Pigeonpea (Cajanus cajan) responses to Helicoverpa armigera herbivory and simulated herbivory at different crop stages

Abstract

The ability of a plant to tolerate herbivory depends on the characteristics of the herbivore, the plant itself, and the environment. Understanding how crop plants tolerate pest herbivory is fundamental to development integrated pest management strategies. Pigeonpea (Cajanus cajan) is a major multi-purpose pulse crop cultivated throughout the tropics and subtropics. Pigeonpea yields are heavily constrained by insect pests that attack the crop during its reproductive stages. In this study we investigated the response of pigeonpea to herbivory at two stages of crop development: peak flowering and late podding. We compared how plants respond to herbivory by larvae of pigeonpea's key pest Helicoverpa armigera (Lepidoptera: Noctuidae) and to simulated H. armigera herbivory. Across two semi-field experiments we obtained slightly different results, suggesting that environmental conditions affected the plant responses to herbivory. At flowering, plants tolerated actual and simulated herbivory in the first experiment. But in the second, while plants tolerated simulated herbivory, actual herbivory resulted in yield loss. At podding, all herbivory treatments caused direct yield loss in the first experiment but in the second experiment simulated herbivory did not, although it increased the number damaged (i.e. unmarketable) seeds. Flowering plants were able to tolerate herbivory by redirecting yield to side branches and increasing mean seed weight, whereas there appear to be no tolerance mechanisms available for pigeonpea plants at the late podding stage. Future research should investigate how environmental factors influence tolerance expression, and such investigations will underpin the development of economic thresholds for H. armigera in pigeonpea.