In various organisms, parents invest a large amount of energy to supply food and take care of their brood. Some of them raise their young by exploiting nests of conspecifics or heterospecifics to reduce their energy consumption (Davies et al. 1989). This phenomenon, called social parasitism, is well known in birds such as cuckoos (Payne 1977) and social insects such as ants, bees, and wasps (Wilson 1971).

Social parasitism is classified into intraspecific or interspecific and facultative or obligate. Facultative parasitism is an alternative strategy to rear the brood, while obligate parasitism is a complete dependence on another species for this purpose (Cervo 2006). Among social insects, several polistinae species and Vespa dybowskii (Vespidae) are well known as social parasites (Matsuura and Yamane 1984, Cervo and Dani 1996, Carpenter 1997, Cervo et al. 2004, Cervo 2006).

Among Polistinae, obligate social parasitism is present in Polistes sulcifer, Polistes atrimandibularis, and Polistes semenowi (Carpenter 1997), whereas facultative social parasitism is present in the following pairs of species: the nests of Polistes fuscatus are usurped by Polistes apachus (Snelling 1952), those of Polistes instabilis are usurped by Polistes canadensis (O’Donnell and Jeanne 1991), those of Polistes versicolor by Polistes lanio, (Giannotti 1995), those of Polistes dominulus by Polistes nimphus (Cervo et al. 2004), and those of Polistes gallicus by Polistes donimulus (Cervo and Dani 1996). This social parasitism occurs at a rate of 0.3-7.6% in the field (Cervo et al. 2004). Elaborate intruder’s tricks such as morphological, chemical, and behavioral similarities to the host account for the mechanism of social parasitism (Dapporto et al. 2004, Cervo 2006, Cini et al. 2011, Montagna et al. 2012).

In South Korea, cases of social parasitism have been reported in V. dybowskii and Vespula austriaca (Choi et al. 2013), but little is known about social parasitism among Polistes species. In this study, we report for the first time social parasitism found in South Korea between two Polistes species.

Two P. djakonovi nests were observed in the rural areas of Machi-ri (N 35°47′35.05″ E 127°15′30.48″), Wanju-gun, Jeonbuk Province (nest 1) and Bongsan-myeon (N 36°9′5.77″ E 128°4′0.57″), Gimcheon-si, Gyungbuk Province (nest 2), South Korea.

The inquilines in the two nests were P. djakonovi and P. mandarinus, but Nguyen and Kojima (2014) have suggested that P. mandarinus may need reidentification in Korea. Therefore, we use the term Polistes sp. until identification of this species is confirmed. The images of specimens were taken using a Leica M125 stereomicroscope (Leica MICROSYSTEMS, Wetzlar, Germany) equipped with a Leica DFC450 camera (Leica MICROSYSTEMS).

Polistes djakonovi Kostylev and Polistes sp. are widespread in South Korea and are frequently collected in forests, rural areas, and green suburban areas. These species nest on shrub branches (Choi et al. 2012a, 2012b, 2014).

In this study, two nests of P. djakonovi exploited by Polistes sp. were found, representing a case of interspecific facultative social parasitism. Nest 1 (65 cells) contained 5 females of P. djakonovi and 4 females of Polistes sp. on 30 July 2014. Nest 2 (102 cells) contained 12 females and 16 males of P. djakonovi and 3 females of Polistes sp. on 28 August 2013 (Fig. 1).

Most cases of social parasitism occur during the preemergence period (Cervo et al. 2004). The timing of the life cycle and nesting sites have to be matched between the intruder and host for social parasitism to take place (Cervo 2006, Montagna et al. 2012). We believe that social parasitism between these two species is possible because their life cycles and nesting sites overlap in South Korea. Although we found the two nests in July and August, P. djakonovi seems to have been exploited by Polistes sp. at the end of the preemergence period (early and mid-June).

Typical P. djakonovi nests have white cocoon caps (Fig. 2a), whereas those of Polistes sp. have yellow cocoon caps (Fig. 2b). The two nests found in this study had mainly white cocoon caps with several yellow ones (Fig. 2c), which is a clear evidence of social parasitism.

Social parasitic species are more phylogenetically closely related to their hosts than to other species (Emery 1909, Lowe et al. 2002). Therefore, morphological similarities between the two Polistes species must have been a prerequisite for social parasitism. P. djakonovi and Polistes sp. are very similar in their morphology except the differences in the mark on the clypeus and their size (Fig. 3).

To usurp its host, an intruder usually needs to have a larger body size than the host; even if the intruder has a smaller body than the host, body parts used for fighting such as the head, first femur, and posterior tibia are more developed (O‘Donnell and Jeanne 1991, Cervo 1994, Giannotti 1995, Cervo and Dani 1996, Cervo et al. 2004). However, the results for the two species in this study were opposite: the host had a lager body than the intruder (Fig. 3a and 3b). Although this study could not fully address the mechanism of the relationship between these social parasitic species, their ecology should be studied in the future.