Abstract
Kairomones are chemical signals that mediate interspecific interactions beneficial to organisms that detect the cues. These attractants can be individual compounds or mixtures of herbivore-induced plant volatiles (HIPVs) or herbivore chemicals such as pheromones, i.e., chemicals mediating intraspecific communication between herbivores. Natural enemies eavesdrop on kairomones during their foraging behaviour, i.e., location of oviposition sites and feeding resources in nature. Kairomone mixtures are likely to elicit stronger olfactory responses in natural enemies than single kairomones. Kairomone-based lures are used to enhance biological control strategies via the attraction and retention of natural enemies to reduce insect pest populations and crop damage in an environmentally friendly way. In this review, we focus on ways to improve the efficiency of kairomone use in crop fields. First, we highlight kairomone sources in tri-trophic systems and discuss how these attractants are used by natural enemies searching for hosts or prey. Then we summarise examples of field application of kairomones (pheromones vs. HIPVs) in recruiting natural enemies. We highlight the need for future field studies to focus on the application of kairomone blends rather than single kairomones which currently dominate the literature on field attractants for natural enemies. We further discuss ways for improving kairomone use through attract and reward technique, olfactory associative learning, and optimisation of kairomone lure formulations. Finally, we discuss why the effectiveness of kairomone use for enhancing biological control strategies should move from demonstration of increase in the number of attracted natural enemies, to reducing pest populations and crop damage below economic threshold levels and increasing crop yield.
Highlights
The foraging behaviour of a parasitoid or predator is a process by which it searches for oviposition sites and feeding resources for its survival, growth, and reproductive success [reviewed in Kramer (2001)]
The identification of attractant compounds from herbivore-induced plant volatiles (HIPVs) and ovipositioninduced plant volatiles (OIPVs) is mostly specific to the tri-trophic system herbivore-plant-natural enemy and associated chemical interactions indicated in Figure 1 (Mumm and Dicke, 2010; Kaplan, 2012b; McCormick et al, 2012)
Kairomones have been validated for the recruitment of natural enemies in crop fields across diverse agro-ecosystems (James and Price, 2004; Tóth et al, 2009; Jones et al, 2016; Cai et al, 2017; Peñaflor, 2019)
Summary
The foraging behaviour of a parasitoid or predator is a process by which it searches for oviposition sites and feeding resources for its survival, growth, and reproductive success [reviewed in Kramer (2001)]. The identification of attractant compounds from HIPVs and OIPVs is mostly specific to the tri-trophic system herbivore-plant-natural enemy and associated chemical interactions indicated in Figure 1 (Mumm and Dicke, 2010; Kaplan, 2012b; McCormick et al, 2012). Unlike HIPVs or OIPVs, pheromones are species-specific (Aukema and Raffa, 2005; Ruther, 2013), and in most cases they are generally distinct from plant background odours and reliable indicators for natural enemies searching for herbivorous insects (Vet and Dicke, 1992; Rodriguez-Saona and Stelinski, 2009). The blend that contained benzaldehyde, a compound which dominated the tea background odour, was no longer attractive to tea leafhoppers in crop fields (Cai et al, 2017)
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