Ants suppressing plant pathogens: a review

Manipulated ants: inducing loyalty to sugar feeders with an alkaloid

BACKGROUND

Wood ants are promising biocontrol agents in fruit plantations because they prey on pest insects and inhibit plant diseases. However, these ants also attend plant-feeding homopterans to harvest their honeydew secretions, thereby increasing their numbers. This problem can be solved by offering ants alternative sugar sources that are more attractive than honeydew. From natural interactions, it is known that some species manipulate mutualistic partners toward loyalty by adding alkaloids to the food they offer their mutualists. Inspired by this, the addition of alkaloids might be used to make ants loyal to artificial sugar feeders and thus used to reduce populations of ant-farmed homopterans in ant-mediated biological control. We aimed to explore whether wood ants (Formica polyctena) would develop a taste preference for morphine-containing sugar solutions in two-choice laboratory tests.

RESULTS

After having fed on a morphine/sugar solution for 1 week, ants showed a significant preference for morphine solutions compared with equal concentration sugar solutions without morphine. Furthermore, ants lost this preference after 6-9 days on a morphine-free diet.

CONCLUSION

The results show that wood ants react to morphine in their food, enabling chemical manipulation of their behavior, most likely through a taste preference. Thus, ants are susceptible to manipulation by mutualistic partners in natural interactions and furthermore may be manipulated artificially in biocontrol programs to avoid ant-mediated build-up of homopteran populations. © 2024 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Foraging ants affect community composition and diversity of phyllosphere fungi on a myrmecophilous plants, Mallotus japonicus

Many microorganisms inhabit the aboveground parts of plants (i.e. the phyllosphere), which mainly comprise leaves. Understanding the structure of phyllosphere microbial communities and their drivers is important because they influence host plant fitness and ecosystem functions. Despite the high prevalence of ant-plant associations, few studies have used quantitative community data to investigate the effects of ants on phyllosphere microbial communities. In the present study, we investigated the effects of ants on the phyllosphere fungal communities of Mallotus japonicus using high-throughput sequencing. Mallotus japonicus is a myrmecophilous plants that bears extrafloral nectaries, attracting several ant species, but does not provide specific ant species with nest sites like myrmecophytes do. We experimentally excluded ants with sticky resins from the target plants and collected leaf discs to extract fungal DNA. The ribosomal DNA internal transcribed spacer 1 (ITS1) regions of the phyllosphere fungi were amplified and sequenced to obtain fungal community data. Our results showed that the exclusion of ants changed the phyllosphere fungal community composition; however, the effect of ants on OTU richness was not clear. These results indicate that ants can change the community of phyllosphere fungi, even if the plant is not a myrmecophyte.

Ant identity determines the fungi richness and composition of a myrmecochorous seed

Myrmecochory-seed dispersal by ants-is a mutualistic interaction in which ants attracted by seed appendices take them away from the parental plant location, where seeds usually have better development odds. Not all ant species benefit plants, and the mechanisms of those divergent outcomes are still unclear, especially from the perspective of microbial third parties. Here, we explore the effects of seed manipulation on fungi communities promoted by two ant species with contrasting effects on seed germination and antimicrobial cleaning strategies. We hypothesize that: i) fungi richness is higher in seeds manipulated by Acromyrmex subterraneus (species that negatively affect seed germination), followed by unmanipulated seeds and seeds manipulated by Atta sexdens (ant species that increase seed germination) and ii) seeds manipulated by A. sexdens, Ac. subterraneus and unmanipulated seeds present dissimilar fungi compositions. We identified fungal morphotypes in three groups of seeds: i) manipulated by A. sexdens; ii) manipulated by Ac. subterraneus; iii) unmanipulated. Seeds manipulated by Ac. subterraneus exhibited higher fungal richness than those manipulated by A. sexdens and unmanipulated seeds, indicating that the ant species known to impair germination increases the fungal load on seeds. Additionally, we found that A. sexdens ants were unable to reduce fungal richness compared to unmanipulated seeds. Furthermore, fungal composition differed among all three treatments. Our results underscore the significance of ant species identity in shaping the fungal communities associated with myrmecochorous seeds. Given the potential influence of microbial infection on seed fate, we suggest considering manipulation strategies when evaluating the overall quality of an ant as a seed disperser.

Fight and rescue or give up and flee? Behavioural responses of different ant species tending the mutualist walnut aphid Panaphis juglandis to native and exotic lady beetles

Mutualism between ants and honeydew-producing hemipterans is a highly successful evolutionary innovation that attains the status of ecological keystone across many terrestrial ecosystems, involving a multitude of actors through direct or cascading effects. In these relationships, ants often protect their hemipteran partners against their arthropod natural enemies, sometimes interfering with the biological control of pest species. However, the dynamics of these interactions are highly variable based on the specific identity of all the actors involved, and baseline data remain scarce. We performed a field experiment exposing colonies of the walnut aphid Panaphis juglandis attended by five European ant species (Camponotus piceus, Ca. vagus, Crematogaster scutellaris, Dolichoderus quadripunctatus, Lasius emarginatus) to a native and an exotic lady beetle (Adalia bipunctata and Harmonia axyridis), documenting the behavioural interactions between these insects and the performance of ants in the protection of the aphids. Our results reveal a significant behavioural diversity among the ant species involved, with D. quadripunctatus and L. emarginatus being the most aggressive and having the best performance as aphid defenders, and Ca. piceus being least effective and often fleeing away. Cr. scutellaris displayed a rare rescue behaviour attempting to pull away the aphids that the lady beetles grabbed. On the other hand, behavioural responses to A. bipunctata and H. axyridis were similar. Further investigations are needed to understand the eco-ethological implications of these differences, while a better understanding of ant behavioural diversity may help refine biological control strategies.

Implementing wood ants in biocontrol: Suppression of apple scab and reduced aphid tending

BACKGROUND

Ants can become efficient biocontrol agents in plantation crops as they prey on pest insects and may inhibit plant pathogens by excreting broad-spectrum antibiotics. However, ants also provide a disservice by augmenting attended honeydew producing homopterans. This disservice may be avoided by offering ants artificial sugar as an alternative to honeydew. Here we tested the effect of artificial sugar feeding on aphid abundance in an apple plot with wood ants (Formica polyctena, Förster), and tested the effect of ant presence on apple scab (Venturia inaequalis, Cooke) disease incidence.

RESULTS

Over a 2-year period, sugar feeding eliminated ant-attended aphid populations on the apple trees. Furthermore, scab symptoms on both leaves and apples were reduced considerably on ant trees compared to control trees without ants. The presence of ants on the trees reduced leaf scab infections by 34%, whereas spot numbers on fruits were reduced by between 53 and 81%, depending on apple variety. In addition, the spots were 56% smaller.

CONCLUSION

This shows that problems with wood ant-attended homopterans can be solved and that ants can control both insect pests and plant pathogens. We therefore propose wood ants as a new effective biocontrol agent suitable for implementation in apple orchards and possibly other plantation crops. © 2023 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Predatory Abilities of Two Mediterranean Ants on the Eggs and Larvae of the Codling Moth Cydia pomonella

The predatory ability of ants (Hymenoptera, Formicidae) against insect pests can offer an important service to agricultural activities and may sometimes be directly exploited in biological control strategies. The codling moth Cydia pomonella (Lepidoptera, Tortricidae) is a major agricultural pest of fruit orchards, whose biological control is complicated by the fact that the larvae spend most of their life protected within the fruits they damage. In a recent experiment in Europe, pear trees in which ant activity was artificially increased by the addition of sugary liquid dispensers (artificial nectaries) suffered less damage caused by the larvae to their fruits. While some ants were already known to prey upon the mature larvae or pupae of C. pomonella in the soil, prevention of fruit damage would require predation upon eggs or newly hatched larvae, which have not yet excavated into the fruits. We verified whether two different Mediterranean ants frequently observed in fruit orchards, Crematogaster scutellaris and Tapinoma magnum, were able to prey upon C. pomonella eggs and larvae in laboratory conditions. Our experiments demonstrated that both species similarly attacked and killed young C. pomonella larvae. On the other hand, the eggs mostly attracted the attention of T. magnum but were never damaged. Further field assessments are required to understand whether ants may also interfere with oviposition by adults or whether larger ant species, although generally rarer in orchards, may also prey upon eggs.

An approach to the plant body: Assessing concrete and abstract aspects

The paper aims at proposing a representation of plants as individuals. The first section selects the population of plants to which this study is addressed. The second section describes the effective architecture of plants as modular systems with fixed and mobile elements, in other words, plants and their extensions. The third section presents how plants integrate the fixed and mobile modules into functional units through three areas of particular relevance to plant growth and development: nutrition, defence and pollination. Based on the tangible elements introduced in the previous sections, the fourth section presents the main issue of the proposal which is not apparent at first glance, namely, the local-global relationship in plants' architecture that determines their individuality as organisms. Finally, in the conclusion, we issue the challenge of developing a collective presentation of plants which satisfies their complementary dimension.

Gram-negative bacteria associated with a dominant arboreal ant species outcompete phyllosphere-associated bacteria species in a tropical canopy

Ants have efficient and well-studied social immunity mechanisms, which prevent the colony contamination. Little is known about how workers keep their outside territory clear of diseases. We investigated the interactions between Azteca chartifex ants, their associated bacteria and bacteria on the phyllosphere of Byrsonima sericea trees, comparing plants patrolled and not by the ants. The hypothesis is that bacteria associated with the worker's exoskeleton may outcompete the leaf bacteria. Culturable bacteria were isolated from ants, from the main and satellite nests, and from phyllosphere of B. sericea taken from trees that had A. chartifex nests and from trees without nests. The isolates were grouped by Gram guilds and identified at the genus level. There was a higher percentage of Gram-negative isolates in the ants and on the leaves patrolled by them. There was a higher growth rate of ant bacteria from the main nest compared to those found in ants from the satellite nests. The most representative genus among ant isolates was Enterobacter, also found on leaves patrolled by ants. Under favourable in vitro conditions, A. chartifex Gram-negative bacteria outcompete leaf bacteria by overgrowth, showing a greater competition capacity over the Gram-positive bacteria from leaves with no previous interaction with ants in the field. It was demonstrated that ants carry bacteria capable of outcompeting exogenous bacteria associated with their outside territory. The leaf microbiota of a patrolled tree could be shaped by the ant microbiota, suggesting that large ant colonies may have a key role in structuring canopy plant-microbe interactions.

First Report on the Acrobat Ant Crematogaster scutellaris Storing Live Aphids in Its Oak-Gall Nests

Simple Summary

Galls represent an amazing microcosm which contains a variety of multiple interactions among different actors, and therefore, offers the opportunity to observe and investigate phenomena belonging to different areas of biology: from the development process, connected to the interaction between the galligenous agent and the host plant, to the moment of their colonization by different species, since some ants may provide defense against pathogens, certain phytophagous insects or favor mutualists. In the present work we describe some aspects of oak-gall colonization by different ant species, highlighting how the gall’s height on the plant influences ant colonization and how different ant species produce different nest architectures. The most relevant aspect, however, is the discovery of a novel ant-aphid relationship: the transport of living aphids into oak-gall nests. We found no evidence of immediate predation of these aphids inside the galls, so they are likely stored to overwinter due to a mutualistic relationship and/or serve as food storage. This is not only an interesting report on the mutualisms involving ants and their insect partners, but it may also have important consequences on the aphids’ phenology with the host plants. Once more, ants show their relevant impact on multitrophic interactions and ecosystem dynamics.

Abstract

This study provides new data about the role of ants in mutualistic interactions with aphids mediated by galls. We focused our investigation on galls induced by the cynipid Andricus kollari by conducting a survey and a subsequent experiment in an Italian oak forest. The ants Crematogaster scutellaris, Colobopsis truncata and Temnothorax italicus frequently used the galls as nests: Crematogaster scutellaris occupied galls which were located higher on the oak trees, while C. truncata and T. italicus were located at lower positions. In addition, galls occupied by C. scutellaris showed varied internal architecture in relation to the colony composition. Importantly, field surveys revealed for the first time that C. scutellaris nest galls also contained live individuals of the non-galligenous aphid Panaphis juglandis. Field experiments suggested that the ants actively seek, collect and stock live aphids. No signs of predation and injuries were detected on the stored aphids, which were probably kept for safe overwintering, though we cannot exclude a possible occasional use as food. This report reveals a possible novel relationship which could have important consequences on the phenology and presence of aphids on the host plant.

The effect of edaphic factors on the distribution and abundance of ants (Hymenoptera: Formicidae) in Iran

The current study is aimed at investigating the effect of edaphic factors on the distribution and abundance of ants in different habitats of the central areas of Iran, while considering the vegetation. During 2018 to 2019, 20 stations from four habitats, including deserts, mountainous and submontane, plains and rural areas and urban areas, were selected. In general, a total of 311 sample units were collected from all the stations, out of which, 32 species belonging to 13 genera, nine tribes and three subfamilies were identified. The biological distribution and abundance of species were argued by computing the physical and chemical parameters of the soil, such as salinity, pH, total nitrogen, organic carbon, calcium and vegetation. The present study has demonstrated that the calcium content significantly affects the species richness of ants, although the impact of this element on various genera is different. We found that increasing in the abundance and richness of plant species has a positive impact on the abundance and richness of ants. Our results also show that some genera are meaningfully adaptable to a variety of habitats. In Kahak station, which is an urban habitat, with enormous diversity, 14 species were found, while in Sadrabad Historic Karvansara, a desert habitat, only Cataglyphis lividus (André, 1881) was collected. Cataglyphis bellicosus (Karavaiev, 1924), as the most abundant species, collected from 12 stations, was the most dominant species.

The ecosystem services provided by social insects: traits, management tools and knowledge gaps

Social insects, i.e. ants, bees, wasps and termites, are key components of ecological communities, and are important ecosystem services (ESs) providers. Here, we review the literature in order to (i) analyse the particular traits of social insects that make them good suppliers of ESs; (ii) compile and assess management strategies that improve the services provided by social insects; and (iii) detect gaps in our knowledge about the services that social insects provide. Social insects provide at least 10 ESs; however, many of them are poorly understood or valued. Relevant traits of social insects include high biomass and numerical abundance, a diversity of mutualistic associations, the ability to build important biogenic structures, versatile production of chemical defences, the simultaneous delivery of several ESs, the presence of castes and division of labour, efficient communication and cooperation, the capacity to store food, and a long lifespan. All these characteristics enhance social insects as ES providers, highlighting their potential, constancy and efficiency as suppliers of these services. In turn, many of these traits make social insects stress tolerant and easy to manage, so increasing the ESs they provide. We emphasise the need for a conservation approach to the management of the services, as well as the potential use of social insects to help restore habitats degraded by human activities. In addition, we stress the need to evaluate both services and disservices in an integrated way, because some species of social insects are among the most problematic invasive species and native pests. Finally, we propose two areas of research that will lead to a greater and more efficient use of social insects as ES providers, and to a greater appreciation of them by producers and decision-makers.

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

Knowledge of the role of ants in many agroecosystems is relatively scarce, and in temperate regions the possibility to exploit ants as biocontrol agents for crop protection is still largely unexplored. Drawing inspiration from mutualistic ant–plant relationships mediated by extrafloral nectaries (EFNs), we tested the use of artificial nectaries (ANs) in order to increase ant activity on pear trees and to evaluate the effects on the arthropods, plant health and fruit production. While EFNs secrete a complex solution mainly composed of sugars and amino acids, ANs were filled with water and sucrose only. The results suggest that ANs can be used as manipulative instruments to increase ant activity over long periods of time. High ant activity was significantly linked to lower incidence of the pathogen fungus Venturia pyrina (pear scab) on pear leaves, and of the presence of Cydia pomonella (codling moth) caterpillars on pear fruit production. These results further encourage exploring underrated possibilities in the development of new tools for conservation biological control (CBC).