Raiding behavior of the dulotic antChalepoxenus muellerianus (Finzi) in the field (Hymenoptera: Formicidae,Myrmicinae)

Summary Little information is available regarding the raiding behavior in nature of dulotic ants belonging to the tribeLeptothoracini. Between July 17 and 24, 1991, several raids ofChalepoxenus muellerianus (Finzi) were observed in nature near Tignale/Lago di Garda, Italy. Apparently, this species raids frequently during its summer raiding season (5 raids were observed during 7 days spent observing 8 colonies). A singleChalepoxenus colony sometimes raids more than one host colony more or less simultaneously. Observations during which oneChalepoxenus colony raided another and captured slavemaker brood indicate that intraspecific raids can occur either accidentally, or as a result of competition or territoriality when there is a sufficient dense slavemaker population.     

Life cycle and social behavior in a heathland population of the allodapine bee,Exoneura bicolor (Hymenoptera: Apidae)

Summary Previous studies of the allodapine beeExoneura bicolor have utilised a predominantly social population from tall montane forests. In this report we examine aspects of the sociality and biology of a population from subcoastal heathland in western Victoria, Australia. Entire colonies, nesting inXanthorrhoea minor, were collected at monthly intervals over 12 months between February 1989 and January 1990. In addition, three collections fromMelaleuca squarrosa were made over three consecutive winter months. The heathland population differs from the montane population in several aspects associated with sociality: (i) it shows lower levels of co-occupancy in both newly founded and re-used nests; (ii) foundress dispersal occurs over a larger proportion of the year; and (iii) there may be a partical second brood produced in late summer, during and after adult eclosion of the first brood. The occurrence of a partial second brood gives rise to opportunities for sib-rearing and eusociality. Colony sizes tend to be larger inM. squarrosa thanX. minor, but still smaller than colonies from the montane population. Egg-laying commences several weeks earlier in multifemale colonies than in single female colonies, before spring dispersal begins. It appears that multifemale colonies are more successful at preventing brood loss than single females and that the mean number of brood per female is larger in multifemale colonies than in single female colonies. These two populations, which display differences in several life history traits, provide an ideal system for studying the mechanisms maintaining social behavior.     

Intraspecific slavery inPolyergus rufescens Latr. (Hymenoptera,Formicidae): field and laboratory observations

Summary Four intraspecific slave raids involving 3 colonies of the European amazon antPolyergus rufescens Latr. occurred during hot and sunny afternoons in July 1992 near Parma, Italy. Generally, no fighting between the resident ants and the invaders was recorded, and pillage of the brood lasted just a few minutes. The brood captured during 3 raids was collected and transported ot the laboratory where slavemaker callow workers eclosed and were accepted by both the raiders and hosts present in artificial mixed colonies. This is the first case of functional intraspecific slavery recorded in the field for the obligatory dulotic formicine antP. rufescens.     

An experimental analysis of pleometrotic advantage in the desert seed-harvester antMessor pergandei (Hymenoptera; Formicidae)

Summary Starting colonies of the desert seed-harvester antMessor pergandei are clumped in the field and face severe intraspecific competition through brood raiding. Single foundress laboratory colonies ofM. pergandei are more likely to succeed at brood raiding with conspecific colonies if they are given additional workers and mature pupae several days prior to brood raiding. Per foundress fecundity remains constant across laboratory starting colonies established with 1, 3 and 5 foundresses. These results suggest that the selective advantage of cooperative colony foundation (pleometrosis) in this and similar species may derive directly from the ability of multiple foundresses to produce a larger brood raiding force.     

Ecology of spiders inhabiting abandoned mammal burrows in South African savanna

Summary We marked the sites chosen by 338 foundress queens of two desert ant species (Veromessor pergandei and Myrmecocystus flaviceps) and monitored changes in the spacing of both species and the foraging activity at V. pergandei young nests. Although the long established colonies of both species tend intraspecifically toward regular dispersion, queens of both species were intraspecifically clumped. After 3 months, when the first workers emerged, the young colonies (reduced to a total of 42 colonies) were randomly spaced intraspecifically. We also followed the spatial patter of queens with respect to established colonies of both species. Queens founded nests away from the nests of all established colonies on the site. After three months, the young colonies were dispersed away from conspecifics only. During June through August 1986, we censused the number of foragers at the surviving V. pergandei nests. Young colonies that were more active also tended to be far from established conspecifics in July and August. There was no correlation of forating activity with distance to heterospecific established colonies in any month. These results indicate that established conspecifics may reduce the survivorship of young ant colonies.     

<Emphasis Type="Italic">Camponotus punctulatus</Emphasis> ant’s demography: a temporal study across land-use types and spatial scales

Agricultural activities promote the explosion of diverse pest populations. In Argentina, the ant Camponotus punctulatus invades agricultural fields after production ceases. The temporal demography and spatial distribution of colonies of C. punctulatus were studied over a five year period using replicated plots of different land use types representing a gradient of increasing agricultural disturbance. We experimentally tested the hypothesis that the increase in C. punctulatus colony density was related to increasing levels of agricultural disturbance. Abandoned rice fields represented the situation with greatest disturbance. Sown pastures were intermediate. Natural grasslands represented no agricultural disturbance. The predictions were (1) the greater the soil disturbance produced by agriculture, the greater the susceptibility for invasion by C. punctulatus, (2) rice fields offers greater opportunities for establishment of colonizing species than sown pastures, and (3) disturbed land use areas that were more recently colonized as well as land use areas with greater soil disturbance will exhibit patterns of colony aggregation at a small scale but with time the patterns will become uniform. Initially, colonies in the abandoned rice fields had a higher annual mortality and larger turnover than in sown pastures. Over five years, abandoned rice fields sustained higher densities of colonies than sown pastures. The colonies were the largest and had the longest lifespans in abandoned ricefields. Natural grasslands had the lowest colony density, survivorship, and size but had variable levels of colonization. More than one type of spatial distribution was found in field replicates. At small spatial scales across disturbed land use types, replicates exhibited regular distributions. At greater spatial scales, spatial distributions were mostly random in sown pastures, there were many cases of aggregation in rice fields, although some cases of uniform distributions were also found in all disturbed land uses. These results highlight significant intraspecific variation in ant demography across types of land use, space, and time, and show a clear predisposition of C. punctulatus to invade and successfully establish in the most disturbed land use types. Hypotheses that can account for the changes in demography across land use types are discussed. Received 20 January 2006; revised 3 August 2006; accepted 26 October 2006.     

Brood raiding and the population dynamics of founding and incipient colonies of the fire ant, Solenopsis invicta

Abstract. 1. One of the first activities of minim workers in incipient fire ant nests is mutual brood raiding, the amalgamation of nests through the reciprocal stealing of brood and defection of workers.
2. Discrete mating flights created cohorts of incipient colonies. About 25% of founding nests survived the claustral period of 16–48 days (depending on season). Early incipient colony mortality was 5–6% per day. Over 60% of this mortality was accounted for through brood raiding. Most colonies raided when each cohort first became active, and many raided more than once. Raid size (number of nests, duration, distance) tended to increase during the summer as colonies grew through raiding. After the raiding period, nest mortality rate dropped 3–10-fold. Only 1–3% of founding nests were still alive at this time.
3. Queens from losing or failed nests tended to abandon their nests and attempted to enter successful ones, often following raiding trails to do so. This emigration was at least as successful as non-emigration in ultimately achieving the status of reproductrix of a successful nest (about 4%).
4. Brood raiding is a dominant process in early population dynamics, probably accounting for most of the early nest mortality. Its effect is to change the venue and unit of competition from nest-against-nest to a shifting aggregation of queens, workers and brood involving entire local populations. Nest thinning is thus very rapid, and the boost to the size of winning nests very large, allowing raiding colonies to win the competition for territory, and to achieve the early colony maturity so important to this, and other, weedy species. The importance of winning brood raids may also have driven increased minim production through the evolution of pleometrosis.     

Effect of Crown Asymmetry on Size-Structure Dynamics of Plant Populations

The effect of crown asymmetry on the size–structure dynamicsof populations was evaluated using a spatial competition modelincorporating crown asymmetry. Computer simulations were carriedout with various combinations of density levels, spatial patterns,and degrees of asymmetry in competition to assess how they modifythe effect of crown asymmetry on size–structure dynamics. In the model, crown asymmetry is expressed by the crown-vector,or the vector linking the stem base and the centre of the projectedarea of the crown on the horizontal plane. Crown-vectors areassumed to develop in the manner by which crowns repel eachother. As crown-vectors develop, the positions of the crown-centresmove. Competition between individuals is expressed by a neighbourhoodmodel, in which individual growth is determined by the distancefrom, and size of, the neighbours' crown-centres. Generally, populations of individuals which developed asymmetriccrowns had larger survivorship, larger mean size, smaller coefficientsof variation and skewness, and a more regular spatial patternthan populations of individuals which developed symmetric crowns.The effect of crown symmetry is generally stronger in populationswith high density and a clumped spatial pattern. The effectof mortality caused by one-sided competition on size-structuredynamics was similar to that of crown asymmetry; mortality increasedmean size, reduced size hierarchy, and made the spatial patternmore regular. Because mortality was heavier in populations withoutcrown asymmetry, its effect on size-structure dynamics cancelledout, or overwhelmed, the effect of crown asymmetry in latergrowth stages. If crown asymmetry is associated with a reductionin growth, the effect of crown asymmetry is reduced. Nevertheless,the resultant population structure is different from that ofpopulations without crown asymmetry. Competition; crown asymmetry; morphological plasticity; neighbourhood interference model; size-structure dynamics     

Changing colony growth rates inCamponotus floridanus as a behavioral response to conspecific presence (Hymenoptera: Formicidae)

Previous work with the antCamponotus floridanus demonstrated that perception of competition can be clearly differentiated from effects of mortality and decreased resources. That is, brood biomass in ant colonies decreases as a consequence of a behavioral decision(s) rather than because of limited food availability or reduced numbers of brood tenders. The experiments presented here extend that work. Under experimental conditions, colony growth inC. floridanus is modified by distance between brood and unrelated conspecifics and by worker age distribution. When nonnestmates are encountered at the nest versus at a separate foraging site, less brood is maintained by a colony. Although colonies with older workers maintain a brood biomass similar to that of colonies with younger workers, that biomass is concentrated in fewer, larger, more rapidly maturing larvae. These effects seem to be due entirely to worker control.     

Factors influencing seasonal absconding in colonies of the African honey bee,Apis mellifera scutellata

Summary This study investigated the effects of colony growth and development, food storage, foraging activity and weather on the migration behavior of African honey bees in the Okavango River Delta, Botswana. Four observation colonies were studied during the honey bee migration season (November–May), at which time the availability of blooming species was reduced. Two of the colonies (colonies 1 & 2) migrated during the study period, while the remaining two (colonies 3 & 4) did not. During the 4–6 weeks preceding the onset of migration preparations, colonies 1 & 2 exhibited increasing population sizes, high levels of brood production with low brood mortality, relatively large stores of food, and increasing mass. In contrast, the populations of colonies 3 & 4 did not increase, brood-rearing activity was erratic and lower, brood mortality was higher, food stores became depleted and colony mass declined. Both colonies 3 & 4 ceased rearing brood, and colony 3 died of starvation. Colony foraging activity was examined by monitoring waggle-dance activity 2–3 days each week. For 4–6 weeks before the onset of migration in colonies 1 & 2, daily foraging areas and mean daily foraging distances became increasingly large and variable. Colonies 3 & 4 exhibited foraging patterns similar to those observed for colonies 1 & 2 preceding migration. There was no clear association between 7 weather parameters examined and migration behavior. These data suggest that migration is influenced by an interaction of intra-colony demographics, food reserves and foraging patterns. Migration may be feasible only for those colonies that possess (1) a population of appropriate size and age structure to compensate for the natural attrition of older workers during the emigration process, and (2) sufficient food reserves for long-distance travel and the establishment of a new nest. Changing foraging patterns may reflect a deteriorating foraging environment, which may trigger the onset of migration preparations, provided that colony demographics and food reserves are conducive. Colonies that show decreased brood production, higher brood mortality and reduced food stores may be incapable of migrating, even when experiencing deteriorating foraging conditions. Rather, such colonies may have a greater chance of survival if they attempt to persist in a given area.     

Observations on the social waspRopalidia montana from peninsular India

Four colonies ofRopalidia montana collected in August in the Nilgiri Hills of southern India had adult populations of 32,000-61,000. Queens represented 0·46–1·40% of the populations. All colonies contained males, but in widely varying proportions (1·47–27·00%). The large adult populations and the fact that the nests were largely filled with brood in all stages of development indicate that the colonies were in a stage of active growth in this season. In December, however, colonies have been reported to have smaller adult populations and little brood. Thus brood production appears to be seasonal in southern India. Predation by the hornetVespa tropica was observed in 4 of 31 active colonies.Ropalidia montana adults were defenseless in the face of these depredations, which appear to continue at a low level for days or weeks on a given nest.     

Demography and life history characteristics of two honey bee races (Apis mellifera)

Summary Intra-colony demography and life history characteristics of neotropical Africanized and temperate European honey bearaces were compared under simulated feral conditions. Major differences in colony demography were found which nevertheless resulted in some similar reproductive characteristics. European colonies were larger than Africanized colonies, had more rapid initral growth rates of worker populations, showed better survivorship of brood and adult workers, and differed in patterns of worker age distribution. However, both races were similar in the brood and adult populations when colonies swarmed, the frequency and timing of swarming, and the number of workers in prime swarms. The factors most important in determining these colony growth and reproductive patterns were likely worker mortality rates, climate, and resource availability.     

Ergonomics in stingless bees: changes in intranidal behavior after partial removal of storage pots and honey inMelipona favosa (Hym. Apidae,Meliponinae)

Summary The ergonomics of task allocation in stingless bees was examined in two laboratory colonies ofMelipona favosa containing individually marked workers. Performance of intranidal behavior patterns, brood cell production and the processing and storage of nectar, were studied during a control and a wax-deprived period. Experimental wax deprivation consisted of regular controlled removal of storage pots and their contents. Stored carbohydrate concentration and storage pot mass declined during the wax-deprived period. Behavior was measured by sampling for a complete ethogram at seven intranidal areas.In comparison withApis mellifera, M. favosa appeared to be more sensitive to stress, with a distinct reallocation of effort between tasks. The significant increase under stress of brood production and brood-rearing behavior patterns observed was previously also found inApis. Self-oriented behavior patterns declined under stressed circumstances.Involucrum construction declined in wax-deprivedMelipona colonies. Relatively low rates of walking behavior in general inMelipona suggest a low mean free distance between tasks. These results are of particular interest in relation to the basically different nest structure of stingless bees, e.g. very distinct areas for brood production and the processing and storage of food. This architecture of stingless bee nests appeared to influence strongly their specific responses to wax deprivation.     

Microsatellite analysis reveals strong but differential impact of a social parasite on its two host species

The speed and the dynamics of the co-evolutionary process strongly depend on the relative strengths of reciprocal selection pressures exerted by the interacting species. Here, we investigate the influence of an obligate social parasite, the slave-making ant Harpagoxenus sublaevis, on populations of the two main host species Leptothorax acervorum and Leptothorax muscorum from a German ant community. A combination of genetic and demographic data allowed us to analyse the consequences of raiding pressure on the hosts' life history and possible host preferences of the parasite. We can demonstrate that slave raids during which the social parasite pillages brood from neighbouring host colonies are both frequent and extremely destructive for both host species. Microsatellite analysis showed that, on average, a single slave-maker colony conducts more than three raids per year and that host colonies mostly perish in the aftermath of these parasite attacks. Only in few cases, surviving nests of previously raided host colonies were found in the surroundings of slave-maker colonies. As a consequence of the high prevalence of parasites and their recurrent and devastating slave raids on host colonies, the life expectancy of host colonies was severely reduced. Combining our results on host-specific parasitic colony founding and raiding frequencies with the post-raid survival rate, we can demonstrate an overall higher mortality rate for the smaller host species L. muscorum. This might be caused by a preference of H. sublaevis for this secondary host species as demographic data on host species usage indicate.     

Spatial autocorrelation and dispersal in mistletoes: field and simulation results

The infections of two species of mistletoes in Baja California, Mexico were investigated for spatial patterns of abundance, and for an effect of the dispersal patterns of mistletoe seeds on these spatial patterns. Mistletoe distributions were mapped and the dispersal of mistletoe seeds was observed. Most mistletoes seeds were dispersed locally to the parent tree or to nearby trees. While mistletoe distributions were highly clumped at the level of the individual tree, no spatial pattern was found above the scale of the individual tree. Infected trees were no more clumped than the overall host population, and infected trees had no more mistletoes on nearby surrounding trees than did uninfected trees. Trees showed no spatial autocorrelation in the number of mistletoes they supported. Simulations using a spatially explicit simulation model with local dispersal and stochasticity in seed dispersal, host mortality, and mistletoe mortality were used to interpret the field results. Simulation results suggest that dispersal patterns affect the overall level of variance in the number of mistletoes per tree but do not lead to spatial patterns in abundance above the scale of the tree. Thus, both simulation and field systems give the surprising result that local dispersal does not lead to spatial autocorrelation in the numbers of mistletoes per tree.Abbreviations AI = Arroyo Inspiracion - VSR = Valle San Rafael     

Patterns of colonization and incipient nest survival inAcromyrmex niger andAcromyrmex balzani (Hymenoptera: Formicidae)

Summary Patterns of colonization by queens and incipient nest survival of the leaf-cutting antsAcromyrmex niger andAcromyrmex balzani were studied by means of belt transects and individually marked incipient nests. No relation was found between colony density and the number of colonization attempts. Both species are not claustral, and high rates of queen mortality were attributed to conspecific executions and predation. Of other discernable mortality factors, failure of fungal garden establishment was the most important. Only 34 of 296A. balzani and 13 of 154A. niger marked colonies were alive at the end of one year. These figures are higher than those reported for species ofAtta. These results are contrasted with those of claustral-foundingAtta species. Small colonies are occasionally raided by larger colonies which robbed brood.     

Density effects in a colonial monoculture: experimental studies with a marine bryozoan (Membranipora membranacea L.)

Summary Naturally occurring monocultures of plants and animals are not common, despite recent emphasis on the analysis of density effects in artificial plant monocultures. In natural populations, Membranipora membranacea, an encrusting marine bryozoan, usually forms monospecific, nearly even-aged stands on kelp blades. We experimentally manipulated the density of M. membranacea colonies and monitored the responses of individual colonies on settling panels. Colonies undergo a sub-annual cycle of growth, stasis and reproduction, shrinkage, and death. However, crowding by conspecifics accelerates the transition to stasis, triggers early onset of reproduction, and results in increased stage-specific mortality. Unlike many interactions involving colonial invertebrates, overgrowth rarely occurs at boundaries of M. membranacea colonies. Instead, colonies stop growing when they contact conspecifics; therefore more dense assemblages are populated with smaller individual colonies. At the peak in colony size during August, the mean size among colonies grown at high population densities was 300 mm² less than colonies grown at low densities or approximately 62% smaller. Mortality was concentrated in small size classes; at the end of the season colonies gradually shrank to the smallest size classes and then died. We summarized the demography of M. membranacea colonies on low- and high-density panels using size-classified transition matrices and used loglinear analysis to examine the effects of density and time on the transition patterns. As the amount of free space on panels declined, so did the frequency of upward size-class transitions. Our analysis revealed that free space declined more rapidly on panels in the high density treatment and that the transitional probabilities were sensitive to density of conspecifics and seasonal change, but only for some size classes and during some time periods.     

Spatial patterns of goldenrod aphids and the response of enemies to patch density

Summary The two aphid species feeding on goldenrod (Solidago altissima) in northern Florida (U.S.A.) exhibited behavioral differences that resulted in characteristic spatial patterns. Uroleucon nigrotuberculatum alates (winged forms) aggregated when colonizing stems and subsequent non-winged generations were relatively sedentary, resulting in a clumped spatial pattern. U. tissoti colonized stems singly and was more mobile; these behaviors resulted in its more random spatial pattern within fields of goldenrod.Manipulations of aphid density in the field revealed that although patches with high densities of aphids accumulated more predators than patches with few aphids, predation pressure (measured as number of predators per aphid) was lower in dense patches. As a result, aphids in dense patches had a higher per capita change in density than aphids in sparse patches. However, when the fungal pathogen, Neozygites fresenii, became the dominant mortality agent, the influence of aphid density on mortality was reversed; aphids in dense patches were then more vulnerable than aphids in sparse patches. Thus the spatial patterns exhibited by the U. nigrotuberculatum and U. tissoti resulted in differences in their relative vulnerability to different natural enemies.     

Structural heterogeneity and tree spatial patterns in an old-growth deciduous lowland forest in Cantabria, northern Spain

Old-growth deciduous forests in western Europe, for the most part, consist of small tracts that often may be atypical due to human disturbance, poor soil productivity or inaccessibility. In addition, very little information on tree age distributions, structural heterogeneity and tree spatial patterns appears to be available for west-European forests. Characterization of the structural features of tree populations in these old-growth stands can provide the basis to design conservation plans and also inform on how present forests might look in the absence of human interference. Four old-growth stands in a deciduous forest in the Cantabrian lowlands, northern Spain, were surveyed to determine forest structure and spatial patterns. Live and dead trees were identified, measured and mapped, and live trees were cored for age estimation. Structural heterogeneity was analyzed by means of the spatial autocorrelation of tree diameter, height and age, and the uni- and bivariate spatial patterns of trees were analyzed. The dominant species, Fagus sylvatica and Quercus robur, showed reverse-J shaped size distributions but discontinuous age distributions, with maximum ages of 255–270 yr. Tree ages suggested that the forest was largely modified by past changes in forest-use, especially by temporal variation in grazing intensity. Spatial autocorrelation revealed that former parkland stands were heterogeneous with respect to tree height only, while high forest stands were composed of patches of even-aged and even-sized trees. Young trees were clumped at varying distances and establishment occurred preferentially in canopy gaps, except for Ilex aquifolium that mainly occurred beneath mature Quercus trees. Surviving trees became less intensely clumped in the dominant species, and more strongly clumped in understorey ones, which may have been due to the effects of intraspecific competition and of canopy trees on tree survival, respectively. The spatial associations between species varied within the forest, probably as a consequence of specific establishment preferences and competitive interactions.     

Kin recognition of worker brood by worker honey bees,Apis mellifera L.

Experimental colonies of honey bees consisting of two patrilines were observed as they reared worker brood. Seven behavior patterns that relate to brood care were recorded. Worker bees biased the care they provided to eggs and larvae destined to become workers on the basis of brood patrilines. Both patrilineal and antipatrilineal preferences in various behavioral patterns were observed. There was variation among colonies that may have been the result of the frequencies of brood of each patriline and the total amount of brood available to be reared. In addition, there were some differences between workers of the two patrilines in the way that they cared for the two patrilines of brood.