective juvenile of Steinernema riobrave

Males, first generation:

Generalmorphology (FIG.1), similar to female. With a single reflexed testis. The spicules are paired, sickle-shaped, with a distinct dark golden coloration. Spicule head elongated tapering anteriorly (FIG1.SEM). Shaft present but short. Lamina moderately curved with two internal ribs tapering gradually to a bluntly pointed tip. Calornus/larnina angle averaging 100 degrees (range 90-100). Velum present, gubernaculum 0.7 times as long as the spicules, boat shaped in lateral view and ventrally curved with a proximal knob or hook. In ventral view, the gubernaculum neck has almost the same width as posterior corpus. Cuneus Y-shaped or needle-shaped, long, pointed posteriorly. Twenty five genital papillae (twelve pairs and a single ventral preanal) were observed under scanning electron microscope. (FIG2.SEM) Seven pairs are consistently found anterior to cloaca, subventral in position, pair eight is lateral. A single ventral papilla present consistently just anterior to cloaca. Occasionally, there is a small additional papilla in lateral position, near mid-body. Pairs nine, ten, eleven and twelve are postanal. Pair nine is subventral; pairs ten and eleven are ventral and subterminal. and pair twelve is subdorsal. The ventral portion of the tail is usually straight. Anterior flap of cloaca present. Mucron in tail tip absent.

 Measurements (n=10): Length=1,700 um (1,500-1900); Width=133 um (116-160); EP=103 um (94-111); NR=115 um (106-134); ES=144 um (128-154); testis flexure=226 um (185-257); tail=31 um (29-35); ABW=59 um (50-64); mucron= absent; spicule length=67 um (63-75); spicule width=12.4 um (11.2-13.7); gubernaculum length=51 um (47.5-56.2);gubernaculum width=8.1 um (7.1-8.7); D=0.71 (0.6-0.8)

Abbreviation: um=micrometer; EP=distance from anterior end to excretory pore; NR=distance from anterior end to nerve ring; ES=esophagus length; ABW=anal body width; D=distance from anterior end divided by esophagus length.

Males, second generation: Similar to but smaller than first generation males.

Females, first generation:

Cuticle smooth, head rounded, continuous with body (FIG.2). Six distinct lips each with one papilla . Four cephalic papillae. Amphids distinct, located behind lateral cephalic papillae (FIG2.SEM). Stoma partially collapsed. Esophageal collar absent, esophagus extending to near mouth opening. Cheilorhabdions represented by a thickening sclerotized structure just beneath the lips. Below this, there is another sclerotized ring that represents the prorhabdions. Meso-, meta and telorhabdions are vestigial and would occur in the collapsed area of the stoma .

Esophagus muscular with a cylindrical procorporal area followed but a slightly swollen non-valvated metacorpus, a narrow isthmus, and a basal bulb with a valve. Nerve ring usually surrounding isthmus. Excretory pore opening usually anterior to nerve ring, but its location variable. Lateral fields and phasmids inconspicuous. Gonads amphidelphic, reflexed. Vulva a transverse slit, usually protruding slightly from the body surface. The vagina short leading into paired uteri. Eggs deposited initially, but later hatching inside the females and the juveniles boring their way out. Tail with rounded projection terminally. Pigmy form occurred in some instances.

Measurements (n=10): Length=6,500 (3,700-8300) um (micrometers), Width=275 um (200-390), stoma length=5.7 um (4.3-6.3), stoma width=8.0 um (7.1-8.8), EP=96 um (80-118), NR=147 um (131-168), ES=193 um (171-211), tail length=45 um (41-50), ABW=93 um (63-115), V%=52 (49-56), D=0.49 (0.42-0.62).

 Abbreviations: um=micrometer; EP=distance from anterior end to excretory pore; NR=distance from anterior end to nerve ring; ES=esophagus length; ABW=anal body width.D=distance from anterior end divided by esophagus length.

Females, second generation: Similar but smaller than first generation females. Tailstraight and pointed, with a prominent postanal swelling.

 Infective juveniles:

Body narrower than the corresponding parasitic juvenile. Labial region with 2 hoen-like structures (FIG3.SEM). Mouth and anus closed and esophagus degenerate. Tail pointed, usually curved ventrally when relaxed forming an angle 110 degrees with the body. The highest number ofincisures in lateral field is eight, the formula of lateral field is 2, 7, 8, 6, 2 (FIG4.SEM). The length of the infective stage (n=25) was 622 um (561-701), the greatest width, 28 um (micrometers) (26-30); distance from the head to the excretory pore, 56 um (51-64); distance from the head to the nerve ring 87um (84-89); distance from the head tobase of the esophagus 113 um (I 09-116), length of the tail, 53.5 um (46-59); width at anus, 16 um (I 5-16.5).

Measurements: (n=20) Body length=622 um (561-701); body width=28 um (26-30); EP=56 um (51-64); NR=87 um (84-89); ES=114 um (109-116); tail=54 um (46-59); ABW=16 um (15-17);
a=22.5 (20.1-23.5); b=5.4 (4.9-6.0); c=11.6 (10.1-12.4); D%=49 (45-55); E%=105 (93-111).


The natural host currently unknown, S. riobrave was recovered from corn earworms, HelicopervaHeliothis) zea (Boddie) (Lepidoptera: Noctuidae) that had been placed in soil as trap insects in a corn field at the United States Department of Agriculture South Farm, in the lower Rio Grande Valley near Weslaco, Texas, USA. The location where the soil samples were taken was recorded by GPS (Global Positioning System) receivers. The Global Positioning System coordinates were latitude 26 degrees 08.155'N, longitude 97 degrees 57.366'W, and altitude 21.7 m above mean sea level (Cabanillas et al, 1994).


Steinernema riobrave has been only recorded from the United States. This nematode has been found parasitizing prepupae and pupae of Helicoperva zea and Spodoptera frugiperda (Raulstonet al, 1992). The soil type where this nematode was isolated was a Hidalgo sandy clay loam (47.9% sand, 3 5.6% clay, 16.5% silt, 1. 1 % organic mater, pH 8.3, 3 9.95 meq/100 g CEC. Daytime soil temperatures at 5 cm deep during corn growing season were about 35 degrees C.


The life cycle of S. riobrave is comparable to that of other species of Steinernema. It include the egg, four juvenile stages (separated by molts) and the adult. The third- stage infective juveniles enter the hemocoel of the insects deliver their associated bacteria, complete usually two generations, and then emerge from the insect cadaver as infective juveniles. Infective juveniles transferred to insect blood drops (G. mellonella and H. zea) reached the adult stage in 48h and produced eggs in 72 h at 24 degrees C. In Petri dish assays, S. riobrave reached the preadult and/or adult stages in 48h in fall armyworm larvae (9 day- old) exposed to infective juveniles at 29.5 degrees C (Cabanillas et al., 1994). This indicates that the development of this nematode is markedly influenced by temperature. Experiments in type locality showed that this nematode can move downward and upward throughout soil profile and survive well in the dry and hot (35 degrees) environment such as in Texas, USA.


The mutualistic bacteria associated with S. riobrave was isolated on nutrient agar and produced a brownish crearn color. The primary form of the bacterium was characterized by its adsorption of bromothymol blue from NTBA (nutrient bromothymol blue-triphenyl tetrazolium chloride agar), and adsorption of neutral red from Mac Conkey agar (red colonies). These characteristics do not differ from the description of Xenorhabdus nematophilus, but further studies need to be conducted (Cabanillas et al., 1994).


The efficacy of S. riobrave to control corn earworm ( Helicoperva zea) prepupae has been evaluated in greenhouse and field experiments. It was shown that the efficacy of S. riobrave to control corn earworm prepupae was influenced by the nematode concentration and placement method under greenhouse conditions (Cabanillas & Raulston, 1995). These studies indicated that S. riobrave is effective for suppressing corn earworm populations, under field conditions of high soil temperature with irrigation and proper timing (Cabanillas &,Raulston, 1995, 1996). Recently, this nematode has been commercialized to control different insects in the United States.


Cabanillas H. E., G. O. Poinar, and J. R. Raulston 1994. Steinernema riobravis n. sp. (Rhabditida: Steinernematidae) from Texas. Fundamental and Applied Nematology 17:123-131.

This document was constructed and is maintained by KHUONG B. NGUYEN
Entomology & Nematology Department
University of Florida