Colony-level non-associative plasticity of alarm responses in the stingless honey bee, Tetragonisca angustula

Olfactory Strategies in the Defensive Behaviour of Insects

Most animals must defend themselves in order to survive. Defensive behaviour includes detecting predators or intruders, avoiding them by staying low-key or escaping or deterring them away by means of aggressive behaviour, i.e., attacking them. Responses vary across insect species, ranging from individual responses to coordinated group attacks in group-living species. Among different modalities of sensory perception, insects predominantly use the sense of smell to detect predators, intruders, and other threats. Furthermore, social insects, such as honeybees and ants, communicate about danger by means of alarm pheromones. In this review, we focus on how olfaction is put to use by insects in defensive behaviour. We review the knowledge of how chemical signals such as the alarm pheromone are processed in the insect brain. We further discuss future studies for understanding defensive behaviour and the role of olfaction.

Defense in Social Insects: Diversity, Division of Labor, and Evolution

All social insects defend their colony from predators, parasites, and pathogens. In Oster and Wilson's classic work, they posed one of the key paradoxes about defense in social insects: Given the universal necessity of defense, why then is there so much diversity in mechanisms? Ecological factors undoubtedly are important: Predation and usurpation have imposed strong selection on eusocial insects, and active defense by colonies is a ubiquitous feature of all social insects. The description of diverse insect groups with castes of sterile workers whose main duty is defense has broadened the purview of social evolution in insects, in particular with respect to caste and behavior. Defense is one of the central axes along which we can begin to organize and understand sociality in insects. With the establishment of social insect models such as the honey bee, new discoveries are emerging regarding the endocrine, neural, and gene regulatory mechanisms underlying defense in social insects. The mechanisms underlying morphological and behavioral defense traits may be shared across diverse groups, providing opportunities for identifying both conserved and novel mechanisms at work. Emerging themes highlight the context dependency of and interaction between factors that regulate defense in social insects.

The Ant Who Cried Wolf? Short-Term Repeated Exposure to Alarm Pheromone Reduces Behavioral Response in Argentine Ants

Simple Summary

A significant challenge of chemical communication between ants is to maintain accurate communication of information in a variety of contexts. Argentine ants use volatile (airborne) compounds for a variety of functions, but one very important function is to elicit alarm via alarm pheromones. Given the importance of accurately responding to this signal, we expected Argentine ants to consistently show an alarm response to repeated exposure of alarm pheromones from their nestmates. However, we instead observed a reduction in their alarm behaviors over time. We speculate that a consistent response to repeated alarm signaling might require reinforcement from an actual alarming stimulus (e.g., the presence of predators or rival colonies). Argentine ants are considered a pest and several integrated pest management regimes use pheromones (i.e., mating disruption, aggregation pheromones, etc.) to reduce pest populations. Our results could be important to consider in the development of such control strategies because if ants habituate to their alarm pheromone over continuous exposure (without actually alarming stimuli) it might prove to be an ineffective strategy to repel them.

Abstract

In this study we test whether Argentine ants (Linepithema humile) progressively reduce their response to a salient stimulus (alarm pheromone) with increased exposure over time. First, we used a two-chamber olfactometer to demonstrate three focal behaviors of Argentine ants that indicate an alarmed state in response to conspecific alarm pheromone and pure synthetic iridomyrmecin (a dominant component of L. humile alarm pheromone). We then measured how these behaviors changed after repeated exposure to conspecific alarm pheromone from live ants. In addition, we investigate whether there is a difference in the ants’ behavioral response after “short” (3 min) versus “long” (6 min) intervals between treatments. Our results show that Argentine ants do exhibit reduced responses to their own alarm pheromone, temporarily ceasing their response to it after four or five exposures, and this pattern holds whether exposure is repeated after “short” or “long” intervals. We suggest alarm pheromones may be perceived as false alarms unless threatening stimuli warrant a continued state of alarm. These results should be kept in mind while developing pheromone-based integrated pest management strategies.

Changing of the guard: mixed specialization and flexibility in nest defense (Tetragonisca angustula)

Task allocation is a central challenge of collective behavior in a variety of group-living species, and this is particularly the case for the allocation of social insect workers for group defense. In social insects, both benefits and considerable costs are associated with the production of specialized soldiers. We asked whether colonies mitigate costs of production of specialized soldiers by simultaneously employing behavioral flexibility in nonspecialist workers that can augment defense capabilities at short time scales. We studied colonies of the stingless bee Tetragonisca angustula, a species that has 2 discrete nest-guarding tasks typically performed by majors: hovering guarding and standing guarding. Majors showed age polyethism across nest-guarding tasks, first hovering and then changing to the task of standing guarding after 1 week. Colonies were also able to reassign minors to guarding tasks when majors were experimentally removed. Replacement guards persisted in nest defense tasks until colonies produced enough majors to return to their initial state. Tetragonisca angustula colonies thus employed a coordinated set of specialization strategies in nest defense: morphologically specialized soldiers, age polyethism among soldiers within specific guarding tasks, and rapid flexible reallocation of nonspecialists to guarding during soldier loss. This mixed strategy achieves the benefits of a highly specialized defensive force while maintaining the potential for rapid reinforcement when soldiers are lost or colonies face unexpectedly intense attack.