Klaus Jaffe

How plants shape the ant community in the Amazonian rainforest canopy: the key role of extrafloral nectaries and homopteran honeydew

Abstract. Ant-plant interactions in the canopy of a lowland Amazonian rainforest of the upper Orinoco, Venezuela, were studied using a modified commercial crane on rails (Surumoni project). Our observations show a strong correlation between plant sap exudates and both abundance of ants and co-occurrence of ant species in tree canopies. Two types of plant sap sources were compared: extrafloral nectaries (EFNs) and honeydew secretions by homopterans. EFNs were a frequent food source for ants on epiphytes (Philodendron spp., Araceae) and lianas (Dioclea, Fabaceae), but rare on canopy trees in the study area, whereas the majority of trees were host to aggregations of homopterans tended by honeydew-seeking ants (on 62% of the trees examined). These aggregations rarely occurred on epiphytes. Baited ant traps were installed on plants with EFNs and in the crowns of trees from three common genera, including trees with and without ant-tended homopterans: Goupia glabra (Celastraceae), Vochysia spp. (Vochysiaceae), and Xylopia spp. (Annonaceae). The number of ant workers per trap was significantly higher on plants offering one of the two plant sap sources than on trees without such resources. Extrafloral nectaries were used by a much broader spectrum of ant species and genera than honeydew, and co-occurrence of ant species (in traps) was significantly higher on plants bearing EFNs than on trees. Homopteran honeydew (Coccidae and Membracidae), on the other hand, was mostly monopolised by a single ant colony per tree. Homopteran-tending ants were generally among the most dominant ants in the canopy. The most prominent genera were Azteca, Dolichoderus (both Dolichoderinae), Cephalotes, Pheidole, Crematogaster (all Myrmicinae), and Ectatomma (Ponerinae). Potential preferences were recorded between ant and homopteran species, and also between ant-homopteran associations and tree genera. We hypothesize that the high availability of homopteran honeydew provides a key resource for ant mosaics, where dominant ant colonies and species maintain mutually exclusive territories on trees. In turn, we propose that for nourishment of numerous ants of lower competitive capacity, Philodendron and other sources of EFNs might be particularly important.

Chemical ecology of the palm weevil Rhynchophorus palmarum (L.) (Coleoptera: Curculionidae): attraction to host plants and to a male-produced aggregation pheromone.

Attraction to host plants by adultRhynchophorus palmarum (L.) palm weevils was studied in the field and in the laboratory. Chemical analysis revealed the presence of ethanol and ethyl-acetate in stems of coco palms and in pineapple fruits and of pentane, hexanal, and isopentanol in coco stems. In the olfactometer, the first two compounds and isoamyl-acetate were attractive to the insects and the last three compounds, although not attractive by themselves, increased attractiveness when mixed with the first two compounds. Mixtures of these compounds, in proportions similar to the one occurring in attractive plant tissue, were as attractive as natural coconut tissue. In the field, the chemical compounds, either presented alone or as a mixture, did not attract the weevil. Males produce an aggregation pheromone when smelling ethyl-acetate. Rhynchophorol, 2(E)-6-methyl-2-hepten-4-ol, the known active component of the aggregation pheromone, attracts weevils in the olfactometer and in the field only if plant tissue, ethyl-acetate, or the above-mentioned odor mix are present. We propose that a complex mix of ethanol, ethyl-acetate, pentane, hexanal, isolamyl-acetate, and/or isopentanol serve as a short-range orientation cue to fresh wounds on the plant and that additional host odors, attracting weevils from a distance, have still to be discovered. Rhynchophorol can be considered to be a Synergist, having an anemotactic action at a distance. We recommend the use of retention traps baited with rhynchophorol, ethyl-acetate, and sugar cane as an alternative control method for the pest.

Ant wars: combat strategies, territory and nest defence in the leaf-cutting antAtta laevigata

Two laws have been highlighted when discussing combat strategies in ants: the linear law (in a series of one-to-one conflicts, a few good fighters are better than many poor fighters) and the square law (if all individuals are vulnerable to attack, numerous fighters are more advantageous than a few good fighters). The leaf-cutting ant,Atta laevigata, responded to a simulated vertebrate threat by recruiting many soldiers (large workers), but responded to conspecific and interspecific ant threats by recruiting mainly small ants. In staged intraspecific interactions in the field between adjacent nests, ants were reluctant to fight on unmarked land and often retreated onto established marked trails, retreated to the nest entrance which they then capped or guarded, or immediately began marking the battle site with Dufours gland secretion. These results indicate that the territorial behaviour ofA. laevigatais part of a complex system to defend resources, where the defence of one resource (food) can develop into the defence of another (the nest). During the build-up of a war, once individuals from two nests met, recruitment changed from medium and large ants to mainly small ants. It is suggested thatA. laevigatauses the soldier caste to defend the nest against large organisms and recruits numerous small ants in response to conspecific and interspecific ant threats following the square law.

AN AGGREGATION PHEROMONE MODULATES LEKKING BEHAVIOR IN THE VECTOR MOSQUITO AEDES AEGYPTI (DIPTERA: CULICIDAE)

ABSTRACT Males of Aedes aegypti mosquitoes formed swarms in the laboratory, triggered by the onset of the photophase or by the presence of odors from a rat (which is a potential host for females). The swarm attracted both males and females and increased mating activity. The number of copulas per mosquito was positively correlated with the number of mosquitoes in the swarm and with the duration of the swarm. Swarming and mating activity increased with the presence of a host for females. Young sexually immature males, less than 24 h old, flew but did not swarm nor copulate. Observations using an olfactometer showed that swarming males produced a volatile pheromone that stimulates the flying activity of females at a distance. Females also produce a volatile attractant. The results suggest that males, and possibly also females, produce an aggregation pheromone that attracts males and females towards the swarm. The characteristics of the pheromone-mediated swarm may be described as a 3-dimensional lek. Our results suggest that the development of pheromone-based control systems and/or pheromone traps for the monitoring of vector populations is feasible, adding a new tool to combat this vector of several human pathogens.

On foraging, recruitment systems and optimum number of scouts in eusocial colonies

SummaryA numerical model of an eusocial colony foraging for food showed that, for each set of values of resource density, resource size and recruitment system employed, a given optimal proportion of scouts in the colony maximize the amount of resources retrieved by a colony during a fixed period. The model predicts that ants using mass recruitment systems should have larger colonies with small foragers, and should forage on large food sources. Retrieval of small food sources by small colonies is best achieved with large workers using individual foraging strategies. For mass foragers, several food sources are best retrieved using democratic decision-making systems in recruitment, whereas for very large food sources at very low mean food patch density, autocratic decision-making systems are optimal. Some of the experimental evidence available is discussed in the light of these findings, as they confirm the prediction that large colonies with small workers have mass recruitment systems, whereas workers of small colonies with large workers are generally lone foragers.

Sex ratio at birth deviations in modern Venezuela: The Trivers‐Willard effect

This study evaluates the impact of the Trivers-Willard (T-W) effect on human populations, using demographic data collected from vital registration data in Venezuela. The evaluation of the sex ratio at birth (SRB) and of fetal and infant deaths supports the existence of T-W effect in the Venezuelan population in extreme conditions. This T-W effect was observable in the SRB but not at later ages and is related to the marital status of the mother. The results indicate that the investment in females associated with environmental adversity is greater than the investment in males associated with good environmental conditions.

On Nest Densities of the Leaf-Cutting Ant Atta cephalotes in Tropical Primary Forest1

Atta nest densities in six undisturbed primary rain forest ecosystems in the Orinoco-Amazon basin have been estimated. The only species encountered was Atta cephalotes which occurred at a maximal density of 0.045 nests per ha. Three possible explanations for these low densities in contrast to those found in disturbed habitats are discussed. The number of clearings in the forest is proposed to be the limiting factor for colonization of new forest sites by A. cephalotes.

Colony-specific territorial marking with the metapleural gland secretion in the ant Solenopsis geminata (Fabr)

Abstract S. geminata workers mark the areas they explore with a secretion from the metapleural gland. The territorial mark lasts for more than 6 h. Territories without the chemical mark induce the workers to initiate recruitment to that territory. Ants on their own territory or on territories impregnated with metapleural gland extract from workers of their colony, initiate more intraspecific combats than ants on territories of a different colony or on territories without the chemical mark.

Nestmate recognition signals of the leaf-cutting ant Atta laevigata

Behavioral tests with field colonies of Atta laevigata were performed in order to identify the source of the odors used in nestmate recognition. We tested the postpharyngeal (PPG) and mandibular glands (MG) as putative organs producing chemical signals for nestmate recognition. Chemical analyses of PPG were also undertaken. With a series of bioassays, we confirmed that nestmate recognition is based on cephalic odors and that these odors come mainly from the mandibular gland secretion. We show chemical evidence that odors from MG are dispersed all over the cuticle. Although odors from PPG elicited colony-specific behavioral responses, the types of behaviors they elicited differed from those of nestmate recognition of whole ants or MG extracts. PPG secretion was characterized by long-chain alkanes and methyl branched alkanes of low volatility, whereas MG contained volatile ketones and alcohols.

Comparative Study of Brain Morphology in Ants

Brain morphology, with special attention to the three dimensional form of the corpora pedunculata, was studied in thirteen species of ants, representing four subfamilies of Formicidae. The results can be summarized as follows. (1) The neural systems processing optic and olfactory information differ in the evolutionary history among the studied taxa. A positive correlation can be demonstrated in the phylogenetic history of the corpora pedunculata, central body, cerebral bridge and olfactory lobe, but not the optic lobe. Ant species with very large eyes and thus, probably, highly developed vision, show gigantic optic lobes, with no exaggeration of any other brain structure. (2) More social species have more complex chemical communication systems and better developed corpora pedunculata (more surface in foldings of the calyces) and olfactory lobes; however, this tendency seems to reverse in highly social species with a sophisticated polymorphic caste system: individuals of these species are generally less developed neurally. (3) There are differences between the form of the internal and external calyces of the corpora pedunculata, and these differences are proportionately more pronounced in species with complex social organization. (4) Individuals from different worker castes of the same species differ in their brain morphology, but each species shows a different pattern of variation among their castes. (5) Brain structure shows characteristic marks due to the different phylogenetic developments in the taxa studied. For example the trends in brain volume-body size ratio are different for the various subfamilies, suggesting a divergent phylogenetic history.