Halictillus reticulatus (Gravel)

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 : 1856-1860

Halictillus reticulatus

( Figures 3a , 7 , 8c ; Tables S12–S13)

Nest site

Three nests were found in a slope 4 m wide × 1 m long in the Challhuaco Valley (Río Negro) ( Figure 7a ). The soil was dry and slightly sandy, and it contained some stones. The nest entrances were 3–4 cm apart. Two additional nests of this species were found in the same flat ground near Lake Lácar where the nests of Co. ampliata were studied ( Figure 3a , see Co. ampliata for site description). Nests from both sites were dug by midsummer.

Nest structure

The nests from the Challhuaco Valley had very small entrances (x = 2.0 mm SD = 0.5, n = 3), but the burrow widened considerably immediately below (x = 3.4 mm SD = 0.2, n = 3) (Table S12). Each nest had 4–9 (x = 7.0 SD = 2.7, n = 3) cells, which were at 4.1 cm (SD = 0.4, n = 3) from the surface. The cells were in a comb-like cluster, horizontally orientated, with their openings facing the tunnel, but the chamber was not clearly defined. One nest (Table S12: nest 1) had two cells on each side of the tunnel, while another nest (Table S12: nest 3, Figure 8c ) had two cells with faeces and filled with soil located opposite a cluster with seven cells (four active, closed cells, and three empty, open cells). All the nests had some open cells with faeces. The entrances of the cells that were closed were smooth on the outer side (facing the burrow) and rough on the inner side. The cells were very fragile and only a few were extracted intact. The cells were ovoid in shape, with the following measurements: 2.7–3.0 mm maximum width; 5.5–5.6 mm length (n = 2). The main tunnel turned beyond the cells, and ended in a blind burrow (x = 7.5 cm SD = 1.3, n = 3).

The nests of H. reticulatus from the site near Lake Lácar had a radial tumulus of loose soil like the nests of Co. ampliata , but the tumulus was smaller (20 mm diameter) in this species, and the bees did not appear or re-make the tumulus immediately after it was removed. The nests had an entrance of 3.2 mm (SD = 0.3, n = 2) in diameter, and they consisted of a vertical burrow that ended 15 cm (SD = 1.4, n = 2) from the surface (Table S13). In one of the nests the deepest 5 cm of the tunnel was slightly wider, suggesting a poorly defined chamber. No cells were detected in these nests, but some old cells may have gone unnoticed due to weather conditions at the moment of the excavation (see details in Co. ampliata ).

Nesting behaviour

Unfortunately no adults were observed in the nests found in the Challhuaco Valley, but I was able to associate these nests to the species due to the advanced degree of sclerotisation of the pupae (Table S12). One of the nests found near Lake Lácar had 11 females; only one of them had fully worn mandibles and wings, while the remaining specimens had entire mandibles and only some of them had one or two notches in the wings (Table S13: nest 2). Two females were found in the other nest (Table S13: nest 1), both with very low mandible and wing wear. I did not detect clusters or cells in these nests, and none of the females had well-developed ovaries. Data suggest that the female with worn mandibles and wings from Nest 2 could have been the founder of the nest and the mother of the remaining females. As in Co. ampliata , although this could be evidence of eusocial behaviour due to overlap of generations, given the time of the year when the nests were studied it is more plausible that the bees were using their natal nest as a refuge, and were soon to enter diapause. Unfortunately the data are insufficient to infer whether H. reticulatus has some form of social behaviour.

Only well-developed pupae were found in the nests dug at the end of January, and 5 days later (2 February 2011) several dozen males were found in flowers of Chrysanthemum sp. (Asteraceae) near the area studied ( Figure 7b,c ). These data may suggest an explosive peak of males in midsummer, as happened with Ca. aureoviridis , although I observed an unbiased sex ratio in the pupae of the few nests studied (5 f: 5 m). Since males appear in the field at the beginning of January (Table S14), it is inferred that H. reticulatus would be univoltine, like the other species of the Corynura group that inhabit the area.

Figures

Figure 3. Nesting site of Co. ampliata and H. reticulatus near Lake Lácar. (a) Habitat; (b) nest entrances with the characteristic tumulus of soil (in this case, Co. ampliata).

Figure 7. Halictillus reticulatus nesting site in Challhuaco Valley. (a) Habitat; (b) male on Chrysanthemum sp.; (c) seven males feeding on Chrysanthemum sp., near the nesting site.

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.