Different species interact with each other in complex and diverse ways. Why and how do we study these interspecies interactions? How do we introduce students to these interactions through real-world observations of plants, aphids, and ants from their own backyards?
Children begin observing and wondering about interspecies interactions in and around their home and school long before their science textbook introduces them to related concepts. Giving students the opportunity to study some of these interactions may help teachers connect abstract concepts in the textbook to real-world observations from their backyard. For example, have you come across a plant in your garden, school compound, or fields that seems to be covered with tiny creatures? Does it look like these parts of the plant are bustling with ants? If your answer to both these questions is ‘yes’, then you may be at the right place to witness a fascinating three-way interaction between ants, aphids, and plants (see Box 1).
Box 1. Finding and observing plant–aphid–ant interactions:
Interactions among plants, aphids, and bursts, flowering, and fruiting). Since the for ants on plant parts and tracking their ants are often opportunistic or facultative environmental conditions in winter limit movement can help provide important in nature.
This means that ants may or the activities of aphids and ants, such clues to the location of such interactions. may not form associations with the aphids interactions are best observed in summer Many a time, aphid infestation can also be throughout the year. The aphid – ant (March/April – July) and post-monsoon seen on the apical portions of host plants. association is strongly dependent on the (September/October – November).
While once such an interaction is identified, availability of food resources, seasonality, such interactions may be difficult to the plant – aphid – ant interaction can be requirements of the ant colony, and observe on high branches or tall trees, they observed for a variety of features (see phenology of the host plant (changes in can be easily observed on 3 – 5 m tall plants Checklist for observation of plant–aphid– the timing of seasonal events like bud or on branches at a lower height. Looking ant interactions).
The plant – aphid interaction
As you may have guessed, the tiny white, yellow, green, or black creatures are insects called aphids (see Fig. 1).
Like mealybugs, whiteflies, and plant hoppers, aphids have needle-like mouth parts (called stylets) that they inject into the tender parts of the plant to feed upon the phloem sap that the plant makes for its own nourishment (see Box 2).1 The loss of nutrition caused by these phytophagous (phyto= plants, phagy = to eat) or sap-sucking insects, affects the health of the plant, and causes wilting and yellowing of its parts. It may also affect the reproductive fitness of the plant, reducing its fruit and seed set.
Aphids may also act as vectors of deadly plant viruses such as cucumber mosaic virus and potato virus. These viruses can enter their host plant through the saliva of a feeding aphid. Weakened by the aphid infestation, the plant becomes more susceptible to these and other diseases. Since the aphid benefits at the expense of the plant, the plant – aphid interaction offers a real-world example of an antagonistic (parasitic) interaction.
The aphid – ant interaction
While the aphids feed on phloem sap, they excrete droplets of a sticky, sugary, nutrient-rich liquid, called honeydew, from their rectum. This attracts certain species of ants to the host plant. How? Studies show that these ants ‘smell’ the presence of certain volatile organic compounds (VOCs) in the honeydew through sensory organs called olfactory lobes that act like our nose (see Box 2).
These VOCs are produced by the action of specific bacteria that reside in the inner gut wall of the aphids.2 The ants feed on the honeydew, and herd and tend to the aphids (see Fig. 2). Some ant species also deter the natural enemies of aphids (see Box 2).3,5
These could include the larval and adult stages of insects such as ladybird beetles, hoverflies, and parasitic wasps that either feed on the aphid or lay eggs within its body (see Fig. 3). Since both partners benefit from this association, the aphid – ant interaction offers a real- world example of mutualism.3, 4, 5 This type of mutualism is referred to as ‘food for protection’ mutualism.5
Box 2. Glossary:
- Phloem sap: A nutrient-rich food resource produced in plants. The name is derived from the mode of transportation that occurs through the phloem (a pipeline for transporting food across different parts of the plant). Rich in sugars and amino acids, it provides nourishment for the growth and development of the plant.
- Volatile Organic Compounds (VOCs): Chemical compounds that rapidly evaporate on contact with air. Secreted by organisms during interspecies interactions, these chemicals help in interspecies communication.
- Natural enemies: The organisms that prey on or parasitize a particular species.
- Myrmecophilous aphids: Derived from the combination of two words — myrmeco meaning ‘ants’, and phily meaning ‘loving’, this term refers to aphid species that are tended to by ants.
However, like many other interspecies interactions, the ant-aphid association is more complex and dynamic than it may seem. Most ants are opportunistic foragers — eating almost anything they can find to fulfill the needs of their colony. This may lead one to wonder if ants also prey on aphids?
They do, when their protein requirement exceeds their carbohydrate requirement (which can be met by honeydew). Under such circumstances, the aphid – ant interaction changes from being mutualistic to becoming antagonistic. However, studies also suggest that ants prefer to prey upon aphid species that are non-myrmecophilous rather than those that are myrmecophilous (see Box 3).6
Box 3. Identifying the ant and myrmecophilous aphid species in your backyard:
For ants, this may be helpful: https://www.antweb.org/
For aphids, check these out: http://aphid.aphidnet.org/credits.php or https://influentialpoints.com/Blog/How_to_identify_aphids_from_ photos – the_basics.htm
This raises the question — how do ants distinguish between the two kinds of aphids? Studies suggest that the body odour of every aphid has species-specific chemical signatures known as cuticular hydrocarbons (CHCs). Depending on the nature of these CHCs, the relationship between an aphid species and its tending ant species may be obligatory (both species depend entirely on, and cannot survive without services from each other) or facultative (the ant species is partly dependent on the aphid species). CHCs also help a partner ant species to differentiate between myrmecophilous and non- myrmecophilous aphids.7
The plant – ant interaction
Recent studies on interactions between plants, ants, and aphids have shown that some species of ants protect the host plant against other non-sap-sucking insect herbivores such as caterpillars and beetles.8
Other studies show that accumulation of honeydew can attract fungal infection. By cleaning plant parts of honeydew, ants protect the host plant against these infections.9, 10 This suggests that despite the harm caused by aphids to the host plant, the presence of protective ants might save the plant from further damage.
Detailed studies on interspecies interactions like the plant – aphid – ant interaction have helped reveal the dynamics and complexity of such interactions in general, as well as their role in maintaining ecological balance.
For example, several studies show that when ants are blocked from access to aphids (using a sticky insect barrier, like Tanglefoot), aphid colony size reduces. This also causes an increase in abundance of the natural enemies of aphids and herbivory on host plants, which reduces both aphid and host plant fitness. The excluded myrmecophilous ants tend to show a preference for insect prey, suggesting a shift from carbohydrate-seeking to protein-seeking foraging behaviour.
Introducing students to some of the most interesting insights that ecologists have learnt about plant – aphid – ant interactions through scientific investigation can spark their interest and curiosity in related life science topics from their curriculum. It can also strengthen and expand their understanding of the nature and process of scientific inquiry.
Here’s a checklist for observation of plant-aphid-ant interactions:
- Real-world observations of plant – aphid – ant interactions in their immediate surroundings can be used to introduce students to textbook concepts around interspecies interactions and their role in maintaining ecological balance.
- The plant – aphid relationship offers a relatable example of an antagonistic interaction, while the aphid – ant relationship offers a fascinating example of a mutually beneficial interaction.
- Since the aphid – ant interaction remains mutualistic only as long as the benefit that the ant gains from it is high, the context-dependent nature of this interaction can be used to illustrate the complex and dynamic nature of interspecies interactions.
- Sharing details of the kind of experiments used to understand these interactions can help expand student understanding of the nature and process of scientific inquiry.
Acknowledgements: The authors are grateful to the Department of Biotechnology (DBT), India for funding (Project No. DBT-NER/Agri/24/2013 dated 30/03/2015). We are indebted to our collaborators — the National Centre for Biological Science (NCBS), Indian Institute of Science (IISc.) and University of Agricultural Science (UAS), Bangalore for organizing workshops and training programs for students. Our sincere thanks go to Dr. Shannon B. Olsson, Dr. Joyshree Chanam, Dr. Lucy Nongbri, Dr. Dibyendu Adhikari, Dr. Raghuvar Tiwary, Satyajeet Gupta, and Anita Gupta for their valuable suggestions during the preparation of the manuscript. We are thankful to Prof. Uma Ramakrishnan, Dr. Dhruba Sharma, and Dr. Arkamitra Vishnu for coordinating and providing assistance whenever required. The authors would like to extend their sincere thanks to the Head, Department of Botany, North-Eastern Hill University, Shillong, and Director, National Botanical Research Institute, Lucknow for providing necessary facilities.
Source of the image used at the beginning of the article: https://commons.wikimedia.org/wiki/File:Ant_guards_its_Aphids.jpg#/media/File:Ant_guards_its_Aphids.jpg
Credits: viamoi, Wikimedia Commons. License: CC-BY.
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- Fischer CY, Lognay GC, Detrain C, Heil M, Grigorescu A, Sabri A, et al. Bacteria may enhance species association in an ant – aphid mutualistic relationship. Chemoecology. 2015; 25(5): 223 – 32.
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- Lang, C., and Menzel, F. Lasius niger ants discriminate aphids based on their cuticular hydrocarbons. Animal Behavior 2011; 82: 1245 – 1254.
- Buckley RC. Interactions involving plants, Homoptera, and ants. Ann Rev Ecol Syst. 1987; 18: 111 – 35.
- Renault CK, Buffa LM, Delfino MA. An aphid – ant interaction: effects on different trophic levels. Ecol Res. 2005; 20(1): 71 – 74. https://doi.org/10.1007/s11284-004‑0015‑8
- Völkl W, Woodring J, Fischer M, Lorenz MW, Hoffmann KH. Ant-aphid mutualisms: the impact of honeydew production and honeydew sugar composition on ant preferences. Oecologia. 1999; 118: 483 – 91.
About the Authors
Rudra Prasad Banerjee is a doctoral student at CSIR-National Botanical Research Institute (CSIR-NBRI), Lucknow where he studies spatio-temporal variations in the tri-trophic interaction between plants, aphids, and ants. He can be contacted at firstname.lastname@example.org
Renee M Borges is Professor at the Centre for Ecological Sciences at Indian Institute of Science (IISc), Bangalore. Her research team is interested in understanding the coevolutionary dynamics between plants, insects, other invertebrates, and microbes. She can be reached at email@example.com.
Saroj Kanta Barik is Director of CSIR-National Botanical Research Institute (CSIR-NBRI), Lucknow. His research interests include ecology and environment, conservation biology, and chemical ecology. He can be reached at firstname.lastname@example.org.
Prem Prakash Singh is pursuing his doctoral degree at Department of Botany, North-Eastern Hill University, Shillong. His research is related to the impact of climate change on plant diversity and conservation of threatened plants. He can be contacted at email@example.com.
Madhoolika Agrawal is Professor in the Department of Botany at Banaras Hindu University (BHU), Varanasi. She studies the physiological adaptations of important agronomic plants in the context of global warming and climate change. She can be reached at: firstname.lastname@example.org.