Systematics of the tribes scymnini and stethorini (coleoptera: coccinellidae) from South India

Abstract

Family Coccinellidae, the lady beetles, belong to the superfamily Coccinelloidea of the order Coleoptera and comprises approximately 6000 described species worldwide. Tribes Scymnini and Stethorini of the subfamily Scymninae of Coccinellidae are economically important predators successfully used in the biological control programmes of sucking pests of crops. Members of Scymnini are mainly predators of aphids, mealybugs, whiteflies and scales, whereas Stethorini are specific to mites. Though the fauna of Scymnini and Stethorini in India is species rich, it is poorly studied. The present investigation on “Systematics of the tribes Scymnini and Stethorini (Coleoptera: Coccinellidae) from south India” was carried out during 2015-17. The objectives include the following: (1) taxonomy of the tribes Scymnini and Stethorini associated with sucking pests in different agricultural ecosystems of south India, (2) prepare a key to the species of Scymnini and Stethorini and (3) generate DNA barcode for different species of Stethorini. Purposive surveys were undertaken across different districts of Kerala, Karnataka and Tamil Nadu covering 47 locations and 64 crops under different agricultural ecosystems. Beetles and immature stages of Scymnini and Stethorini were collected along with the associated prey. Immature stages of Scymnini and Stethorini were reared to adults. The specimens were then mounted, labeled and preserved as per standard procedures. The beetles were dissected and the taxonomic characters viz., antenna, mouth parts, prosternum, tarsus, post coxal line, male and female genitalia were studied and illustrated. The specimens were identified up to species level. Descriptions, illustrations and key to genera and species of Scymnini and Stethorini of southern India were prepared. The study encompasses 28 species of Scymnini of which six are putative new species. Two species are recorded for the first time in India and two species each from south India and Kerala are new records. Scymnini include Axinoscymnus, The genera treated under Cryptolaemus, Horniolus, Nephus, Sasajiscymnus and Scymnus. The genus Scymnus, represented by 20 species, of which 18 species are placed in three subgenera viz., Scymnus, Neopullus and Pullus. Two species are treated separately, as these differ distinctly from the known subgenera. The genus Nephus is represented by three species, Axinoscymnus with two species and Cryptolaemus, Horniolus and Sasajiscymnus with one species each. Stethorini are represented by 10 species in two genera: Stethorus and Parastethorus. Two putative new species of the tribe are described. Six species of Stethorini are new reports for Kerala. Two subgenera recognized under the genus Stethorus are Stethorus and Allostethorus. represented by four species each. Stethorus keralicus, one of the most common species, has been treated separately in this study, as its characters are not in agreement with the known subgenera. The genus Parastethorus is represented by only one species. The prey ranges of Scymnini and Stethorini were documented. Species distribution map were prepared for the species studied. Twenty one species of prey in four families viz., Aphididae, Pseudococcidae, Aleyrodidae and Diaspididae were recorded in association with the tribe Scymnini. Two new prey records for Scymnini are Toxoptera odinae for Scymnus pyrocheilus and Saccharicoccus sacchari for Nephus tagiapatus. Among Scymnini, Scymnus coccivora and S. saciformis are widely distributed with wider host range of six species as prey, while species of Axinoscymnus, though widely distributed, is specific to whiteflies. Among Stethorini, Stethorus pauperculus was found to be the predominant species with wider host range, while Stethorus keralicus was specific to Raoiella indica. New prey records for Stethorini includes Tetranychus okinawanus and T. truncatus for Stethorus forficatus; T. macfarlanei for S. pauperculus and Eutetranychus orientalis for Parastethorus indira. For barcoding the species of Stethorini, DNA was isolated using Qiagen DNeasy blood and tissue kit and the COI locus was amplified and sequenced. The sequences were aligned and characteristic barcode gaps were identified for Parastethorus indira, Stethorus forficatus, S. pauperculus, S. rani, S. keralicus, S. (Allostethorus) sp. 1 and S. (Stethorus) sp.1. Pairwise distances between the sequences were analysed which showed that intraspecific divergence ranged between 0.00 to 0.03, while the interspecific distance ranged between 0.14 to 0.24. A phylogenetic tree was constructed with 21 sequences in MEGA 7 using the maximum likelihood tree method. The sequences were submitted to GenBank (NCBI) and to BOLD for the generation of species specific barcodes. The study identified 38 species of predatory coccinellids in the tribe Scymnini and Stethorini in association with aphids, mealybugs, whiteflies, scales and mites, which are serious pests of crops. Knowledge on the taxonomy of these predators and their prey range throws light on the potential of the above groups in biocontrol of sucking pests.