A shift in colony founding behaviour in the obligate plant-ant Crematogaster (Decacrema) morphospecies 2

Summary. A shift in colony founding behaviour from single queen (haplometrosis) to multiple queens (pleometrosis) was observed locally in the obligate plant-ant Crematogaster (Decacrema) morphospecies 2, which is associated with Macaranga trees in Borneo. In addition, about a quarter of all mature colonies (27 of 95 trees examined) were found to be multiple queen colonies. They arose either directly from pleometrotic founding colonies or secondarily by adoption of additional queens. Using microsatellite markers, we showed that queens in colonies founded through pleometrosis are unrelated and each queen participates in producing worker offspring, albeit with significant skew in a third of the colonies. In mature polygynous colonies, all resident queens contributed to the production of workers and sexual offspring. Relatedness of queens in mature polygynous colonies was not significantly higher than in foundress associations. We hypothesize that increased nest site limitation in this specific interaction trigger the observed shift in colony founding behaviour. Crematogaster msp. 2 inhabits the light demanding pioneer plant species Macaranga pearsonii that is typical for early successional stages of secondary forests. Thus suitable host-plants for colonisation are abundant for only a short time in highly disturbed sites and become increasingly sparse when secondary forest matures.Received 6 September 2004; revised 29 November; accepted 10 December 2004.     

Six weeks in the life of a reproducing army ant colony: male parentage and colony behaviour

The army ant Eciton burchellii is one of the most conspicuous ant species in New World tropical forests, but studies of colony life histories have been hampered by the nomadic lifestyle of these ants, which alternate between a nomadic phase when the colony relocates frequently, and a statary phase when the colony remains at a fixed site. Here we report on a colony from Venezuela that we studied continuously for six weeks, from the time that the queen produced a reproductive brood until the adult reproductives emerged and the colony entered the next cycle. Our findings support the contention that reproductive larvae develop faster than worker larvae, and that the nomadic phases of colonies with reproductive broods are significantly shorter than those of colonies with worker broods. This strongly suggests that the onset of pupation is linked to the onset of the statary phase. We used microsatellite genotyping to accurately identify male and queen larvae and we describe how they can be distinguished morphologically. Using the same genetic markers, we determined the parentage of 81 males produced by this colony. Only one of the males had a genotype that could not be directly derived from the observed queen genotype, but this mismatch is most probably due to a single mutation at one of the microsatellite loci, rather than this male being a worker son. We therefore conclude that this colony provides no evidence that workers lay eggs that develop into adult males in the presence of the queen, confirming the results of an earlier study on male parentage in an Old World army ant. Received 16 November 2006; revised 15 January 2007; accepted 16 January 2007.     

Number of queens in founding associations of the ponerine ant Pachycondyla villosa

In the ant Pachycondyla villosa, new colonies are usually started cooperatively by two or more young queens who establish a dominance order with a division of labour. Co-founding can lead to primary polygyny, where queens stay together after workers have emerged. Here we show that two queens associations are the most common (47%) and also the most stable in the field. When offered additional nest sites in the laboratory, two-queen associations did not split, whereas larger associations did so. We also show that solitary foundresses always accepted experimentally introduced alien queens, while these were attacked and sometimes killed in queen associations. The removal of dominant alpha queens from three-queen associations resulted in beta queens obtaining the dominant role and sometimes the destruction of the existing eggs. It appears that two queens suffice for a successful association and that pleometrosis is favoured by ecological constrains, such as nest-site limitation. Received 28 February 2005; revised 20 April 2005; accepted 22 April 2005.     

Cooperative foundation of colonies by unrelated foundresses in the antPolyrhachis moesta

Summary Field studies ofPolyrhachis moesta revealed that colonies were facultatively founded by more than one queen, even though each established colony was inhabited by only one queen with her workers. In pleometrotic colonies, it was found that cofoundresses frequently exhibited cooperative behaviors, including trophallaxis. The results of DNA fingerprinting showed that multiple foundresses were not genetically to one another. It seems that they aggregate randomly and independently of their relatedness. A discussion of the reasons why foundresses ofP. moesta do not aggregate preferentially with genetically related females.     

Parallel diversification of Australian gall-thrips on Acacia

The diversification of gall-inducing Australian Kladothrips (Insecta: Thysanoptera) on Acacia has produced a pair of sister-clades, each of which includes a suite of lineages that utilize virtually the same set of 15 closely related host plant species. This pattern of parallel insect-host plant radiation may be driven by cospeciation, host-shifting to the same set of host plants, or some combination of these processes. We used molecular-phylogenetic data on the two gall-thrips clades to analyze the degree of concordance between their phylogenies, which is indicative of parallel divergence. Analyses of phylogenetic concordance indicate statistically-significant similarity between the two clades. Their topologies also fit with a hypothesis of some degree of host-plant tracking. Based on phylogenetic and taxonomic information regarding the phylogeny of the Acacia host plants in each clade, one or more species has apparently shifted to more-divergent Acacia host-plant species, and in each case these shifts have resulted in notable divergence in aspects of the phenotype including morphology, life history and behaviour. Our analyses indicate that gall-thrips on Australian Acacia have undergone parallel diversification as a result of some combination of cospeciation, highly restricted host-plant shifting, or both processes, but that the evolution of novel phenotypic diversity in this group is a function of relatively few shifts to divergent host plants. This combination of ecologically restricted and divergent radiation may represent a microcosm for the macroevolution of host plant relationships and phenotypic diversity among other phytophagous insects.     

Selection of salt-tolerant Rhizobium isolates of Acacia nilotica

Among 35 Rhizobium isolates of Acacia nilotica, from different agro-climatic zones, two, ANG4 and ANG5, tolerated up to 850 mm NaCl and one, ANG3, was sensitive to NaCl above 250 mm. Nodulation and nitrogenase activity of the three isolates decreased with increasing concentration of salt up to 150 mm. Nodulation by ANG3 was 15% at 75 mm NaCl and nil at 100 mm. With ANG4 and ANG5, nodulation was only slightly decreased at 150 mm NaCl. Nitrogenase activity associated with plants inoculated with ANG3 was halved at 25 mm NaCl compared with salt-free controls, whereas isolates ANG4 and ANG5 retained 25% and 15% activity, respectively, even at 100 mm NaCl. Salt-tolerant Rhizobium isolates can therefore nodulate and fix N₂ in saline soils.     

A newly-discovered mode of colony founding among fire ants

Summary Queen ants start new colonies either unassisted by workers (independent founding), assisted by workers from their natal nest (dependent founding), or assisted by the workers of other species (dependent, socially parasitic). The monogyne form of the fire ant,Solenopsis invicta, founds independently in summer, but in the fall it also produces a few sexuals some of which overwinter, then fly and mate in early spring. These overwintered queens lack the nutritional reserves and behaviors for independent colony founding. Rather, they seek out unrelated, mature, orphaned colonies, enter them and exploit the worker force to found their own colony through intraspecific social parasitism. Success in entering orphaned colonies is higher when these lack overwintered female alates of their own. When such alates are present, orphaning causes some to dealate and become uninseminated replacement queens, usually preventing entry of unrelated, inseminated replacement queens. Such colonies produce large, all-male broods. Successful entry of a parasitic queen robs the host colony of this last chance at reproductive success. Only overwintered sexuals take part in this mode of founding.     

Sociality in the Australian Allodapine Bee Brevineura elongata: Small Colony Sizes Despite Large Benefits to Group Living

Allodapine bees (family Apidae, subfamily Xylocopinae) provide substantial material for investigating the evolution of sociality because of their wide variation in colony size, life history traits, and caste differentiation. Two recent studies have shown that the Australian allodapine genus Brevineura exhibits a strong increase in per capita brood production (PCBP) in social colonies compared to single-female nests. However both species previously examined, B. xanthoclypeata and B. froggatti, show relatively few multi-female nests, which is puzzling considering the apparently large advantages for group-living in these species. Here we show that in a third species, B. elongata, there are also substantial benefits for group living, involving increased PCBP and a greatly reduced likelihood of nests without brood. As expected from these observations, we also found strongly female biased sex allocation. Nevertheless only a small percentage of nests contained more than one adult female, similar to the other two Brevineura species, raising the question of why multifemale colonies are not more common in this genus. Solving this puzzle will throw light on conditions that constrain sociality when group living apparently provides major advantages.     

Life-history parameters of western flower thrips on susceptible and resistant cucumber genotypes

The life-history parameters reproduction rate, developmental time and age specific survival of the western flower thrips,Frankliniella occidentalis (Pergande) [Thysanoptera: Thripidae], were determined on susceptible and resistant cucumber (Cucumis sativus L.) genotypes. Both newly emerged andF. occidentalis females of mixed ages showed a substantial reduction (36 to 50%) of the reproduction rate on all resistant genotypes, in particular after the second day. On the resistant genotypes 9127 and 9140,F. occidentalis had a prolonged developmental period. This was primarily due to a prolongation of the second larval stage. On all resistant genotypes,F. occidentalis suffered from high (82 to 97%) preadult mortality, predominantly at the second larval stage. It is conclude that the resistant genotypes do not cause an immediate intoxication of adult nor preadult thrips stages.     

Biological control of thrips and whiteflies by a shared predator: Two pests are better than one

We studied the capacity of one species of predator to control two major pests of greenhouse crops, Western flower thrips (Frankliniella occidentalis (Pergande)) and the greenhouse whitefly (Trialeurodes vaporariorum (Westwood)). In such a one-predator–two-prey system, indirect interactions can occur between the two pest species, such as apparent competition and apparent mutualism. Whereas apparent competition is desired because it brings pest levels down, apparent mutualism is not, because it does the opposite. Because apparent competition and apparent mutualism occurs at different time scales, it is important to investigate the effects of a shared natural enemy on biological control on a time scale relevant for crop growth. We evaluated the control efficacy of the predatory mites Amblyseius swirskii (Athias-Henriot) and Euseius ovalis (Evans) in cucumber crops in greenhouse compartments with only thrips, only whiteflies or both herbivorous insects together. Each of the two predators controlled thrips, but A. swirskii reduced thrips densities the most. There was no effect of the presence of whiteflies on thrips densities. Whitefly control by each of the two predators in absence of thrips was not sufficient, yet better with E. ovalis. However, whitefly densities in presence of thrips were reduced dramatically, especially by A. swirskii. The densities of predators were up to 15 times higher in presence of both pests than in the single-pest treatments. Laboratory experiments with A. swirskii suggest that this is due to a higher juvenile survival and developmental rate on a mixed diet. Hence, better control may be achieved not only because of apparent competition, but also through a positive effect of mixed diets on predator population growth. This latter phenomenon deserves more attention in experimental and theoretical work on biological control and apparent competition.     

Ecological barriers to early colony establishment in three coexisting acacia-ant species in Kenya

In black cotton uplands in East Africa, four symbiotic acacia-ant species compete for possession of a single swollen thorn tree species, Acacia drepanolobium, and yet coexist at fine spatial scales. Three of the four ant species produce independent foundress queens that establish colonies claustrally within swollen thorns, most often on small saplings. We conducted surveys of such saplings at two sites in 2001 and 2004, and examined foundresses and incipient colonies within their swollen thorns to determine what factors influence their success. Competition among foundresses for nest initiation sites was intense, with an average of over one founding attempt per swollen thorn in all samples, and with living and dead queens significantly hyper-dispersed among available thorns. Combat with other foundresses was the most common cause of death among claustral queens, especially for Tetraponera penzigi. In interspecific battles for nest initiation sites, T. penzigi was dominant over Crematogaster nigriceps and C. mimosae, and C. nigriceps won over 80% of its contests with C. mimosae foundresses. For singleton foundresses, brood parasitism by the braconid wasp Trigastrotheca laikipiensis typically results in the death of the entire ant brood. Host queens defend parasite larvae, pupae and eclosed adults, apparently unable to distinguish the wasps from legitimate offspring. Rates of brood parasitism were as high as 15–20% for incipient colonies of both Crematogaster species, but were extremely low for T. nigriceps in all samples. Although T. penzigi and C. nigriceps foundresses are always solitary, approximately 18% of claustral C. mimosae colonies contain cooperating pleometrotic queens. For unparasitized, claustral C. mimosae colonies, brood production per queen did not differ between solitary and cooperating foundresses. However, the per-capita risks associated with parastitism were reduced for pleometrotic queens. Received 8 March 2005; revised 23 May 2005; accepted 3 June 2005.     

Impatiens necrotic spot virus infection and feeding behavior of nematode-parasitized western flower thrips

The effects of parasitism by the nematode Thripinema nicklewoodi Siddiqi (Tylenchida: Allantonematidae) on tospovirus infection and feeding behavior of Frankliniella occidentalis Pergande (Thysanoptera: Thripidae) were studied in the laboratory. In an initial experiment, nematode parasitism reduced the numbers of adult thrips that were positive to Impatiens necrotic spot virus (INSV) by enzyme-linked immunosorbent assay (ELISA) compared to controls. Three hypotheses on possible mechanisms causing this reduction were tested. H₁: nematodes have a tendency to penetrate healthy thrips rather than INSV-infected thrips; H₂: parasitized first instar thrips are less able to acquire virus during feeding; or H₃: INSV replication is suppressed in parasitized thrips. H₁ and H₂ were proven false as we found no difference in nematode attack rate between healthy thrips and thrips that have taken up INSV and no difference in virus uptake or feeding activity between parasitized and non-parasitized larval thrips. H₃ was not supported by data from our tests (P=0.07) but remains the explanation most worthy of future investigation. Interestingly, INSV transmission was not affected by nematode parasitism even though it reduced feeding activity of adult female thrips by 81% on leaves, 38% on pollen, and 22% on honey. However, despite lowered total feeding, probing by parasitized thrips (in honey) was not reduced, and this may explain why lowered feeding did not result in lowered virus transmission.     

Nitrogen fixation in Faidherbia albida,Acacia raddiana,Acacia senegal and Acacia seyal estimated using the 15N isotope dilution technique

A pot experiment was conducted in a greenhouse using the ¹⁵N isotope dilution method and two reference plants, Parkia biglobosa and Tamarindus indica to estimate nitrogen fixed in four Acacia species: A raddiana, A. senegal, A. seyal and Faidherbia albida (synonym Acacia albida). For the reference plants, the ¹⁵N enrichments in leaves, stems and roots were similar. With the fixing plants, leaves and stems had similar ¹⁵N enrichments; they were higher than the ¹⁵N enrichment of roots. The amounts of nitrogen fixed at 5 months after planting were similar using either reference plant. Estimates of the percentage of N derived from fixation (%Ndfa) for the above ground parts, in contrast to %Ndfa in roots, were similar to those for the whole plant. However, none of the individual plant parts estimated accurately total N fixed in the whole plant, and excluding the roots resulted in at least 30% underestimation of the amounts of N fixed. Between species, differences in N₂ fixation were observed, both for %Ndfa and total N fixed. For %Ndfa, the best were A. seyal (average, 63%) and A. raddiana (average, 62%), being at least twice the %Ndfa in A. senegal and F. albida. Because of its very high N content, A. seyal was clearly the best in total N fixed, fixing 1.62 g N plant^–1 compared to an average of 0.48 g N plant^–1 for the other Acacia species. Our results show the wide variability existing between Acacia species in terms of both %Ndfa and total N fixed: A. seyal was classified as having a high N₂ fixing potential (NFP) while the other Acacia species had a low NFP.     

Costs and benefits of induced resistance in a clonal plant network

Plant defense theory suggests that inducible resistance has evolved to reduce the costs of constitutive defense expression. To assess the functional and potentially adaptive value of induced resistance it is necessary to quantify the costs and benefits associated with this plastic response. The ecological and evolutionary viability of induced defenses ultimately depends on the long-term balance between advantageous and disadvantageous consequences of defense induction. Stoloniferous plants can use their inter-ramet connections to share resources and signals and to systemically activate defense expression after local herbivory. This network-specific early-warning system may confer clonal plants with potentially high benefits. However, systemic defense induction can also be costly if local herbivory is not followed by a subsequent attack on connected ramets. We found significant costs and benefits of systemic induced resistance by comparing growth and performance of induced and control plants of the stoloniferous herb Trifolium repens in the presence and absence of herbivores.     

Pollination disruption by European honeybees in the Australian bird-pollinated shrubGrevillea barklyana (Proteaceae)

European honeybees (Apis mellifera) were less efficient pollinators ofGrevillea barklyana than nectar-feeding birds. Nectar-collecting honeybees did not contact reproductive parts of flowers. Pollen-collecting honeybees preferentially visited malestage flowers but rarely visited female-stage flowers. Fruit set on caged inflorescences that allowed access to honeybees but excluded birds was reduced by more than 50% compared to inflorescences that were visited by both types of visitors. Further, fruit set on caged inflorescences was less than on bagged inflorescences that excluded both birds and honeybees, indicating that pollen removal by bees decreased opportunities for delayed autonomous selfing in the absence of birds. Although fruit set was not pollen-limited at the study site, pollen removal by honeybees would decrease fruit set in small populations where birds are scarce. In addition, pollen removal by honeybees would reduce opportunities for outcrossing and reproductive success through male function. Although honeybees have been in Australia for insufficient time to have exerted selection on floral traits, evolutionary shifts in response to these animals are likely to occur in the future.     

Costs and benefits of freezing behaviour in the harvestman Eumesosoma roeweri (Arachnida, Opiliones)

Animals present an enormous variety of behavioural defensive mechanisms, which increase their survival, but often at a cost. Several animal taxa reduce their chances of being detected and/or recognized as prey items by freezing (remaining completely motionless) in the presence of a predator. We studied costs and benefits of freezing in immature Eumesosoma roeweri (Opiliones, Sclerosomatidae). Preliminary observations showed that these individuals often freeze in the presence of the syntopic predatory spider Schizocosa ocreata (Araneae, Lycosidae). We verified that harvestmen paired with predators spent more time freezing than when alone or when paired with a conspecific. Then, we determined that predator chemical cues alone did not elicit freezing behaviour. Next, we examined predator behaviour towards moving/non-moving prey and found that spiders attacked moving prey significantly more, suggesting an advantage of freezing in the presence of a predator. Finally, as measure of the foraging costs of freezing, we found that individuals paired with a predator for 2 h gained significantly less weight than individuals paired with a conspecific or left alone. Taken together, our results suggest that freezing may protect E. roeweri harvestmen from predatory attacks by wolf spiders, but at the cost of reduced food and/or water intake.     

Relationships between the ant Brachymyrmex obscurior (Hymenoptera, Formicidae) and Acacia pennatula (Fabaceae)

Summary. In central Mexico, the ant Brachymyrmex obscurior Forel feeds on nectar produced by extrafloral nectaries of Acacia pennatula (Schlecht. & Cham.) Benth. However, no studies have determined whether the ants visitation is related to plant nectar availability and whether ants protect A. pennatula from herbivory. The objectives of this 2-yr study (2000–2001) were to assess whether seasonal changes in ant visitation coincide with extrafloral nectar productivity in A. pennatula and to determine whether ants protect the plant. At the end of the dry season (April–June) B. obscurior was the only ant species on A. pennatula and extrafloral nectar production is limited to this period. Exclusion experiments, performed at the end of the dry season showed that A. pennatula did not receive a protective benefit when visited by ants. Branches with ants and branches where ants are excluded had similar numbers of the nonmyrmecophile leafhopper Sibovia sp. which was the only herbivore observed under natural conditions.Received 24 March 2004; revised 4 September 2004; accepted 8 September 2004.     

Chromosome numbers and their systematic implications in Australian marine angiosperms: ThePosidoniaceae

Somatic chromosome numbers of 2n = 20 are reported for all eight species of AustralianPosidonia: P. angustifolia, P. australis, P. coriacea, P. denhartogii, P. kirkmanii, P. ostenfeldii, P. robertsonae, andP. sinuosa. All species apparently have five larger and five smaller pairs of chromosomes. There is no evidence that speciation inPosidonia, which is relatively prolific in comparison to other seagrasses, is accompanied by change of chromosome number.     

Dispersal and predation in alien Acacia

Summary I investigated seed removal in the litter layer of alien Acacia stands at bimonthly intervals throughout one year. Both ants (dispersers) and rodents (predators) removed significant quantities of seeds and may compete for seeds in low density Acacia stands. Seed removal from depots was greatest prior to seed-fall (Sept.–Nov.) and lowest during seed-fall (Jan.–Mar.). As rodents may consume a large proportion of the annual seed production at low Acacia densities, I propose that ants have played a critical role in accumulating Acacia seed banks.