A scientific note on Bombus (Psithyrus) insularis invasions of bumblebee nests and honeybee hives in the western USA James Strange, Abby Baur, Jonathan Koch

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James Strange, Abby Baur, Jonathan Koch. A scientific note on Bombus (Psithyrus) insularis inva- sions of bumblebee nests and honeybee hives in the western USA. Apidologie, Springer Verlag, 2014, 45 (5), pp.554-556. ￿10.1007/s13592-014-0271-8￿. ￿hal-01234751￿

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HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Apidologie (2014) 45:554–556 Scientific note * INRA, DIB and Springer-Verlag France, 2014 DOI: 10.1007/s13592-014-0271-8

A scientific note on Bombus (Psithyrus) insularis invasions of bumblebee nests and honeybee hives in the western USA

1 1,2 1,3 James P. STRANGE , Abby BAUR , Jonathan B. KOCH

1USDA-ARS Pollinating Insects Research Unit, Utah State University, BNR Room 261, Logan, UT 84322, USA 2Department of Environment and Society, Utah State University, Logan, UT 84322, USA 3Department of Biology, Utah State University, Logan, UT 84322, USA

Received 11 July 2013 – Revised 22 November 2013 – Accepted 9 January 2014

social parasites / inquilines / Apis mellifera / bumblebees

Bumblebees (Bombus) in the subgenus Psithyrus are were made by Morse and Gary (1961), but of these obligate social parasites that usurp the nests of other records, only 1 of the 28 bumblebees found in 25 bumblebees in order to reproduce. Usurpation occurs honeybee colonies was a social parasite (B. citrinus). after nest establishment by the host species when, The authors attributed these nest invasions to re- upon entry to the nest, the invading Psithyrus kills or source robbing and not nest usurpation. Here, we suppresses the host queen and dominates host report Bombus insularis (Smith) invading the nest of workers to rear her offspring, but considerable two bumblebee species in the subgenus Pyrobombus, behavioral variation occurs in host–parasite interac- Bombus huntii Greene, and Bombus bifarius Cresson, tions among members of the genus (Fisher 1988). and attempted invasions of honeybee hives at four While many Psithyrus species are known to parasitize apiaries by B. insularis. a number of species, few of the relationships are During the summer of 2012, numerous female B. well understood. Of the six North American insularis were observed near bumblebee nest boxes Psithyrus species, most have been recorded from and honeybee hives in Utah, USA. Although obser- the nests of several host species, and it is reasonable vations from previous years were not quantified, it is to assume that each host species reacts differently to uncommon to see dead B. insularis around honeybee an invasion. hives; for example in 2013 at apiary 1, we found no In addition to usurping bumblebee nests, Plath B. insularis at three colonies during weekly inspec- (1927) noted that females of Psithyrus laboriosus tion. The exception was in 2007 when several dead B. (=Bombus citrinus (Smith)) would attempt to invade insularis were observed at hive entrances at apiary 1. honeybee hives (Apis mellifera L.). This was noted as That year, a B. insularis was observed entering a a “dead end,” usually resulting in the death of the honeybee hive and was forced out within a minute. invading bee. Subsequently, records of several When the invader exited the hive, it had several Bombus species attempting to enter honeybee hives honeybee workers attacking her, by biting her legs and tegulae; however, the parasite managed to escape Electronic supplementary material The online version and fly away. The number of invasions in 2012 by B. of this article (doi:10.1007/s13592-014-0271-8) contains insularis provided an opportunity to study aspects of supplementary material, which is available to authorized nest usurpation and hive invasion. We collected dead users. B. insularis from the ground in front of honeybee Corresponding author: J.P. Strange, hives at four apiary sites from 12 June to 12 August [email protected] (Table I). Apiary 1 was inspected weekly for dead Manuscript editor: James Nieh Psithyrus in front of the hives, and apiaries 2–4 were Psithyrus invasions in the western USA 555

Table I. Apidae hosts of B. insularis at five locations Within 4 h of placing nests in the BLG on 19 in northern Utah. June, B. insularis were observed investigating the nest boxes for an entrance. Because one B. huntii nest Site Hosts Coordinates was queenless when deployed, we captured one of the searching B. insularis on 20 June and introduced Apiary 1 B. huntii 41°44.72′N her into the nest. She harassed approximately 35 B. occidentalisa 111 49.74′W workers and males in the nest by biting and pushing A. mellifera–four colonies the workers around the nest, but only one worker Apiary 2 A. mellifera–four colonies 41°45.76′N was killed in the melee. Six days after the nests 111°47.97′W were deployed, on 25 June, there were two dead B. insularis B. Apiary 3 A. mellifera–six colonies 41°45.73′N in the nest box and one live 111°47.76′W insularis. It is unknown if the remaining bee was B. huntii Apiary 4 A. mellifera–two colonies 41°41.16′N the original invader. The nest was then 111°50.08′W abandoned by the inquiline for several days and B. insularis B. bifarius ′ reinvaded by a on 2 July 2012. All Bee lab 41°45.45 N B. insularis B. huntii 111° 48.69′W the we observed harassed workers and B. occidentalisa then appeared to be incubating brood in the nests within a few hours of invasion, a pattern that repeated in Bombus a No invasion attempts were recorded on these nests all the colonies, as shown in the animation (Online Resource). Although the queenless nest of B. huntii was visited one time each in mid-July. We also observed invaded with little loss of life, a queen right nest at female B. insularis invading nests of B. huntii and B. apiary 1 had a different outcome. The first B. bifarius placed in the field at the Logan Bee Lab insularis invasion occurred two days after the nest Gardens (BLG) and apiary 1 (Table I). was placed, on 2 July. Workers had been marked with A total of 47 female B. insularis and one female B. colored tags before the nest was deployed, and all the huntii were found dead in front of honeybee hives at tagged bees either abandoned the nest or were dead four apiaries. The average number of dead B. within a week. The B. huntii queen and two B. insularis was 2.9 per hive across all apiaries (range, insularis were also found dead in the nest within 0.7–6 per hive). Microscopic inspection of the dead 7 days. However, newly emerging B. huntii workers B. insularis showed some cuticular damage to the stayed in the nest and adopted the remaining B. dead bees, most notably extensive removal of setae. insularis invader as queen. This nest was subsequent- Of the 47 B. insularis that were found dead in front of ly invaded by two additional B. insularis, resulting in honeybee colonies, only two were found to have the death of the young, unmarked workers of B. sting remnants in the cuticle, both found in the thin huntii, resulting in no workforce for any B. insularis cuticle between abdominal sterna, indicating that to exploit. Thus, by the end of August, the nest had the sting may not be effective in killing these produced no B. insularis offspring. In all, at least five invaders. Perhaps, the honeybees kill these larger B. insularis attempted to invade the B. huntii nest. and well-armored parasites through balling behavior The B. bifarius nest in the BLG met a similar fate (Papachristoforou et al. 2007); however, the cause to the queen right B. huntii nests. The first B. of death is uncertain. insularis invasion occurred before 25 June when a In June 2012, we deployed nests of B. bifarius, parasite was seen in the nest. There were two dead B. huntii, and Bombus occidentalis Greene that workers and five live workers in the nest, but the had been reared from locally sourced queen original queen and most of her workers were no bumblebees in two-chambered wooden nest boxes longer there, apparently having abandoned it. The (Plowright and Jay 1966). Nests had between 15 next day, a second B. insularis was seen invading the and 30 workers when deployed. Three nests were nest, and the encounter was video recorded. The two placed in the BLG on 19 June, while nests of B. females fought, but the second invader then retreated, huntii and B. occidentalis were placed in apiary 1 leaving the nest to the first. By 27 June, the nest was on 30 June. After observing parasites investigating empty but for a single B. bifarius worker. Eleven the nests, we made approximately weekly observations days later, on 8 July, a single B. bifarius worker and a during the summer to understand the dynamics of B. insularis were observed in the nest, but the B. B. insularis invasion. insularis died that night having not reproduced. 556 J.P. Strange et al.

Considering the volatile history of the usurpation by bumblebee nest entrances by reducing the size of the B. insularis in our observations, it is unknown if the opening seemed to reduce invasions, yet a controlled B. insularis female observed on 26 June is the same study was not conducted. Much needs to be learned individual that died on 8 July. about the ecology, behavior, and management of these Neither B. occidentalis nest deployed in apiary 1 social parasites. nor BLG was observed to have an attempted invasion. Twice we observed B. insularis females flying around a B. occidentalis nest for several Note scientifique sur les invasions de Bombus minutes; however, the parasite did not enter the nest (Psithyrus) insularis dans les nids de bourdons et les nor was there any evidence of an attempted invasion, ruches d’abeilles dans l’ouest des Etats-Unis such as a dead B. insularis outside the nests. The number of B. insularis appearing in the Eine wissenschaftliche Notiz über das Eindringen von summer of 2012 suggests that this species has the Bombus (Psithyrus) insularis in Hummelnester und potential for population outbreaks which are not Honigbienenvölker im Westen der Vereinigten Staaten uncommon for social Hymenoptera (Akre and Reed von Amerika 1981). Perhaps the high population densities leading to multiple invasions are a negative feedback that keeps the parasite population in check if repeated REFERENCES invasions impact the species ability to produce offspring. Alternatively, it may be that B. huntii and B. bifarius are suboptimal hosts in comparison to the Akre, R.D., Reed, H. (1981) Population cycles of yellow jackets other 12 bumblebee species in northern Utah, but this (Hymenoptera: Vespinae) in the Pacific Northwest. Environ. Entomol. 10(3), 267–274 is unclear. The cause of incursions into honeybee colonies is Fisher, R.M. (1988) Observations on the behaviours of three European cuckoo bumble bee species (Psithyrus). Insectes also perplexing. Were these events, as Morse and Soc. 35(4), 341–354 Gary (1961) suggest, simple attempts to rob hive B. Morse, R.A., Gary, N.E. (1961) Insect invaders of the honeybee resources, or are high population densities of colony. Bee World 42, 179–181 insularis driving it to attempt usurpation of novel Papachristoforou, A., Rortais, A., Zafeiridou, G., Theophilidis, species? Questions of host preference and the cause G., Garnery, L., et al. (2007) Smothered to death: hornets of honeybee hive invasion remain to be determined. asphyxiated by honeybees. Curr. Biol. 17(18), 795–796 Finally, considering so many bumblebee workers die Plath, O.E. (1927) Psithyrus laboriosus, an unwelcome guest in during invasions, thus reducing the pollination service the hives of Apis mellifera. Bull. Brooklyn Entomol. Soc. that a colony can provide, is there a management 22, 121–125 strategy to limit the number of bumblebee colony Plowright, R.C., Jay, S.C. (1966) Rearing bumblebee colonies in invasions in outbreak years? Initial attempts to modify captivity. J. Apic. Res. 5, 155–165