Faunistic, ecological, biogeographical and phylogenetic aspects of Coleoptera as gall-inducers and associates in plant galls in the Orient and eastern Palearctic

Abstract

Gall-inducing insects are a cosmopolitan group of herbivores. In spite of a conspicuous habit, the taxonomy of gall-inducing insects, especially in the tropical and subtropical regions of the Orient and eastern Palearctic, is far from the ideal. A correlation between gall-inducing insects and host-plant richness in tropical-rain forests could possibly bridge the gap in the taxonomy of insects that induce galls and those that occur as associates with other gall-inducing arthropods. It is necessary that efforts are directed towards this exploration before we lose several of the fascinating galls, their inducers, and associated organisms. With regard to gall-inducing and gall-associated Coleoptera, available information is scattered, although ample literature deliberates upon their interactions other than gall induction. Many of the gall-inducing taxa are economically important, and several others occur as predators useful to humans as biological-control agents. Attempts to interpret the evolution of gall-inducing Coleoptera have been inconclusive, especially with regard to the Oriental and the eastern Palearctic elements. A cursory survey reveals that the main reason for the same is the complexity and inadequacy of available knowledge. Given this lacuna, in this paper, an attempt has been made to clarify the faunistics of coleopteran gall-inducers, interactions of Coleoptera with other gall-inducing arthropods, picking examples from the Indian subcontinent. Their usefulness in terms of biological control and their phylogeny and evolution, concentrating on select species from the Orient and eastern Palearctic fauna have also been included in this commentary. Close to 150 species of Coleoptera are known to be associated with plant galls. Major gall-inducing species fall under Buprestidae, Cerambycidae, Chrysomelidae, and Curculionoidea. Of these, the curculionids are the most diverse with close to 70 species. Thirty-six species are associated as inquilines, some of them feeding on gall tissues, some others being saprophagous on the decaying galls, or as predators on gall-inducing arthropods. A majority of the Carabidae and Coccinellidae are predators; Nitidulidae, Elateridae, and Tenebrionidae are saprophagous. The cotton-stem weevil Pempherulus affinis Faust, amaranthus-stem weevil Hypolixus truncatulus Fabricius, bean-gall weevil Alcidodes signatus Boheman, sesbania/dainchastem weevil Alcidodes bubo Fabricius, lissora-fruit weevil Baris cordiae Marshall and tamarix-gall weevil Nanophyes sp., are some examples given of gall-inducers that are particularly abundant in the Orient. Interactions of coleopterans with plant galls for biological control have been discussed with examples of Conotrachelus albocinereus Fiedler, Acythopeus burkhartorum O'Brien, and Smicronyx albovariegatus Faust. These examples have been considered mainly because plants like Coccinia grandis Voigt. (Cucurbitaceae), Parthenium hysterophorus Linnaeus (Asteraceae) and parasites like Striga sp., (Scrophulariaceae) is proliferating in the Orient and eastern Palearctic regions. No unique pattern among these associations is evident and hence no definite conclusions can be drawn on either their phylogeny or their coevolution with their respective host plants. However, the evolutionary trends indicate that gall-inducing habit seems to increase in the course of the evolution of Coleoptera, and became established in three phyletic lineages namely Buprestoidea, Chrysomeloidea, and Curculionoidea. The reconstruction of the phylogeny of Phytophaga and the role of angiosperms in beetle diversification has shown that repeated origins of angiosperm-feeding beetle lineages are associated with enhanced rates of beetle diversification, indicating a series of adaptive radiations. Combined evidence from the phylogeny estimates and from the fossil record shows a pronounced conservation in the evolution of beetle-plant association, which is important for the implication that the plants might escape herbivory via key innovative steps. Gall induction or other patterns of association of Coleoptera with plant galls could be such innovative steps towards coevolution. This proposition seems to hold good among the Coleoptera associated with plant galls in the Orient and eastern Palearctic too, similar to those in other parts of the world.