Sap beetles of the genus Omosita Erichson are stored-product pests that are also associated with carrion, potentially making them biosecurity risks and forensic tools. The discovery of a specimen of the Nearctic species Omosita nearctica Kirejtshuk in South Africa prompted an investigation a decade later to determine if this species had established itself in the country, which was confirmed by the collection of further breeding specimens that also facilitated the first description of mature larvae of O. nearctica. A new key to adults of all Omosita species is presented.

Biosecurity, forensic entomology, invasion biology, larval morphology, molecular identification, morphological key

Many insects associated with stored products have been moved between continents following humans’ colonisation of new places. In the case of cryptogenic species, this invasion process has been so thorough that the geographical origin of the insect invaders is no longer clear, e.g. the Hide Beetle, Dermestes maculatus DeGeer, 1774 (Coleoptera: Dermestidae) (Mroczkowski 1968). In other cases, these invasive species are not noticed for years even though they may be well-known pests in other countries, e.g. the Oriental Latrine Fly, Chrysomya megacephala (Fabricius, 1794) (Diptera: Calliphoridae) in South Africa (Williams and Villet 2006; Badenhorst and Villet 2018). Insects, and particularly beetles (Midgley et al. 2009), associated with stored animal products, are often of significance in forensic entomology and biosecurity, and keeping track of new members of the carrion insect community in a particular country is important in both of these contexts.

The sap beetle family Nitidulidae has approximately 350 genera and over 4500 species (Lee et al. 2020). The Nitidulidae in Africa have not been well studied and so what is known of them is limited (Kirejtshuk 2001). There are seven recognised species in the genus Omosita Erichson, 1843 – O. discoidea, O. colon, O. depressa, O. funesta, O. smetanai, O. japonica and O. nearctica (Reitter 1873, 1874; Kirejtshuk 1987; Lee et al. 2015). A specimen of O. discoidea in the Naturhistoriska Riksmuseet, Stockholm^*, was collected by Gustav de Vylder (Lee et al. 2015), probably during his stays in Cape Town in 1871–1873 and 1879–1885 (de Vylder 1998); no other published records of Omosita in the Afrotropical Region were found.

At least some sap beetles of the genus Omosita are relevant in forensic entomological and biosecurity contexts because they are occasional pests of stored products and can be abundant on carrion and corpses (Hinton 1945; Shubeck et al. 1977; Jelínek 1999; Kočárek 2003; Ewing and Cline 2005; Schlechter 2008; Saloña et al. 2010; Lee et al. 2015; Lyu et al. 2016; Torres et al. 2018; Lee et al. 2020). Several Omosita species have been translocated around the globe, e.g. the Palaearctic species Omosita colon (Linnaeus, 1758) and Omosita discoidea (Fabricius, 1775) have been reported from Australia and New Zealand (Blackburn 1903; Carlton and Leschen 2007); O. colon has been recorded on Pitt Island, 800 km east of New Zealand (Alfken 1904; Emberson 1998); and the Mexican species Omosita funesta Reitter, 1873 is reported from Spain (Audisio 1990). Despite their applied significance and widespread distribution, literature about the distribution of Omosita species is demonstrably scattered.

Omosita nearctica Kirejtshuk, 1987 was described from North America (Kirejtshuk 1987), but nothing has yet been published about its biology. Some North American records of O. colon published or identified before 1987 may actually represent O. nearctica, which was only recognised as a separate species in that year (Kirejtshuk 1987).

The collection of a single specimen of Omosita nearctica in August 2001 in South Africa suggested the introduction of O. nearctica to this country. This paper reports this discovery, confirms the breeding of O. nearctica in South Africa, and provides the first description of its larva.

This study presents the first record of Omosita nearctica in South Africa and confirms that it is established as a self-sustaining, breeding alien invasive species in the Eastern Cape Province of South Africa. The COI gene (Fig. 2) agreed with the morphological identification of the beetles as O. nearctica. The sequence from this study grouped together in a clade with three other sequences of O. nearctica that were separated from its sister clade, O. colon, with 100% posterior probability (Fig. 2). One sequence (BOLD Public Record COLON045-10.COI-5P) identified as O. nearctica grouped unambiguously with the O. discoidea clade.

The morphological character states listed in Table 3 may be used to differentiate the mature larva of O. nearctica from that of O. colon, the only other species of Omosita for which the larva has been described (Eichelbaum 1903; Verhoeff 1923; Hinton 1945; Böving and Rozen 1962; Hayashi 1978; Díaz-Aranda et al. 2018). The description of the urogomphi of O. colon by Hayashi (1978) differs from the description by Díaz-Aranda et al. (2018). Hayashi (1978) describes them as short and Díaz-Aranda et al. (2018) states they are half the length of the ninth tergite which appears to be more accurate (Fig. 9b). It must be noted that the pregomphi and urogomphi are all referred to as urogomphi by Díaz-Aranda et al. (2018). It is crucial to recognise that because O. nearctica was recognised only in 1987, prior references to O. colon larvae from the Nearctic (e.g. Hinton 1945; Böving and Rozen 1962) may be in error.

The collection of adults in Makhanda in 2001, 2012 and 2013 confirmed that O. nearctica has probably established in South Africa. This is important in the global context of this species as it has apparently never been recorded outside North America (GBIF.org 2020). The small size, furtive habits and internationally traded diets of sap beetles in general make them good candidates for transport around the world. For instance, at least 32 extralimital species have established in Europe (Jelínek et al. 2016). Most of these species feed on ripening and decaying fruit, but O. funesta was imported from Mexico to Teruel, Spain in 1931, “probably on imported sausages” (Jelínek et al. 2016). Species of Omosita are generally associated with human middens and animal remains (Hinton 1945; Shubeck et al. 1977; Jelínek 1999; Kočárek 2003; Ewing and Cline 2005; Schlechter 2008; Saloña et al. 2010; Lee et al. 2015; Lyu et al. 2016; Torres et al. 2018), so it is less obvious how they were transported to Africa. Omosita nearctica was probably introduced to South Africa on stored products imported through a port or airport. Given the age and remoteness of some austral introductions of Omosita (Blackburn 1903; Alfken 1904; Jelínek et al. 2016), it is possible that the population established well before it was discovered.

Their presence in Makhanda (E. Cape, RSA) suggests that they have been in South Africa for many years, since the town has no international airport and the nearest commercial harbours are over 120 km away. The failure to find specimens in Port Alfred (E. Cape, RSA) is ambiguous evidence of the species’ distribution because the sampling effort was limited.

Nothing is published about the biology of O. nearctica (Kirejtshuk, 1987). That O. nearctica larvae feed on cooked sheep bones suggests that this species feeds on saponified oils and decomposing material, like at least some other species in its genus (Lee et al. 2015; Lyu et al. 2016). In China, O. colon was the only species of beetle observed breaking down adipocere on corpses, potentially giving that species specific significance in medico-criminal forensic entomology (Lyu et al. 2016). The similar diets and close relationship (Fig. 2) of O. colon and O. nearctica imply that the latter species may be similarly useful (Midgley et al. 2009). Beetles associated with stored animal products are often relevant to biosecurity. Further studies on the biology of this species should monitor its global spread and determine its usefulness in forensic entomology. The description of the mature larva will assist in identifying this species where only larvae are found associated with stored products, corpses or carcasses.

We thank Lyndall Pereira for assistance with DNA extraction and amplification; Burgert Muller for assistance with editing of graphics; Gimo Daniel, Alexander Kirejtshuk, Burgert Muller, one anonymous reviewer and especially Riaan Stals for their valuable comments; and the National Research Foundation (NRF) of South Africa for funding. Any opinion, findings and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Research Foundation. Omosita nearctica is neither CITES-listed nor endangered according to regional Red Lists or South Africa’s Threatened or Protected Species (ToPS) legislation and thus did not require special sampling permission.

Key to adults of the species of the genus Omosita (based on diagnostic character states proposed by Reitter 1873, 1874; Kirejtshuk 1987; Jelínek 1999 and Lee et al 2015).