Corynura bruchiana (Schrottky, 1908)

González-Vaquero, Rocío Ana, 2022, Solitary and semisocial behaviour in the Corynura group: new findings in a clade sister to all other Augochlorini bees (Hymenoptera: Halictidae), Journal of Natural History 56 (45 - 48), pp. 1841-1868 : 1852-1853

Corynura bruchiana

( Figures 1b , 4 , 8a ; Tables S6–S7)

Nest site

Nests of this species were found in vertical banks ( Figure 4a ) facing north or west, towards main roads of the National Parks sampled, in the permanent shade of trees. Nest aggregations ( Figure 4b ) contained both active and inactive nests. Bees carrying pollen were observed, flying in close proximity to the bank until they found the entrance of their nest. The nests found in Lake Guillelmo and Villa Mascardi were studied on cloudy, rainy days. Some velvet ants ( Hymenoptera : Mutillidae : Euspinolia sp. ) were observed walking in the nesting area at Puerto Arrayán, and one specimen was found inside an active nest that was occupied by a bee (Table S6: nest 3), but no evidence of parasitism was found in the cells studied. Nests of Caenohalictus sp. ( Halictidae : Caenohalictini ) and anthills of Dolichoderinae and Camponotus sp. (Formicinae) were found in the nesting area of Lake Lolog.

Nest structure

The entrances had a diameter of 5.0 mm (SD = 0.6, n = 15). The nests comprised a horizontal burrow that led to a well-defined chamber (x = 18.8 cm SD = 9.4 from entrance, n = 18) which held an earthen block: the cluster of cells. Since the nests were very close to each other, digging an active nest caused the exposure of tunnels and blocks of nearby nests. Most of these nearby nests were inactive, from past years, and only had open cells with faeces or cells filled with loose soil ( Figure 4c ). These nests were included in the study, though they lack some measurements (e.g. entrance diameter, chamber depth; see Table S6). No nest had branches from the main burrow, although occasionally a blind burrow extended beyond the cluster for a few centimetres. The earthen block containing the cells rested at the bottom of the chamber ( Figure 4d ) or was held in place by roots, but there were no earthen pillars to support it, and therefore it was easy to remove clusters intact. The shape of the block was approximately rounded, the surface was rough and the cells’ outlines were not evident from the exterior. The blocks were opened in the laboratory 7 days after extraction, and measurements and contents were recorded (Table S6); no bee emerged in the intervening period. The blocks were carefully scraped with a knife to find the cells, which were orientated in different directions ( Figures 4c,e ). Some cells contained fungi and dead larvae ( Figure 4f ), suggesting that this species may be particularly sensitive to changes in environmental conditions and/or manipulation, though it is possible that the larvae were already dead before nest excavation. A few blocks were left intact as voucher material. The blocks had the following measurements: 22–36 mm width, 16–28 mm length, 13–24 mm height (n = 5). Each block had at most six cells (x = 3.5 SD = 1.6, n = 17). The cells were ovoid in shape and had the following measurements: 2.8–3.8 mm width at neck, 4.6–6.0 mm maximum width, 9.4–11.4 mm length (n = 14). Two nests had an earthen block with no cells in it (Table S6: nests 7, 9), and the blocks extracted in December were not smaller than those found at the end of January, though the first ones only had one to three cells. This could be evidence that the bees delimit or build the earthen block first and then dig the cells in it, instead of digging the cells as they enlarge the block, as Claude-Joseph (1926) described for Co. apicata . In active nests, both open cells with faeces and cells filled with loose soil were found.

Nesting behaviour

Several indicators suggest that Co. bruchiana is a solitary species, at least in the region studied. I never found more than one adult per nest, all of them had some mandible and wing wear (Table S7), and the earthen blocks had relatively few cells, all features of solitary species.

The nests dug on 20 December had open cells with pollen, and closed cells with pollen and an egg or small larva on it, evidence that the bees had started the nesting season recently. Only one specimen had fully developed ovaries, which was collected in December. Specimens collected at the end of January had little or no evidence of ovary development (Table S7), but this could be due to the fact that oviposition had already ended. Most nests dug in late January had postdefecating larvae, and two had male pupae (Table S6). These data and the fact that there are no male specimens in collections obtained before February (Table S14) indicate that this species is univoltine in the area studied.

Figures

Figure 1. Some of the species studied. (a) Ca. aureoviridis; (b) Co. bruchiana; (c) Co. ampliata removing soil from the nest to the exterior.

Figure 4. Corynura bruchiana, nesting bank in Lake Paimún, Neuquén. (a) Vertical bank with nests; (b) entrances of the nests; (c) exposed, old, earthen block with cells. Half of the block is missing, resulting in the transversal cut of two cells, and the longitudinal cut of another one; (d) earthen block in a chamber, with some cornstarch that was blown in to follow the tunnel; (e) ovoid block, with cells irregularly orientated. The longitudinal axis of the broken cell is approximately 100 degrees from those of the open cells; (f) cell containing a dead larva. Scale bars: b = 5 cm; c, d = 1 cm; e, f = 5 mm.

Figure 8. Profile of a typical nest of the studied species. (a) Co. bruchiana; (b) Co. nahuelita; (c) H. reticulatus (Table S12: nest 3); (d) Ca. aureoviridis; (e) Co. ampliata; (f) H. amplilobus (Table S10: nest 3). Scale bars = 1 cm.