The use of extrafloral nectar in pest management: overcoming context dependence

Abstract

1. Extrafloral nectar (EFN) provides plants with indirect defence against herbivores by attracting predatory insects, predominantly ants. Decades of research have supported the role of EFN as an effective plant defence, dating back to Thomas Belt's description of ants on acacia in 1874. 2. Despite this extensive body of literature, knowledge of the ecological role of EFN has rarely been applied in the field of pest management. We review the existing literature on the use of EFN in agriculture and consider the obstacles that have hindered this transition. 3. Chief among these obstacles is the influence of ecological context on the outcome of EFN-mediated interactions. As such, we consider the options for various agricultural systems in the light of the growth habit of EFN-producing species, focusing first on orchard species and then on herbaceous crops. In each case, we highlight the benefits and difficulties of utilizing EFN as a pest management tool and of measuring its efficacy. 4. Synthesis and applications. We argue that it is time for a shift in extrafloral nectar (EFN) research towards applied settings and seek to address the question: How can a context-dependent and often inducible plant trait be utilized as a reliable tool in agricultural pest management? Breeding crops for increased EFN production, and intercropping with EFN-producing plants, can enhance assemblages of beneficial insects in many agricultural settings. Orchard systems, in particular, provide an ecological context in which the attraction of ants can contribute to cost-effective and sustainable pest management programmes over a broad geographic range.

Managing Floral Resources in Apple Orchards for Pest Control: Ideas, Experiences and Future Directions

“…Alfalfa was not flowering in this experiment. Eventually, benefits from alfalfa for natural enemies in this study would thus have been from alternative prey and any extrafloral nectaries [46,52]. In another study in China, enhanced populations of the anthocorid Orius sauteri (Poppius) in the cover-cropped area (alfalfa and Lagopsis supina (Steph.…”

“…Moreover, vetches (e.g., Vicia sepium L. and Vicia sativa L.,) offer carbohydrates in plant sap from special structures, so-called extrafloral nectaries (EFN) [45] in addition to floral nectar (Figure 2a). These EFN glands are assumed to deliver resources to attract predators in order to create some kind of indirect defence against pests [46] and they can be easily exploited due to their exposed nature. For instance, parasitoid Hymenoptera are known to visit EFN of faba bean [47].…”

“…Additional innovative approaches including non-crop vegetation hold great potential for sustainable apple production. The involvement of EFN-bearing plants belongs to such a concept [46]. EFN-bearing plants are not only interesting because they feed natural enemies.…”

New Tools for Conservation Biological Control: Testing Ant-Attracting Artificial Nectaries to Employ Ants as Plant Defenders

“…Finally, ant activity in agroecosystems can be easily manipulated, for example, by transferring colonies or offering additional food sources and nest sites (e.g., [47,48]). In addition, the manipulation of ant foraging pathways may redirect foragers towards target pests, and can be conducted using plants with EFNs [49].…”

Reduced Responsiveness to Volatile Signals Creates a Modular Reward Provisioning in an Obligate Food-for-Protection Mutualism

“…Although being reasonable (Agrawal and Rutter, 1998 ), surprisingly little evidence has been reported to support this assumption. In fact, the defensive effects of EFN secretion are highly context-dependent (Heil, 2015 ; Jones et al, 2017 ) and EFN secretion can even be counterproductive if ants start to exclude other, more efficient defenders Koptur et al, 2015 ). Inducing EFN secretion with exogenous JA increased the number of defending ants and decreased the number of herbivores showing up on Macaranga tanarius plants (Heil et al, 2001 ), and similar patterns were found on P. lunatus tendrils that were exposed to VOCs or treated with JA to enhance EFN secretion (Kost and Heil, 2008 ).…”