D3217: Intercropping to enhance constitutive phytochemical defense against insect herbivory

Annual monoculture cropping systems can experience high pest pressure, often necessitating the use of pesticides for their control. Polyculture systems embody various mechanisms for pest-resistance including the ability to transfer nutrients and/or defensive signals to neighboring plants through arbuscular mycorrhizal fungi (AMF) networks. Interspecific positive allelopathy mediated by direct root contact or AMF networks could potentially be a relevant ecosystem function for pest reduction in diverse polycultures.

Silphium integrifolium (rosinweed; Asteraceae) is a native, perennial forb currently being domesticated as a perennial oilseed crop. Given its potent chemical defenses, it may also be useful for reducing pest pressure in polyculture systems. However, it is unclear whether S. integrifolium can alter the secondary metabolite profile of neighboring plants, nor whether these potential chemical interactions meaningfully affect pest insect performance.

To investigate whether interspecific positive allelopathy can alter defense response in biculture cropping systems, we conducted a series of greenhouse intercrop experiments where S. integrifolium, was grown in pots with either wheat (Triticum aestivum) or white sweetclover (Melilotus alba); each species was also grown with its conspecific. At 6 weeks growth intercropped and monoculture treatments were either infested with fall armyworm (Spodoptera frugiperda; Lepidoptera:Noctuidae) or were left as controls. We compared the volatile chemical emissions and leaf tissue extracts of infested plants against their uninfested counterparts within the intercropped and monoculture treatments. We also measured impact of intercropping on herbivore performance, by comparing fall armyworm mass gain between interspecific and intraspecific treatments. Results of herbivore performance and plant phytochemicals will be discussed.