A colony of social insects is an excellent model for investigating the organization of responses of subunits (i.e. colony members) with limited skills into sophisticated collective behavior. The defence system of Lasius niger ant colonies is well organized in a context‐dependent way. The proportion of fighting ants to fleeing ants changes gradually according to the importance of the area being defended, and was higher where ants tended honeydew‐rich aphids and on trails for foraging with heavy traffic, than where ants were walking alone or on trails with light traffic. Although there were intrinsic differences in aggressiveness between individual ants, the differences in aggressive responses between defended areas was not due to the presence of highly aggressive or timid individuals in each area. Instead, it was due to a change in aggressiveness of individuals in response to external conditions. The cue that altered individual aggressiveness was the presence of surrounding nest‐mates, rather than the presence of aphids. We concluded that the defence system of this ant species consists of three processes: (i) a recruiting system that allocates more workers to more valuable resources; (ii) individual ants following a simple decision rule to become more aggressive in response to increased numbers of nest‐mates nearby (hence aggressive behavior reflecting the importance of each area to the colony); and (iii) variability in individual responses causing a gradual change in the proportion of fighting ants responding to a threat.

The efficiency of herbivore exclusion by ants on the vetch Vicia angustifolia L. (Leguminosae) with extrafloral nectary, mediated by ant attraction to aphids was investigated in a field census and laboratory experiments. In the field, workers of Lasius japonicus Santschi and Tetramorium tsushimae Emery frequently visited plants of the vetch parasitized by aphids of Aphis craccivora Koch, but only a few workers visited plants without aphids. An increase in the number of ants visiting a plant with increasing numbers of aphids caused a decrease in the number of larvae of the weevil, Hypera postica Gyllenhal. Therefore, the efficiency of herbivore exclusion by ants was higher on plants parasitized by Ap. craccivora aphids than that on plants unparasitized by aphids. In the laboratory experiments, L. japonicus workers frequently patrolled not only shoots with Ap. craccivora aphids but also shoots without them. However, T. tsushimae workers visited mainly shoots with Ap. craccivora aphids but less frequently on shoots without aphids. Therefore, L. japonicus workers excluded herbivores more efficiently on plants of the vetch than T. tsushimae workers. Consequently, the efficiency of herbivore exclusion by ants on the vetch can be influenced directly by differences in ant species and indirectly by the presence of aphids on plants. The present study highlights the significance of indirect interactions between ants and plants with extrafloral nectary, mediated by ant attraction to aphids for herbivore exclusion of plants.

No abstract

Foraging, defense and waste disposal are essential for sustaining social insect colonies. Hence, their nest generally has an open structure, wherein specialized castes called workers and soldiers perform these tasks. However, some social aphids form completely closed galls, wherein hundreds to thousands of insects grow and reproduce for several months in isolation. Why these social aphids are not drowned by accumulated honeydew has been an enigma. Here we report a sophisticated biological solution to the waste problem in the closed system: the gall inner surface is specialized for absorbing water, whereby honeydew is promptly removed via the plant vascular system. The water-absorbing closed galls have evolved at least twice independently among social aphids. The plant-mediated waste removal, which entails insect's manipulation of plant morphogenesis and physiology, comprises a previously unknown mechanism of nest cleaning, which can be regarded as ‘extended phenotype' and ‘indirect social behavior' of the social aphids.

Fungi in soil play pivotal roles in nutrient cycling, pest controls, and plant community succession in terrestrial ecosystems. Despite the ecosystem functions provided by soil fungi, our knowledge of the assembly processes of belowground fungi has been limited. In particular, we still have limited knowledge of how diverse functional groups of fungi interact with each other in facilitative and competitive ways in soil. Based on the high-throughput sequencing data of fungi in a cool-temperate forest in northern Japan, we analyzed how taxonomically and functionally diverse fungi showed correlated fine-scale distributions in soil. By uncovering pairs of fungi that frequently co-occurred in the same soil samples, networks depicting fine-scale co-occurrences of fungi were inferred at the O (organic matter) and A (surface soil) horizons. The results then led to the working hypothesis that mycorrhizal, endophytic, saprotrophic, and pathogenic fungi could form compartmentalized (modular) networks of facilitative, antagonistic, and/or competitive interactions in belowground ecosystems. Overall, this study provides a research basis for further understanding how interspecific interactions, along with sharing of niches among fungi, drive the dynamics of poorly explored biospheres in soil.