Note on the nesting biology of Epimelissodes aegis LaBerge (Hymenoptera: Apidae)

Bees in the genus Epimelissodes (Hymenoptera: Apidae: Eucerini) (formerly Svastra) are generally large, robust bees (8.5–20 mm) found across North and Central America (Michener 2007). Species of Epimelissodes are generally considered polylectic, but museum specimens are commonly collected from host plants in the plant family Asteraceae (Cane 1995; LaBerge 1956; Mitchell 1962). There are currently 20 species in the genus Epimelissodes across four subgenera (Freitas et al. 2023). Information or descriptions of the nesting biology has been documented for only four species in two of the subgenera: Epimelissodes (Epimelissodes) obliqua (Say) (Custer 1928; Rau 1922; Rozen 1964), E. (Epimelissodes) ageis atrimitra (LaBerge) (Cane 1995), E. (Idiomelissodes) duplocincta (Cockerell) (Rozen 1991), and E. (Epimelissodes) sabinensis (Cockerell) (Rozen 1983, 2016]). All four species are known to host kleptoparasitic bees in the genus Triepeolus (Hymenoptera: Apidae: Epeolini) (Rightmeyer 2008).

A nesting site of Epimelissodes aegis was discovered in late September 2023 at the University of Florida North Florida Research and Extension Center located in Qunicy, Florida (Gadsen county) at approximately 63 m a.s.l. E. aegis has been reported as a specialist on Asters, with collection records from Balduinia Nutt., Chrysopsis (Nutt.) Elliot, Gaillardia Foug., Helenium L., Helianthus L., Symphotrichium Nees, and Vernonia Schreb. (Fowler 2020; Fowler and Droege 2020; LaBerge 1956; Mitchell 1962). Mitchell (1962) also includes Dalea (=Kuhnistera) (Fabaceae) as a floral record. Other than floral host records, this species is relatively understudied considering its broad range across the Gulf and Atlantic coasts in the southeastern United States (LaBerge 1956). The nesting aggregation was located in an area of the research farm planted with cotton and used for a nitrogen rate and fungicide trial in 2023 and previously planted with peanuts in 2022. Cotton (Delta Pine 1646) was strip-tilled into wheat residue (Figure 1A). Fertility other than nitrogen, insects, nematodes, weeds, and diseases were managed with local practices as needed. Insecticide applications included an at-plant in-furrow application of aldicarb for nematodes and thrips and foliar applications of pyrethroids for control of plant bugs and stink bugs (Hemiptera). Plant growth regulators and harvest chemicals such as defoliants and boll openers were applied as needed. Aldicarb, in particular, is known to be water soluble, to translocate within plants and soil, and to be toxic to bees (Johansen et al. 1984; Mink et al. 1989).

Figure 1:

Images from a nesting aggregation of Epimelissodes aegis in a cotton trial located at the North Florida Research and Education Center in Quincy, Florida. A) Cotton plot where nesting aggregation was observed, post defoliation, B) Triepeolus concavus female about to enter nest, C) sealed nest, D) unsealed nest, E) female E. aegis, and F) male E. aegis. All images were taken on 23 September 2023 by Isaac L. Esquivel.

The aggregation of nests was discovered by one of the authors while taking data on the trial. Initial observations of the aggregation were made on 20 September 2023 at 10 am. Nests were in rows two, three, four, and five of an eight-row cotton plot that had already been defoliated and provided no floral resources at the time of observation. Nests appeared to be randomly built, some in between two cotton rows and some in line with the cotton plants. At least 20 nests were active in an area of 27 m² within the 0.1 ha trial. No nesting activity was found in the rest of the trial or surrounding area including field margins. Three nests were aggregated around a dirt and residue mound (1 m²) which initially was thought to be a single nest, but entrances were hidden under cotton plant debris. Fifteen of the 20 nests were open, and five were sealed with a layer of soil (Figure 1C and D). Each nest entrance was marked, labeled, photographed, and measured once on 29 September 2023 with digital calipers. Nest entrances ranged from 7.61–9.35 mm, with one outlier of 19.79 mm in diameter. On average, entrances were 8.54 mm in diameter, excluding the 19.79 mm nest, which was likely enlarged by equipment passing through the rows. Visual observations involved a single observer walking the area of the cotton plot for approximately 30 min, recording behaviors and taking photographs. Observations started on the day of discovery and were done at least once a week until 20 October 2023, when activity appeared to cease. In addition, two cameras mounted on tripods were set up to record four individual nests for 15 min. Recordings took place once on 22 September 2023 at approximately 10 am with sunny skies and a temperature of 22 °C.

Both male and female E. aegis (Figure 1E and F) were active concurrently, with males appearing to outnumber females (greater than 5:1, but exact numbers were not recorded). Males darted rapidly over the area of the nest aggregations fairly low (<60 cm), under the top of the cotton canopy between rows. Sometimes, males would stop and hover around a nest entrance and occasionally enter the nest. Females were not as abundant as males but were observed returning to nests with visible pollen loads. Females were often seen with their heads just visible in the nest entrance, waiting before darting out. Similar to other male Epimelissodes species, males actively patrolled the nesting aggregation. Occasionally a male, or multiple males, would pounce on a female and attempt to mate, with some males appearing to wrestle each other and occasionally with a female. Males also were seen entering the nesting site after a female before quickly being forced out.

Nesting behavior of E. aegis appears to be similar to that of other published accounts within the genus and within the broader tribe Eucerini. Much of the literature and data on nesting behavior about bees are scattered as single species accounts with a few exceptions (Orr et al. 2022; Rozen 1991). Some species nest communally, like Epimelissodes (Epimelissodes) sabinensis (Cockerell) (Rozen 1983), while others, like Epimelissodes atripes (Cresson), are solitary nesters (Cane 1995). In the case of E. aegis, observations are inconclusive to whether they nest communally or solitarily. Individual visual observations suggest that only a single female entered any given nest at a time; however, after reviewing the longer video recordings, two distinct females were seen entering a single nest on multiple occasions.

Each of the previously described nesting aggregations of Epimelissodes has hosted different species of kleptoparasitic Triepeolus (Cane 1995; Custer 1928, 1929]; Rozen 1964, 1983], 1991], 2016]). In the current study, T. concavus (Cresson) were observed resting near nest entrances, walking around entrances, occasionally peeking in, entering briefly before exiting, and sometimes disappearing into the nest entrance for longer periods of time (Figure 1B). At most, four individual T. concavus were seen around a single nest by video recording. At the same time nests were being recorded, other individuals of T. concavus were observed at other nests by visual observation. T. concavus also has been reported from nests of E. obliqua (Rightmeyer 2008), but collection from nests of E. aegis is a novel host association. No other bees were found nesting at the site. Voucher specimens of E. aegis and T. concavus were collected and identified and are housed in the collection located at United States Department of Agriculture, Agricultural Research Service, Pollinator Health in Southern Cropping Ecosystems Research Unit in Stoneville, Mississippi. No other species of bees were observed around the aggregation with the exception of Apis mellifera L. (honey bees) occasionally flying by.