Megalurothrips usitatus Directly Causes the Black-Heads and Black-Tail Symptoms of Cowpea along with the Production of Insect-Resistance Flavonoids

The thrip (Megalurothrips usitatus) damages the flowers and pods of the cowpea, causing “black-heads and black-tails” (BHBT) symptoms and negatively affecting its economic value. However, the mechanism by which BHBT symptoms develop is still unknown. Our results showed that the microstructure of the pod epidermis was altered and the content of the plant’s resistance-related compounds increased after a thrip infestation. However, the contents of protein and free amino acids did not change significantly, suggesting that the nutritional value was not altered. Pathogens were found not to be involved in the formation of BHBT symptoms, as fungi and pathogenic bacteria were not enriched in damaged pods. Two herbivory-induced flavonoids—7,4′-dihydroxyflavone and coumestrol—were found to exert insecticidal activity. Our study clarified that BHBT symptoms are directly caused by the thrip. Thresholds for pest control need to be reconsidered as thrip herbivory did not degrade cowpea nutrition.