Pathogen spillover, broadly defined as the transfer of one or more pathogens from one reservoir host to a recipient host is one of the major factors influencing disease spread in natural populations. Spillover often occurs when infected host species invade novel habitats and transmit exotic parasites to native hosts, which often experience substantial reduction in reproduction and survival [1, 2]. Pathogen spillover occurs commonly in bee colonies transported from one region to another for commercial purposes because commercially-reared colonies often harbor parasites harmful to wild bees, resulting in disease epidemics, and native bee declines in wild populations [3,4,5]. In this regard, Bombus terrestris Linnaeus (Apidae) (Fig. 1a), one of the most extensively used bumblebee species for crop pollination worldwide has been suggested to transmit their parasites to recipient native bees in southern South America [6, 7]. One of the parasites commonly harbored by B. terrestris is Crithidia bombi Lipa & Triggiani, a flagellated trypanosome species that infects exclusively species of the Bombus genus. This protozoan is horizontally transmitted by ingestion of infectious cells present in the feces deposited on flowers by infected workers of other colonies or bumblebee species, by contaminated honey stores and nest material, and through vertical transmission from one generation to another within the same colony [8, 9]. Like C. bombi, the microsporidium Nosema bombi Fantham & Porter, infects exclusively species of the Bombus genus. The spores (infective stages) locate mostly in the digestive tract of the bumble bees, and after proliferation within the host, are released into the environment [10, 11]. As in C. bombi, this parasite transmits vertically across generations in bumble bees [12]. Finally, Apicystis bombi Liu, Macfarlane & Pengelly, is a neogregarine protozoan that in addition to Bombus species, infects Apis mellifera, and Osmia bicornis Linnaeus (Megachilidae). It locates in the host adipose tissue, reducing the colony establishment success [12]. Its transmission is horizontal through shared flower resources, but vertical transmission has also been suggested [13].
In Chile, B. terrestris was imported for tomato pollination in 1997/1998. After greenhouse confinement escape, it has spread and become established in several localities of southern Chile and Argentina (see reviews in [6, 14]). In 2009, the presence of C. bombi and A. bombi was reported for the first time in South America [15]. Since then, further reports have confirmed that invasion of B. terrestris to novel habitats in southern Chile and Argentina carried exotic intestinal parasites [6, 16], hence increasing the chance of pathogen spillover to other bee species. For example, after B. terrestris invasion, C. bombi was detected in the native bumblebee Bombus dahlbomii Guérin-Méneville in Chile [6], and A. bombi was recorded in the honeybee Apis mellifera in Argentina [17, 18], which strongly suggests parasite spillover from B. terrestris to other bee species in southern south America [2].
A recently reported invasive bee species in Chile is the South American carpenter bee Xylocopa augusti Lepeletier (Fig. 1b and c). Originally distributed in Argentina, Brazil, Uruguay, and Paraguay, this species was recorded for the first time in central Chile in 2013, probably transported through undetected nests in packaging structures from Argentina [19]. After its arrival, new ecological scenarios of coexistence between otherwise allopatric X. augusti and B. terrestris are frequently found in central Chile, providing new opportunities for parasite transmission. It is known that parasite transmission is an expected result when founding populations in novel habitats are genetically depauperate [20, 21]. In principle, judging by the pathogens found in B. terrestris and other Xylocopa species, X. augusti has the potential to become infected by some viruses such as DWV (Deformed wing virus), BQCV (Black queen cell virus), and SBPV (Slow bee paralysis virus), all of them detected in B. terrestris also [22, 23]. However, as most studies inquiring on the potential transmission of parasites from B. terrestris to other bee species in southern South America have focused almost exclusively on intestinal parasites, and molecular diagnosis techniques for viral testing require a methodology different to that used in this study [24, 25], we restricted the analysis to the molecular detection of the protozoans C. bombi and A. bombi, and the microsporidium N. bombi in B. terrestris and X. augusti.