Effect of Argentine ant invasions on ground-dwelling arthropods in northern California riparian woodlands

Although the Argentine ant (Linepithema humile) is a widespread invasive species that displaces native ants throughout its introduced range, the effects of these invasions on arthropods other than ants remain poorly known. This study documents the consequences of Argentine ant invasions on ants and other ground-dwelling arthropods in northern California riparian woodlands. Baits and unbaited pitfall traps were used to sample different components of the arthropod communities at five pairs of uninvaded and invaded sites. Sites occupied by Argentine ants supported almost no native epigeic ants except for the winter-active Prenolepis imparis. Sites with Argentine ants averaged four to ten times more ant workers than did sites with native ants, but ant worker biomass did not differ between uninvaded and invaded sites. Argentine ants recruited to baits in invaded areas, on average, in less than half the time of native ants in uninvaded areas. Despite the loss of epigeic native ants, higher Argentine ant worker abundance, and faster recruitment by Argentine ants at invaded sites, pitfall trap samples from uninvaded and invaded areas contained similar abundances and diversities of non-ant arthropods. These findings suggest that Argentine ants and the native ants they displace interact with the ground-dwelling arthropods of these habitats in a similar manner. Received: 24 February 1997 / Accepted: 9 November 1997     

Nest mounds of red wood ants (Formicaaquilonia): hot spots for litter-dwelling earthworms

A previously undocumented association between earthworms and red wood ants (Formicaaquilonia Yarr.) was found during an investigation of the influence of wood ants on the distribution and abundance of soil animals in boreal forest soil. Ant nest mounds and the surrounding soil of the ant territories were sampled. The ant nest mound surface (the uppermost 5-cm layer) harboured a much more abundant earthworm community than the surrounding soil; the biomass of the earthworms was about 7 times higher in the nests than in the soil. Dendrodrilusrubidus dominated the earthworm community in the nests, while in soils Dendrobaenaoctaedra was more abundant. Favorable temperature, moisture and pH (Ca content), together with abundant food supply (microbes and decomposing litter) are likely to make a nest mound a preferred habitat for earthworms, provided that they are not preyed upon by the ants. We also conducted laboratory experiments to study antipredation mechanisms of earthworms against ants. The experiments showed that earthworms do not escape predation by avoiding contact with ants in their nests. The earthworm mucus repelled the ants, suggesting a chemical defence against predation. Earthworms probably prevent the nest mounds from becoming overgrown by moulds and fungi, indicating possible mutualistic relationships between the earthworms and the ants. Received: 21 November 1996 / Accepted: 3 April 1997     

Experimental tests of the mechanism for ant-enhanced growth in an ant-tended lycaenid butterfly

In a previous laboratory study, larvae of the ant-tended lycaenid butterfly Hemiargus isola developed into larger adults when reared with the ant Formica perpilosa than when reared without ants. Ants neither fed butterfly larvae nor significantly delayed developmental duration. We investigated two non-exclusive hypotheses for the mechanism of this effect: larvae tended by F. perpilosa (1) consume more food, and (2) digest the food they consume more efficiently, than those reared without ants. Larvae reared in the laboratory with F. perpilosa ants became significantly heavier adults but produced a significantly lighter fecal mass than their untended counterparts, suggesting that greater food consumption was not the primary mechanism for the higher growth rates of ant-tended larvae. Tended and untended larvae were equally proficient at digesting the contents of pollen (a major natural food source) throughout the tended portion of the life cycle. Taken together, the results suggest that neither greater consumption nor higher assimilation accounts for the larger size of F. perpilosa-tended larvae. We propose that tended larvae may expend less energy than their untended counterparts. Received: 3 January 1997 / Accepted: 18 June 1997     

Detection of magnetism in the red imported fire ant (Solenopsis invicta) using magnetic resonance imaging

Red imported fire ant (Solenopsis invicta) workers, queens, and alates were analyzed by magnetic resonance imaging (MRI) for the presence of natural magnetism. Images of ants showed distortion patterns similar to those of honey bees and monarch butterflies, both of which possess ferromagnetic material. The bipolar ring patterns of MRI indicated the presence in fire ants of small amounts of internal magnetic material, which may be used in orientation behaviors, as in the honey bees. Bioelectromagnetics 18:396–399, 1997. © 1997 Wiley-Liss, Inc.     

Morphophysiological Differences between the Metapleural Glands of Fungus-Growing and Non-Fungus-Growing Ants (Hymenoptera, Formicidae)

The metapleural gland is an organ exclusive to ants. Its main role is to produce secretions that inhibit the proliferation of different types of pathogens. The aim of the present study was to examine the morphophysiological differences between the metapleural gland of 3 non-fungus-growing ants of the tribes Ectatommini, Myrmicini, and Blepharidattini and that of 5 fungus-growing ants from 2 basal and 3 derived attine genera. The metapleural gland of the non-fungus-growing ants and the basal attine ants has fewer secretory cells than that of the derived attine ants (leaf-cutting ants). In addition, the metapleural gland of the latter had more clusters of secretory cells and sieve plates, indicating a greater storage capacity and demand for secretion in these more advanced farming ants. The glands of the derived attine ants also produced higher levels of polysaccharides and acidic lipids than those of Myrmicini, Blepharidattini, and basal attines. Our results confirm morphophysiological differences between the metapleural glands of the derived attines and those of the basal attines and non-fungus-growing ants, suggesting that the metapleural glands of the derived attines (leaf-cutting ants) are more developed in morphology and physiology, with enhanced secretion production (acidic lipids and protein) to protect against the proliferation of unwanted fungi and bacteria in the fungal garden, it is possible that leaf-cutting ants may have evolved more developed metapleural glands in response to stronger pressure from parasites.     

Path efficiency of ant foraging trails in an artificial network

In this paper we present an individual-based model describing the foraging behavior of ants moving in an artificial network of tunnels in which several interconnected paths can be used to reach a single food source. Ants lay a trail pheromone while moving in the network and this pheromone acts as a system of mass recruitment that attracts other ants in the network. The rules implemented in the model are based on measures of the decisions taken by ants at tunnel bifurcations during real experiments. The collective choice of the ants is estimated by measuring their probability to take a given path in the network. Overall, we found a good agreement between the results of the simulations and those of the experiments, showing that simple behavioral rules can lead ants to find the shortest paths in the network. The match between the experiments and the model, however, was better for nestbound than for outbound ants. A sensitivity study of the model suggests that the bias observed in the choice of the ants at asymmetrical bifurcations is a key behavior to reproduce the collective choice observed in the experiments.     

The unusual social organization of the antPachycondyla tridentata (Formicidae,ponerinae)

Colonies of the ponerine antPachycondyla tridentata from Malaysia occur with and without queens. In a total of 7 colonies we found more than 80% of the workers to be mated, irrespective of the presence or absence of queens. This is a hitherto unknown social organisation in ants. Queens and workers competed equally for reproduction. In the colonies investigated several ants were laying eggs. Behavioral observations revealed persistent dominance interactions between colony members. A few ants, but not necessarily a queen, occupied top positions. Removal of the most dominant ants led to a new hierarchy in which subordinate ants with developed ovaries were attacked significantly more frequently than non-reproductive ants. On the average, callows were more aggressive than older subordinate ants, displacing most of the older laying workers in one colony. Nestmate recognition tests revealed that non-reproductive ants were much more aggressive towards foreign ants than were ants with developed ovaries.     

The interaction of thorns and symbiotic ants as an effective defence mechanism of swollen-thorn acacias

Evidence is provided for the interaction of ants (Crematogaster spp.) and thorns as a means of defence against browsing mammals for one species of African myrmecophyte, Acacia drepanolobium. Two experiments were conducted using goats as representative mammalian browsers. In the first experiment, the defences of individual branches were manipulated in order to assess the effectiveness of ants and thorns both on their own and together as anti-herbivore defences. It was shown that ants on their own are more effective defences for a single branch than having neither ants nor thorns, but ants from a single branch do not add significantly to the effectiveness of thorns as an anti-herbivore defence. The second experiment looked at the effect of a whole tree of ants and how they interacted with thorns in the defence of the tree. It was shown that ants from a whole tree do significantly add to the effectiveness of thorns as an anti-herbivore defence. In all cases, the goat refused to go back to and feed from a tree whose ants had just attacked it. Thorns on their own, however, do not act as total browsing deterrents. They slow down the rate of feeding but animals may compensate for this by feeding for longer periods of time. The interaction of a whole tree of ants and thorns is a very effective browsing deterrent which causes the animal to stop feeding almost immediately, therefore keeping the amount of foliage lost to a minimum. These results provide support for the hypothesis that the ant-acacia relationship (in the case of A. drepanolobium) evolved at least partly because of pressure from browsing mammals. Received: 20 October 1997 / Accepted: 28 February 1998     

Ant aggression and evolutionary stability in plant-ant and plant-pollinator mutualistic interactions

Mutualistic partners derive a benefit from their interaction, but this benefit can come at a cost. This is the case for plant-ant and plant-pollinator mutualistic associations. In exchange for protection from herbivores provided by the resident ants, plants supply various kinds of resources or nests to the ants. Most ant-myrmecophyte mutualisms are horizontally transmitted, and therefore, partners share an interest in growth but not in reproduction. This lack of alignment in fitness interests between plants and ants drives a conflict between them: ants can attack pollinators that cross-fertilize the host plants. Using a mathematical model, we define a threshold in ant aggressiveness determining pollinator survival or elimination on the host plant. In our model we observed that, all else being equal, facultative interactions result in pollinator extinction for lower levels of ant aggressiveness than obligatory interactions. We propose that the capacity to discriminate pollinators from herbivores should not often evolve in ants, and when it does it will be when the plants exhibit limited dispersal in an environment that is not seed saturated so that each seed produced can effectively generate a new offspring or if ants acquire an extra benefit from pollination (e.g. if ants eat fruit). We suggest specific mutualism examples where these hypotheses can be tested empirically.     

Parasitoids deter foraging by Argentine ants (Linepithema humile) in their native habitat in Brazil

The Argentine ant, Linepithema humile, has invaded sites across Africa, Australia, Europe, and North America. In its introduced ranges it eliminates native ants and tends agricultural pests. Few studies have examined the ecology of Argentine ants in their native habitat. This study examined the effects of parasitoid flies, genus Pseudacteon, on the foraging behavior of Argentine ants in part of their native range in southern Brazil. Pseudacteon parasitoids commonly attacked Argentine ants, but not other ant species, in daylight at temperatures above 18°C. Argentine ants abandoned food resources and returned underground in the presence of parasitoids. Parasitoid attack rates diminished as Argentine ants retreated underground. Where parasitoids were present, Argentine ants were abundant at food resources only during times of day when parasitoids were inactive. Where parasitoids were absent, Argentine ants were abundant at food resources throughout the day. Overall, the presence of parasitoids explained observed variation in Argentine ant foraging far better than temperature, although temperature had some effect. The results suggest that Pseudacteon parasitoids inhibit the ability of Argentine ants to gather food resources in their native habitat in Brazil. Received: 11 December 1997 / Accepted: 12 June 1998     

Compound mimicry and trading predators by the males of sexually dimorphic Batesian mimics

Sexual dimorphism is pronounced in Myrmarachne, a large genus of ant-like jumping spiders (Salticidae) and one of the major animal groups in which Batesian mimicry of ants has evolved. Although adult females and juveniles of both sexes are distinctly ant-like in appearance, Myrmarachne males have elongated chelicerae that might appear to detract from their resemblance to ants. Experimental findings suggest that the Myrmarachne male's solution is to adopt compound mimicry (i.e. the male's model seems to be not simply an ant worker but a combination of an ant and something carried in the ant's mandibles: an "encumbered ant"). By becoming a mimic of a particular subset of worker ants, Myrmarachne males may have retained their Batesian-mimicry defence against ant-averse predators, but at the price of receiving the unwanted attention of predators for which encumbered ants are preferred prey. Two salticid species were used as predators in the experiments. Portia fimbriata is known to choose other salticids as preferred prey and to avoid unencumbered ants and their mimics (Myrmarachne females). In experiments reported here, P. fimbriata avoided encumbered ants and Myrmarachne males. Ants are the preferred prey of Chalcotropis gulosus. In our experiments, C. gulosus chose safer encumbered ants in preference to more dangerous unencumbered ants, chose Myrmarachne males more often than Myrmarachne females and showed no evidence of distinguishing between Myrmarachne males and encumbered ants. The cost of reconciling sexual dimorphism with Batesian mimicry appears to be that Myrmarachne males attract the unwanted attention of specialist predators of their compound model.     

Factors governing rate of invasion: a natural experiment using Argentine ants

Predicting the success of biological invasions is a major goal of invasion biology. Determining the causes of invasions, however, can be difficult, owing to the complexity and spatio-temporal heterogeneity of the invasion process. The purpose of this study was to assess factors influencing rate of invasion for the Argentine ant (Linepithema humile), a widespread invasive species. The rate of invasion for 20 independent Argentine ant populations was measured over 4 years in riparian woodlands in the lower Sacramento River Valley of northern California. A priori predictors of rate of invasion included stream flow (a measure of abiotic suitability), disturbance, and native ant richness. In addition, baits were used to estimate the abundance of Argentine ants and native ants at the 20 sites. A multiple regression model accounted for nearly half of the variation in mean rate of invasion (R ² = 0.46), but stream flow was the only significant factor in this analysis. Argentine ants spread, on average, 16 m year⁻¹ at sites with permanent stream flow and retreated, on average, −6 m year⁻¹ at sites with intermittent stream flow. Rate of invasion was independent of both disturbance and native ant richness. Argentine ants recruited to more baits in higher numbers in invaded areas than did native ants in uninvaded areas. In addition, rate of invasion was positively correlated with the proportion of baits recruited to by native ants in uninvaded areas. Together, these findings suggest that abiotic suitability is of paramount importance in determining rate of invasion for the Argentine ant. Received: 16 September 1997 / Accepted: 8 February 1998     

Juvenile hormone in adult eusocial hymenoptera: Gonadotropin and behavioral pacemaker

Studies on the role of juvenile hormone (JH) in adult social Hymenoptera have focused on the regulation of two fundamental aspects of colony organization: reproductive division of labor between queens and workers and age-related division of labor among workers. JH acts as a gonadotropin in the primitively eusocial wasp and bumble bee species studied, and may also play this role in the advanced eusocial fire ants. However, there is no evidence that JH acts as a traditional gonadotropin in the advanced eusocial honey bee or in the few other ant species that have recently begun to be studied. The role of JH in age-related division of labor has been most thoroughly examined in honey bees. Results of these studies demonstrate that JH acts as a “behavioral pacemaker,” influencing how fast a worker grows up and makes the transition from nest activities to foraging. Hypotheses concerning the evolutionary relationship between the two functions of JH in adult eusocial Hymenoptera are discussed. Arch. Insect Biochem. Physiol. 35:559–583, 1997. © 1997 Wiley-Liss, Inc.     

Effects of harvester ants on plant species distribution and abundance in a serpentine grassland

Seed harvesting ants can have important effects on the composition and structure of plant communities. We investigated two effects of Messor andrei, the black seed-harvesting ant, on a serpentine grassland plant community in northern California. First, to determine if selective seed predation by ants affects plant community composition, we excluded harvester ants from 1-mediameter circular plots of grassland. Abundances of all species on these plots and on control plots were measured before and after exclosure. Second, to determine if M. andrei nest mounds affect plant community composition, we compared plant species abundances on and off nest mounds. M. andrei deposit large amounts of organic matter on their nest mounds over a foraging season, so mounds may alter the edaphic environment. The exclusion of seed-harvesting activity did not cause changes in the plant community. Nest mounds had a strong effect on plant communities: there were many more grasses and fewer forbs on ant mounds, although at least one forb, Lepidium nitidum, produced twice as many seeds when it grew on nest mounds. We found that nest mounds formed islands of higher-temperature soil in the serpentine grassland. Received: 31 March 1997 / Accepted: 6 May 1997     

Ant defence system: A mechanism organizing individual responses into efficient collective behavior

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.     

Visual cues for the retrieval of landmark memories by navigating wood ants

BACKGROUND: Even on short routes, ants can be guided by multiple visual memories. We investigate here the cues controlling memory retrieval as wood ants approach a one- or two-edged landmark to collect sucrose at a point along its base. In such tasks, ants store the desired retinal position of landmark edges at several points along their route. They guide subsequent trips by retrieving the appropriate memory and moving to bring the edges in the scene toward the stored positions. RESULTS: The apparent width of the landmark turns out to be a powerful cue for retrieving the desired retinal position of a landmark edge. Two other potential cues, the landmark's apparent height and the distance that the ant walks, have little effect on memory retrieval. A simple model encapsulates these conclusions and reproduces the ants' routes in several conditions. According to this model, the ant stores a look-up table. Each entry contains the apparent width of the landmark and the desired retinal position of vertical edges. The currently perceived width provides an index for retrieving the associated stored edge positions. The model accounts for the population behavior of ants and the idiosyncratic training routes of individual ants. DISCUSSION: Our results imply binding between the edge of a shape and its width and, further, imply that assessing the width of a shape does not depend on the presence of any particular local feature, such as a landmark edge. This property makes the ant's retrieval and guidance system relatively robust to edge occlusions.     

Time-courses of memory decay in vector-based and landmark-based systems of navigation in desert ants, Cataglyphis fortis

In foraging and homing, desert ants of the genus Cataglyphis employ two different systems of navigation: a vector-based or dead-reckoning mechanism, depending on angles steered and distances travelled, and a landmark-based piloting mechanism. In these systems the ants use either celestial or terrestrial visual information, respectively. In behavioural experiments we investigated how long these types of information are preserved in the ant's memory, i.e. how long the ants are able to orient properly in either way. To answer this question, ants were tested in specific dead-reckoning and piloting situations, whereby the two vector components, direction and distance, were examined separately. The ability to follow a particular vector course vanishes rapidly. Information about a given homing direction is lost from the 6th day on (the time constant of the exponential memory decay function is τ = 4.5 days). The homing distances show a significantly higher dispersion from the 4th day on (τ = 2.5 days). Having learned a constellation of landmarks positioned at the corners of an equidistant triangle all ants were oriented properly after 10 days in captivity, and 64% of the ants exhibited extremely precise orientation performances even when tested after 20 days. Thus, the memory decay functions have about the same short time-course for information on distance and direction, i.e. information used for dead-reckoning. In contrast, landmark-based information used in pinpointing the nest entrance is stored over the entire lifetime of a Cataglyphis forager. Accepted: 18 January 1997     

Costs and benefits for phytophagous myrmecophiles: when ants are not always available

Costs and benefits in mutualistic associations between ants and phytophagous myrmecophiles are context dependent. We collected information from the literature on costs and benefits of myrmecophily in aphids, coccids, membracids and lycaenids. A key result of the literature survey is that investment in mutualism with ants entails costs paid not only when ants are present (direct costs) but also when they are absent (indirect costs). We incorporated such a trade-off in a model that investigates the fitness consequences of the decision of a potential myrmecophile whether or not to invest in cooperation with ants. The model shows that whether myrmecophily should be favoured depends on the rate of increase of the population, and, if there are indirect costs, on the frequency of habitats with ants. Both direct and indirect costs can limit or prevent the evolution of myrmecophily even when ants are abundant. To understand the patterns of associations in the field we therefore need to measure the benefits and costs of myrmecophily both in the presence and in the absence of ants.     

Ant maze behavior during switch from food motivation to defense motivation

It was shown that scout ants Myrmica rubra are capable for an adequate modification of foraging habit acquired in a multichoice symmetrical maze following a reward change (syrup for the species brood). Under conditions of the high level of food motivation of a family both during learning and "examination" (i.e., brood transportation) all the ants succeed in a conditioned switchover of heterogeneous reflexes. If the high level of motivation during learnings is changed for its low level during examination, some ant do not realize the new task. The unconditioned switch (preliminary 1-3 brood takes at the start ares) facilitates the successful performance. In different experimental sessions a certain ratio was revealed between the ants realizing different ways of correct decision making. Specially developed criteria made it possible to identify different individual types of behavior during examination: transfer of acquired strategy, its gradual adaptation, and fast switchover to the new strategy. The adequate maze method and learning on the basis of specific forms of social behavior of ants (foraging, brood transportation, and exploratory activity) promote the rapid and efficient acquisition of the habit and its modification.     

AntCounter Software: Counting Leaf-Cutting Ants Was never so Precise,fast and Easy

One of the main concerns when collecting data is the bias stemming from the researchers’ expertise and training. The precision and accuracy of the data being collected can vary depending on the focus, concentration, mood, and experience of a researcher, and this can generate errors in data collection that can influence the results. We present a software for counting leaf-cutting ants that tracks the number of ants on previously recorded videos. The software was developed in the Python language and uses an image processing library called OpenCV (Open Source Computer Vision). It is based on a low-resolution video (640 × 480 pixels) taken with a camera mounted on a tripod and orientated perpendicularly to the ground. This open access program uses the movement of ants to identify their position and track them. It counts the number of ants that pass through a given point in two directions with respect to the nest entrance: incoming and outgoing. Comparisons between AntCounter and conventional methods used to count ants show that the program has higher accuracy than traditional counting in real time while reducing data collection time. We also report the error in the data collected by AntCounter. Using AntCounter the researcher reduces both the time of data collection in the field and the time to process data. It is also possible to improve the sampling effort (e.g., increase the number of samples, the number of experimental units), and measure the ants for longer periods of time, and at lower prices.