Diomus was originally established by Mulsant (1850) as a subgenus of Scymnus Kugelann, 1794, based on unique abdominal postcoxal lines that meets the posterior margin of the first abdominal ventrite. Weise (1895) first elevated Diomus to generic status, based on the median fusion of the first and second abdominal ventrite and the peculiar form of the abdominal postcoxal lines. Since then, Diomus has been treated either as a subgenus of Scymnus or a valid genus by various researchers. Gordon (1999) proposed the tribe Diomini to include Diomus and four other Neotropical genera according to a comprehensive study of South American Coccinellidae. Ślipiński (2007) recorded the genera Diomus, Dichaina Weise, 1923 and proposed a new genus Andrzej Ślipiński in this tribe from Australia.

Diomus guilavoguii Duverger, 1994 was described from Guinea by Duverger (1994) and not was discovered in other regions. According to our observation, D. guilavoguii preys mainly on the invasive mealybug, Paracoccus marginatus Williams & Granara de Willink, 1992 (Hemiptera, Pseudococcidae), which infested with papaya and many different kinds of ornamental plants, such as Mussaenda pubescens W. T. Aiton, Barbados nut (Jatropha curcas L.) and Spicy Satropha (Jatropha integerrima Jacq.). Some of these host plants are imported from abroad. It is obvious that D. guilavoguii, as a natural enemy of P. marginatus, has great potential for biological control.

The papaya mealybug, P. marginatus is a globally invasive pest that causes significant yield losses in various crops (Mani et al. 2012). Since its original description in 1992, P. marginatus as a native species of Mexico and Central America has spread rapidly to many other countries including USA, Africa and Asia (Miller and Williams 1999, Ahmed et al. 2015, Finch et al. 2021). It was recorded from China for the first time in 2013 in Guangdong and Yunnan Provinces (Ahmed et al. 2015). The papaya mealybug not only directly affects plant growth by sucking sap, but also indirectly affects the development of sooty mould on plants by excreting honeydew, hindering photosynthesis and gas exchange (Kirsur et al. 2014, Ahmed et al. 2015).

Although the papaya mealybug is native to Mexico and Central America, it is not a pest there due to the presence of its natural enemies keeping it under control. However, when it invaded other countries or regions, it was considered a serious pest mainly due to the lack of natural predators (Ahmed et al. 2015). Saengyot and Burikam (2011) carried out field surveys of host plants in Thailand to investigate the host plants and natural predators of the papaya mealybug. Their investigation indicated that 11 species of natural enemies controlling the papaya mealybug’s populations either by parasitism or predation, including five coccinellids, Cryptogonus orbiculus (Gyllenhal, 1808), Sasajiscymnus quinquepunctatus (Weise, 1923), Scymnus (Pullus) quadrillum Motschulsky, 1858, Scymnus sp. and Stethorus sp. (Saengyot and Burikam 2011). Mani et al. (2012) mentioned that a total of 22 natural enemies were reported on papaya mealybug in different countries. Amongst them, eight species belonged to ladybird beetles (Coleoptera, Coccinellidae), such as Cryptolaemus montrouzieri Mulsant, 1853, Nephus bilucernarius (Mulsant, 1850), Scymnus taiwanus (Ohta, 1929) (= Scymnus (Pullus) quadrillum, Motschulsky, 1858), Brumoides suturalis (Fabricius, 1798), Hyperaspis silvestrii Weise, 1909, Cheilomenes sexmaculata (Fabricius, 1781), Coccinella transversalis Fabricius, 1781 and Chilocorus nigritus (Fabricius, 1798). These ladybird beetles are playing a key role in biological control of the papaya mealybug.

In this paper, Diomus guilavoguii Duverger, 1994 is recorded as being well established in Guangzhou for the first time. Detailed descriptions and illustrations of the adult, pupa and larvae are provided. Diagnostic characters for the genus and species are also given.

Specimens examined were collected from Guangdong Province, China and deposited in the Department of Entomology, South China Agricultural University, Guangzhou, China (SCAU). The terminology used in the descriptions of larva follows Kamiya (1965) and Gordon and Vandenberg (1991) and the descriptions of adults follow Ślipiński (2007) and Ślipiński and Tomaszewska (2010).

Measurements were taken using a micrometer attached to a SteREO Discovery V20 dissecting stereoscope and are defined as follows: (TW) total width, across both elytra at widest part; (TH) total height, at highest part of elytra in lateral view; (TL) total length, from apical margin of clypeus to apex of elytra; (PL) pronotal length, from the middle of anterior margin to the base of pronotum; (PW) pronotal width at widest part; (EW) elytral width, equal to TW; (EL) elytral length, along suture from base to apex including scutellum; (HW) head width, at widest part including eyes.

Male and female genitalia were dissected, cleared in a 10% solution of sodium hydroxide (NaOH) by boiling for several minutes and placed on slides for further study. Photographs of the adult were taken with a digital camera (EOS 5D Mark IV, Canon) and photographs of their genitalia were taken using digital cameras (ZEISS Imager M2 and Axiocam 506 Color) attached to the microscope.

Larvae were reared in 500 ml plastic dishes in rearing chambers at 25 ± 1℃, 70% ± 10% R.H. and 12:12 h L:D. The food, Paracoccus marginatus, was supplied daily to maintain the population stock (Fig. 1).

For morphological studies, the larvae were soaked in 75% alcohol for conservation. The larvae were photographed using digital cameras (EOS 5D Mark IV, Canon), attached to a focus stacking rail (WeMacro Rail). The software Helicon Remote and Helicon Focus were used to capture and render images respectively from the camera. Mouthparts, head and tarsal claw of the larvae were dissected, cleared in a 10% NaOH solution, boiled for half an hour, washed in distilled water and placed on slides. Colour images were captured with digital cameras (ZEISS Imager M2 and Axiocam 506 Color) attached to a dissecting microscope using ZEN 2.3 software. All photographs were edited using Adobe Photoshop CC 2018 and Adobe Illustrator 2020.

Diomus guilavoguii is recorded from China for the first time. The specimens examined in the present paper show some minor variation in antennae however, their external appearance of adult and male genitalia were in agreement with the detailed descriptions and illustrations given by Duverger (1994). Interestingly, the antennae of the specimens collected from Guangzhou have only 10 antennomeres, while the type specimens described by Duverger (1994) have 11 antennomeres. This phenomenon is also present in some species of the genus Diomus from Australia, with some species having 11 antennomeres and a few individuals occasionally having only 10 antennomeres (Pang and Ślipiński 2009, Pang and Ślipiński 2010, Vandenberg and Hanson 2019).

D. guilavoguii closely resembles D. hennessyi Fürsch, 1987 from Zaire, Africa in external appearance and male genitalia. Fürsch (1987) described D. hennessyi, based on the specimens collected from Zaire and Nigeria, showing that the penis guide with a horned projection at the proximal apex, especially the peculiar shape of the penis capsule which matches well with the illustration given by Duverger (1994) (Fig. 5). Gordon (1999) provided more detailed descriptions of D. hennessyi, based on the examination of a large series of specimens from South America and Africa, including the type series of this species. He also pointed out that D. hennessyi is native to South America, but it was introduced into some areas of Africa for biological control of cassava mealybug, Phenacoccus manihoti Matile-Ferrero, 1977 (Gordon 1999). Unfortunately, neither Fürsch (1987) nor Gordon (1999) provided the illustrations of the whole penis. Based on the above mentions, we considered that D. guilavoguii may be a synonym of D. hennessyi. However, since we have not been able to examine the holotype of D. hennessyi deposited in the Musée royal de l'Afrique centrale, Tervuren, Belgium and D. guilavoguii housed in the Museum National d’Histoire Naturelle, Paris, France, further confirmation is needed regarding the relationship between these two species.

There have been no deliberate introductions of D. guilavoguii in China. When and how did this species spread to Guangdong Province, China? As aforementioned, P. marginatus is native to Mexico and Central America. Although this invasive pest has also been reported in Africa, we suspect that D. guilavoguii was likely spread to Guangdong, China from Central America along with P. marginatus. However, its diffusion path remains to be further studied.