Rhynchium oculatum (Fabricius 1781)

Rhynchium oculatum (Fabricius)

This large eumenid wasp occupied six 12.7-mm borings, one each from four different settings (nest Nos. 328, 329, 330, 333) on wooden posts in a garden in Ismailia and one each from settings on a wooden trellis (No. 313) and on a Casuarina trunk (No. 316) in a garden area in Giza. In addition the same wasp probably used two 6.4-mm borings (Nos. 226, 227) from the Casuarina trunk station in Giza; occupants of these nests died as small larvae.

The Ismailia nests were stored and completed during the period 23 May to 13 June. The nests from the Casuarina trunk setting in Giza were stored and completed during the period 25 March–28 April. The nest from the trellis in Giza was stored and completed between 28 April and 12 May.

NEST ARCHITECTURE.—In one each of the 6.4- and 12.7-mm borings at Giza, the mother wasp placed a little mud in the inner end of the boring before bringing in prey. In the other nests the wasps placed the paralyzed caterpillars at the inner end without first coating it with mud.

The six 12.7-mm nests each contained 1–4 provisioned cells (mean 2.5); in length these cells ranged from 18–43 mm (mean 27). Three cells from which females were reared were 24–37 mm long (mean 29), and five male cells were 23–29 mm (mean 26); progeny was not reared from the other seven cells. Each of the 6.4-mm nests had a single provisioned cell 35 and 39 mm long respectively.

Empty intercalary cells were lacking in both 6.4-mm nests and in three 12.7-mm nests; they were present in two of the 12.7-mm nests from Ismailia and in one of the 12.7-mm nests from Giza. In the latter nest there were intercalary cells 13 and 22 mm long between stored cells 2 and 3 and between stored cell 3 and the vestibular cell. In a 4-celled nest from Ismailia there was one intercalary cell 9 mm long between stored cells 2 and 3; the other 2-celled nest (No. 333) had two intercalary cells 22 and 14 mm long between stored cells 1 and 2.

All nests had an empty vestibular cell except one nest from Giza. In the two 6.4-mm nests these cells were 110 mm long; one was divided into two sections by a transverse mud partition. Five vestibular cells in 12.7-mm borings were 13–80 mm long (mean 49); one was divided into two sections by a transverse mud partition. The one nest from Giza which lacked the vestibular cell may have been abnormal; it had only a single stored cell 30 mm long which was sealed by a much thicker partition (4–9 mm) than normal.

The partitions closing the cells and the plug at the nest entrance were made from mud except in the one abnormal (?) nest from Giza which was sealed by a plug made from agglutinated sand. The partitions in 6.4-mm nests were 1.5–3 mm, and the closing plugs were 4–5 mm thick. The partitions closing stored cells in 12.7-mm nests were quite thin in the middle, about 1 mm, but 3–8 mm thick at the edges. The partitions closing empty intercalary cells were slightly thicker than those closing stored cells. The closing plugs in 12.7-mm nests were 2–13 mm thick (mean 7).

PREY.—Rhynchium oculatum stored a large number of rather small caterpillars per cell or a smaller number of larger caterpillars. When I opened nest 313 from Giza, I found a prepupa of oculatum and 103 specimens of a species of Gelechiidae which had not been eaten by the wasp larva. Ninety-nine of the specimens of prey were larvae and four were partially transformed to the pupal stage; the larvae were shriveled and about 5 mm long. Nest 328 from Ismailia contained two dead adult oculatum, and I recovered the head capsule of a small specimen of Pyraloidea from one of the cells.

Nests 329, 330, and 333 from Ismailia were stored with caterpillars of a species of Amphipyrinae (Noctuidae). Fourteen larvae from 333 were 10–15 mm long. Fourteen larvae or fragments thereof were recovered from nest 330, and six from nest 329.

Development of the wasps in all nests of oculatum had progressed so far by the time I opened the nests that it was not possible to ascertain the original number of caterpillars provided in each cell.

LIFE HISTORY.—The empty egg shell in one nest was 3 mm long; it was attached by a delicate filament at a point 3 mm from the inner end and on the upper side of the boring.

The small wasp larvae in the 6.4-mm nests and in one of the 12.7-mm nests from Giza were dead when I opened these three nests on 28 April, probably because of the high temperatures during that period. This suggests that oculatum normally nests in situations offering greater protection from ambient temperatures than afforded by the relatively thin-walled wooden traps.

The other 1-celled nest (No. 313) from Giza contained a creamy, leathery skinned prepupa 14 mm long on 12 May; this nest was provisioned some time after 28 April, probably early in May. There was a pale, pink-eyed pupa in this cell on 17 May, so pupation must have occurred about the 15th. Adult eclosion had not occurred by 26 May, and the nearly fully colored pupa was dead the next time I examined the nest on 16 August.

The four nests from Ismailia were stored during the period 23 May to 13 June. I opened two of them on 29 July and found that the adult occupants had died in the cells before that date. Dead adults were found in their cells in the other two nests when I opened them on 19 August. Three of the Ismailia nests contained both sexes; females were in the innermost, and males in the outermost cells.

The cocoon of this wasp is evanescent or entirely lacking. In the Ismailia nests the larvae in one nest just varnished over the inner wall of the mud partition capping the cells; in two nests the larvae silked over these closing partitions and about 2–3 mm of the cell walls adjacent to the partitions.

Lichtenstein (1869) published the first biological note on this species in southern France. He stated that it nested in rose canes in cavities as short as two to three inches in length. Each cell was provisioned with 8–12 caterpillars of the noctuid moth Plusia gamma (Linnaeus). A female wasp might provision 15–20 cells during her lifetime, storing 150–200 caterpillars in these cells. There was only one generation a year, the immature wasp remaining as a resting larva in the cell during the winter and pupating in April.

Grandi (1961, pp. 47–52, figs. 28–32) summarized his earlier observations on this wasp in Italy. At Pontecorvo he found four nests in dry canes of Arundo donax, arranged vertically to form a wall of a shed for drying tobacco leaves. At the bottom of each nest the wasp coated the whole diaphragm of the internode with a thin layer of mud or made a mud plug 4–5 mm thick if the diaphragm was perforated or broken. A nest in a cane with a calibre of 7 mm had a single stored cell 25 mm long; three nests in canes having a calibre 10–11 mm had 11 stored cells 14–30 mm long (mean 22). Two of the latter nests were 143 and 161 mm long, had five and four stored cells respectively, and vestibular cells of 13 and 26 mm length respectively. One of these nests had a closing plug of mud 14 mm thick. The cells were stored with light green caterpillars of a pyralid moth 25–27 mm long. The wasp egg was 4.3 mm long and was suspended from the cell wall by a filament 2 mm long. Grandi reared the hypermetamorphic rhipiphorid beetle Macrosiagon ferrugineum flabellatum (Fabricius) from resting larvae of the wasp. Later, at Bologna, he found oculatum preying on larvae of the pyralid moth Lypotigris ruralis (Scopoli).