Thrybius togashii, Kusigemati, Kusigemati, 1982

Matsumoto, Rikio & Saigusa, Toyohei, 2001, The biology and immature stages of Thrybius togashii Kusigemati (Hymenoptera: Ichneumonidae: Cryptinae), with a description of the male, Journal of Natural History 35 (10), pp. 1507-1516 : 1508-1513

Life history

Habitat

Adults of T. togashii were usually found resting on leaves of reeds, walking on reed canes, and rarely fl ew away from reed thickets. No specimens of this ichneumonid were captured by sweeping other plants around the observation site and by general collecting at other sites.

Host

On the basis of the following, only larvae of an undescribed species of Tetramesa ( Hymenoptera , Eurytomidae ), which develop as gregarious phytophages (9– 35 larvae per colony on an internode, 26 in average) feeding on the inner tissue of internodes of reeds (figures 2– 4), are considered to be the hosts of T. togashii . The direct observations that larva of T. togashii fed on larvae of Tetramesa were made on 21 October 1998 for two host colonies. The inner wall of internodes containing a mature larva (fi gure 8) or a cocoon of T. togashii was always scraped and roughened, almost certainly through the feeding activity of larvae of Tetramesa , though no larvae of this species were found in these internodal cavities. In addition, these cavities exclusively contained only head capsules of larvae of Tetramesa or their fragm ents.

Ovipositing behaviour

Females of T. togashii were often observed drumming their antennae while walking along the reed canes. When a female located a suitable place for oviposition, she drummed it intensively. Then, she raised her metasoma high and acutely bent it between the fi rst and second segments so that the ovipositor made a right angle with the cane (fi gure 5). At the same time, all legs were stretched out and held the cane tightly. In this posture the body was rocked back and forth, with the antennae stretched out forward, and the ovipositor was inserted into the cane. Movement stopped for about 5 s, at which point an egg was probably laid. All oviposition sequences took 1–2 min.

It is assumed one egg is oviposited because only a single egg or a fi rst instar larva of the parasitoid was found in internodal cavities of randomly collected reeds. In addition, each internode which was attacked by T. togashii contained a single mature larva of this ichneumonid exclusively.

Life cycle

This species is bivoltine in F ukuoka. Adults of the fi rst generation appear from early May to late July. No adults were observed in the fi eld on 25 July 1998. But two pupae and three final instar larvae of T. togashii and three colonies of Tetramesa sp . (containing 12 and 23 larvae and nine pupae, respectively) were found when about 100 reeds were examined. The percentage parasitism of hosts was 62.5% (fi ve ichneumonids to eight host colonies in about 100 reeds). From these pupae the adult wasps of T. togashii and Tetramesa sp. emerged within 7 days. F ield emergence of the second generation also occurred at this time. Adults of the second generation disappeared by late September in the fi eld. The population size of the second generation was somewhat smaller than that of the fi rst generation.

On 21 October 1998 it was observed that most larvae of T. togashii had already matured and had begun to spin cocoons, except for two that were still feeding. The percentage parasitism of hosts was 73.4% (53 ichneumonids to 72 host colonies in 45 reeds). U nder indoor conditions most of these ichneumonid larvae soon pupated, with some adults emerging towards the end of N ovember. The emerged females had some mature eggs in their ovarioles. In the fi eld, no adults were found in late November.

Sixteen ichneumonid larvae were found in 10 reeds that contained three unparasitized host colonies on 5 December 1998. These ichneumonid larvae had already finished feeding and were in the cocoon. Although most of them pupated in 15 days under indoor conditions, no pupae were found in the fi eld on 19 December 1998 (nine ichneumonid larvae were found to 12 host colonies in 10 reeds).

Description

Immature stages

Four larval instars were recognized for T. togashii on the basis of cast skins in internodal cavities, although fi ve instars have usually been recorded for Ichneumonidae (G auld, 1984).

Egg (figures 6, 10). White, 1.2 mm long, elongate and sausage-shaped as usual in Cryptinae , curved and weakly tapered at one end. No diVerences were detected between the ovarian and the oviposited eggs.

First instar larva (figures 11–13). Characteristic of Mandibulate and Polypodeiform type ( Clausen, 1940). Body length about 2.4 mm. H ead capsule well sclerotized, reddish brown, antenna elongate. Body white, somewhat translucent, with ventral transverse tubercles between each segment; each body segment with pair of tubercles ventrally.

Second instar larva (fi gure 14). Head capsule less sclerotized than in fi rst instar; antenna short; stipital sclerite, pleurostoma and hypostoma well developed; labial and maxillary palpi present; mandible without teeth.

T hird instar larva. Similar to second instar but slightly larger.

Final instar larva (figures 15–17). Body length 10.5–18.0 mm. Milky white in colour. Body 13-segmented, cylindrical, weakly tapered posteriorly beyond 2 / 3, with dorsal transverse tubercles on 4th to 10th segments. Vertex with pair of weakly sclerotized longitudinal patches near midline; hypostomal spar present; mandible without teeth; spiracle (fi gure 17) with closing apparatus adjoining atrium.

Pupa (fi gure 18). Body length 11.0– 14.2 mm. Milky white just after pupation, with reddish brown compound eyes. Antenna reaching apex of 2nd metasomal segment; 7th to 8th metasomal segments with a pair of hook-like projections curved posteriorly at tip. Ovipositor obliquely curved dorsally.

Adult morphology

See Kusigemati (1982) for description of the female. This species is dimorphic in colour pattern, with black and red forms, in both sexes. In contrast to the black form whose body is almost entirely black, the red form has 2nd to 4th abdominal segments (only basal half of the 4th in female) and legs almost entirely red. Specimens intermediate between the two forms were collected rarely.

Male, similar to female except as follows: flagellum 24–27-segmented, not clavated, with tyloids on subapical 7–8 segments, without subapical white band; apical segment of flagellum neither elongate nor truncate; fore femur and tibia not inflated unlike female; fi rst metasomal segment more slender apically than in female, 1.9 times as long as wide at apex; subgenital plate (fi gure 20) with rather dense pubescence on apical half; both sides of apical margin of subgenital plate slightly concave; paramere (figures 19, 21) rather broad, with dense setae on outer side apically; apex of distivolsella with rather dense setae; gonolacinia slightly turned outward; aedeagus (fi gure 22) thickened subapically and tapered toward apex; 4th metasomal segment entirely red in red form; 5th to 7th metasomal terga with white markings medio-apically; paramere with longitudinal white marking on outer side.

Length: Body 7.2–12.6 mm, fore wing 4.9–7.9 mm.