Xylosandrus crassiusculus (Motschulsky) on Cocoa Pods (Theobroma cacao L.): Matter of Bugs and Fungi

SIMPLE SUMMARY: During standard monitoring activities aimed at preventing the insurgence of a pest infestation in a cocoa plantation in Karnataka (India), a sudden and unprecedented infestation of cocoa pods was recorded in 2021. The symptoms were the exudation of a brownish gummy substance from mature pods and the presence of pinhead-sized holes on the pod husk. Morphological and molecular characterization identified the cause of the symptoms to be the activity of the native ambrosia beetle Xylosandrus crassiusculus. Given the uniqueness of the attack, research was undertaken to investigate the causes of the infestation, describe the type of damage caused, and characterize possible fungal symbionts that allow the beetle to reproduce. Results showed that the infestation of X. crassiusculus was most likely triggered by previous attacks of Helopeltis theivora (Hemiptera: Miridae), a common pest that causes necrotic injuries to cocoa pods. Furthermore, the success of X. crassiusculus using cocoa as a reproductive substrate appeared to be dependent on its symbiotic fungi and yeasts, possibly involved in the detoxification or enrichment of the substrate, besides being the food source of the beetle. ABSTRACT: Exudation of mucilage from pinhead-sized boreholes in cocoa pods was recorded in Karnataka, India, during 2021. Further investigations showed the association of scolytine beetles with infested pods. The identity of the pest, Xylosandrus crassiusculus, was confirmed through morphological characterization and sequencing of the mitochondrial COI gene. We studied the predisposing factors for its infestation, visible and concealed damaging symptoms, and fungal symbionts. In addition to its well-known symbiotic fungus, Ambrosiella roeperi, a new association of yeast, Ambrosiozyma monospora, was discovered. We also traced the possible role of the mirid bug, Helopeltis theivora, in host selection by X. crassiusculus. Overall results indicated that a ‘mirid bug-ambrosia beetle–pathogen complex’ is responsible for the severe damage to cocoa pods in South India.