New Synthesis: Plant Volatiles as Functional Cues in Intercropping Systems

Abstract

The practice of intercropping has begun to show its enormous potential to translate discoveries in chemical ecology to improve crop pest control and reduce dependence on pesticides (Hassanali et al., 2008). However, the actual mechanism by which intercropping works remains elusive, and the proposed plant-chemistry based hypothesis was recently challenged (Finch and Collier, 2012). A common view proposes that intercrops come in two functional groups, trap plants and repellent plants, which have chemical characteristics making them attractive or repellent to a certain insect pest, thereby reducing pest pressure on the main crop. To identify the exact underlying mechanisms is crucial for the broad application of intercrop pest control strategies.The search for generalizable mechanisms, as reflected in the three studies profiled in this essay, becomes apparent when an intercrop system is as successful in controlling insect herbivores as is the push-pull approach used to control cereal pests in Africa. This approach is based on the use of behavior-modifying stimuli that reduce damage by stem boring Lepidoptera (Noctuidae, Busseola fusca or Crambidae, Chilo partellus) in maize. Maize is intercropped with the legume Desmodium uncinatum or the grass Melinis minutiflora, both of which dramatically reduce plant damage by repelling (pushing) the adult stem-borer moths out of the field, while Napier grass (Pennisetum purpureum) is planted around the field edges to attract (pull) gravid females away from maize. The combined use of these two types of stimuli reduces stem borer damage by two thirds and at least doubles maize yields in comparison with control plots (Hassanali et al., 2008).The success of this approach has been attributed to release of attractive and repellent semiochemicals by the "push" and "pull" companion plants, respectively. Identification of specific volatile organic compounds (VOC) through GC-EAG and corroboration of their behavioral significance suggest attractive as well as repellent functions of intercrop VOCs (Hassanali et al., 2008). One major prediction of this hypothesis is that plant chemicals (e.g. VOCs) act as long-range cues that result in a non-random movement/distribution of pest insects, with reduced densities on the main crop.Finch and Collier (2012) in a recent critique of the mechanism underlying the push-pull system, contrast the functional VOC hypothesis with appropriate/inappropriate landing theory, which assumes random herbivore movement and oviposition/consumption responses based on host plant stimuli that are experienced by insects after landing. In this view, all short-and long range physical and chemical plant characteristics are potentially important for host choice by herbivores, rendering the two hypothetical frameworks non-exclusive. Nevertheless, the controversy reveals major research questions for the future, of which two are particularly apparent:First, Finch and Collier (2012) argue that given the vast diversity of plant VOCs, it would be maladaptive for herbivores to per...

Volatile Exchange between Undamaged Plants - a New Mechanism Affecting Insect Orientation in Intercropping

“…A role for plant volatiles has been both questioned [13] and supported [14]. However, while direct effects of host volatiles on insects have been considered [15], the possibility that volatile interaction between plants can affect insect host choice through changes in the receiving plant has not been addressed.…”

Initialism hypothesis: can gibberellin synthesis inhibitor increase the oat crop competitive ability?

“…The facilitation principle is based on at least four hypotheses. The "stress gradient hypothesis" proposes that different species offer protection against abiotic stresses (Lortie & Callaway, 2006); whereas protection against biotic stresses are justified by the "trap crop hypothesis" (Poveda & Kessler, 2012), or by the "push-pull hypothesis" (Hassanali, 2008), or by the "natural enemy hypothesis" (Hooksa et al 2010). However, no other variables were evaluated in our field experiments to provide evidence for any of these hypotheses.…”

Electrophysiological and oviposition responses of Asian corn borer, Ostrinia furnacalis (Lepidoptera: Crambidae), to compounds rinsed from the surfaces of sugarcane and maize leaves

“…Plant volatiles have crucial roles in mediating insect behaviour such as host plant location and acceptance by phytophagous insects, attracting parastoids or predators of insect pests (Khan et al, 1997), inducing insects to oviposit and influencing plant-plant interactions (Huang et al, 2009;Konstantopoulou et al, 2004;Steinbauer et al, 2004;Ruther & Kleier, 2005). It is proposed that intercrops fall in two functional groups, trap plants and repellent plants, which have chemical characteristics that make them attractive or repellent to certain insect pests, thereby reducing pest pressure on the main crop (Khan et al, 2000;Poveda & Kessler, 2012). Ethological control by applying infochemicals to disrupt feeding, mating and oviposition behaviour may have an important role in such a system (Shelton & Badenes-Perez, 2006;Cook et al, 2007;Zehnder et al, 2007;Guo & Li, 2009).…”