Bruchid egg mortality on Acacia farnesiana caused by ants and abiotic factors

Abstract. 1. This study investigated the effect of arboreal ants on the bruchid beetles that prey upon the seeds of Acacia farnesiana (L.) Willd., a common shrub in the savannas of Santa Rosa National Park, in northwestern Costa Rica.
2. Experiments demonstrated that ants removed an average of about 50% of unhatched bruchid eggs that had been laid on the pods of this leguminous plant. Ants also removed shells of hatched eggs.
3. In addition to the mortality caused by ant predation, about 30% of the eggs disappeared before hatching, presumably due to the combined effects of temperature and wind.
4. The orientation of the branch on which a pod was located did not affect the proportion of eggs lost from that pod.     

Symbiotic ants as an alternative defense against giraffe herbivory in spinescent Acacia drepanolobium

Summary We explore here the occurrence of aggressive ants in an apparently symbiotic relationship with the savanna tree Acacia drepanolobium and their effects on giraffe herbivory on the Athi-Kapiti Plains, Kenya. Trees taller than 1.3 m were more likely to be occupied by aggressive ants in the genus Crematogaster than were shorter trees. Ants wereconcentrated on shoot tips, the plant parts preferred by giraffes. Trees with relatively more foliage had more swarming ants than did trees with less foliage. The feeding behavior of individual freeranging giraffes on Acacia drepanolobium was studied. Giraffe calves exhibited a strong sensitivity to Crematogaster ants inhabiting A. drepanolobium, feeding for significantly shorter periods on trees with a greater number of aggressive ants. Older giraffes were apparently less sensitive to ants, and did not feed for shorter periods on trees with fuller foliage, despite significantly greater ant activity on these plants. The thorns of A. drepanolobium are significantly shorter than are the thorns of A. seyal, a species without symbiotic ants, a pattern that may indicate a trade-off between ants and thorns as defenses.     

Pseudomyrmex ants and Acacia host plants join efforts to protect their mutualism from microbial threats

Plants express numerous ‘pathogenesis-related’ (PR) proteins to defend themselves against pathogen infection. We recently discovered that PR-proteins such as chitinases, glucanases, peroxidases and thaumatin-like proteins are also functioning in the protection of extra-floral nectar (EFN) of Mexican Acacia myrmecophytes. These plants produce EFN, cellular food bodies and nesting space to house defending ant species of the genus Pseudomyrmex. More than 50 PR-proteins were discovered in this EFN and bioassays demonstrated that they actively can inhibit the growth of fungi and other phytopathogens. Although the plants can, thus, express PR-proteins and secrete them into the nectar, the leaves of these plants exhibit reduced activities of chitinases as compared to non-myrmecophytic plants and their antimicrobial protection depends on the mutualistic ants. When we deprived plants of their resident ants we observed higher microbial loads in the leaves and even in the tissue of the nectaries, as compared to plants that were inhabited by ants. The indirect defence that is achieved through an ant-plant mutualism can protect plants also from infections. Future studies will have to investigate the chemical nature of this mechanism in order to understand why plants depend on ants for their antimicrobial defence.Key words: ant-plant mutualism, indirect defense, pathogen resistance, pathogenesis-related proteinPlants fall victim to multiple attackers from various kingdoms and have evolved numerous strategies to defend themselves against herbivores and microbial pathogens. Pathogen resistance is usually based on particular cell wall properties, secondary compounds (phytoalexins) and on pathogenesis-related (PR) proteins.^1,2 Plant resistance to pathogens is thus, mainly based on plant traits that interact with the pathogen and therewith functions as a “direct” defence mechanism. Resistance to herbivores, by contrast, can also be “indirect”: plants house or attract carnivores, which fulfil the defensive function.³ Defensive ant-plant interactions are common mutualisms in which plants provide to ants an array of rewards that comprises both food rewards and domatia (nesting space).^3,4 Ants are attracted or nourished by plant-derived food rewards and defend the plants against herbivores.^3,5 In obligate interactions, myrmecophytes are inhabited by specialised ants during major parts of their life⁴ and the ants are entirely dependent on the food rewards and nesting space that are provided by their host. These ants, in return, defend their host efficiently and aggressively against herbivores and encroaching vegetation. Only recently we found that this protective service might also cover the defence against phytopathogens.In the Mexican Acacia-Pseudomyrmex mutualism, extrafloral nectar (EFN) represents an important plant trait to nourish symbiotic ant colonies. EFN is secreted by special glands called extrafloral nectaries and is rich in mono- and disaccharides and amino acids.⁶ Due to its high content in primary metabolites, EFN appears also attractive to exploiters, which make use of the nectar resource without providing a service to the plant.⁷ For example, EFN can serve as a suitable medium for pathogen growth and is highly prone to microbial infestations, which can have negative consequences on nectar composition.^8,9We recently found that Acacia EFN is biochemically protected from microbial infections.^10,11 More than 50 proteins could be distinguished in EFN of the myrmecophytes, A. hindsii, A. cornigera and A. collinsii, and most of these proteins were annotated as PR-proteins such as chitinases, β-1,3 glucanases, thaumatin-like proteins and peroxidases. In fact, chitinases and glucanases represented more than 50% of the total amount of proteins in EFN of myrmecophytic Acacia plants.^10,11 This dominance of PR-proteins clearly distinguishes the anti-microbial strategy of Acacia-EFN from the strategy of floral nectar of ornamental tobacco. The protection of the latter nectar is mainly based on small metabolites, such as hydrogen peroxide, which are produced by five proteins forming the ‘nectar redox cycle’.^12–14 By contrast, PR-proteins have also been reported from pollination droplets of gymnosperms,¹⁵ although their biological functions remain to be studied. For the Acacia EFN, biotests demonstrated that the chitinases and glucanases are active and that EFN successfully can inhibit the growth of various fungi and oomycetes.¹¹Nevertheless, and although the microorganisms used in these assays were phytopathogens, we have now observed that the protection from microbial infection of the Acacia plant itself depends on certain characteristics of its ant inhabitants. The occurrence of fungi and bacteria in EFN and in the tissue of leaves and nectaries was investigated under natural growing conditions. We collected samples of A. cornigera and A. hindsii plants, which were inhabited by ants or which had been deprived of ants experimentally two weeks before. The tissues were extracted and analyzed for microbial infection by cultivating the extracts on malt agar plates, in order to quantify the abundances of life fungi and bacteria as the numbers of colony-forming units (CFU). Leaves of two species of myrmecophytic Acacia plants showed a significant increase in their bacterial load when they were deprived of the mutualistic P. ferrugineus ants (Fig. 1A). This observation is redolent of an earlier observation made on Macaranga myrmecophytes in Malaysia: lesions of these plants could easily be infected with fungi when the mutualistic Crematogaster ants were absent, but not in the presence of the ants.¹⁶ Similarly, myrmecophytic Piper plants depend, at least in part, on their ant inhabitants to obtain an efficient protection from fungal pathogens.¹⁷ Myrmecophytic Maracanga plants possessed reduced activities of chitinases in their leaves¹⁶ and the same phenomenon was found in Acacia myrmecophytes.¹⁸ Thus, the leaves of the obligate ant-plants of both genera, Acacia and Macaranga, appear not to express PR-proteins at sufficient activities as to protect themselves from pathogens and symbiotic ants are required for this defensive function. The effect is specific for the defending ants, because no significant differences between plants with and without ants could be detected in the case of plants that were inhabited by the non-defending parasite, P. gracilis¹⁹ (Fig. 1B). Most interestingly, even the nectary tissue appears to require ant-mediated protection from phytopathogens, whereas the EFN itself does not: no fungi can usually be isolated from freshly collected nectar of Acacia myrmecophytes under field conditions¹⁰ (Fig. 2A), but the tissue of the nectaries became heavily infected when branches were kept free of the defending ants (Fig. 2B).Open in a separate windowFigure 1Effects of the presence and absence of the symbiotic ant P. ferrugineus and the parasitic ant P. gracilis on the abundance of bacteria [CFU mg dry leaf mass⁻¹] in leaf samples of A. cornigera (A) and A. hindsii (B). Significance levels are indicated: ns p > 0.05, *p < 0.05, **p < 0.01 and ***p < 0.001. (Two-factorial ANOVA applied separately for each plant species; independent variables: ant presence and ant species.)Open in a separate windowFigure 2Effects of presence and absence of the symbiotic ant P. ferrugineus on fungal abundance in EFN (A) and nectar tissue (B) of A. cornigera and A. hindsii. Significance levels are indicated: ns p > 0.05, *p < 0.05, **p < 0.01 and ***p < 0.001. (Two-factorial ANOVA; independent variables: ant presence and plant species).Our new findings highlight the joint efforts of both ant and plant that are required for an efficient protection from pathogens of the myrmecophyte host plants, and, thus, for the prevention of this mutualism from exploitation by microorganisms. It remains open why the myrmecophytic Acacia plants exhibit reduced chitinase activities in their leaves¹⁸ and thus depend on ants for their antimicrobial protection, although they are fully equipped to express functioning PR-proteins and secrete them into their EFN.^10,11 We could assume that an antimicrobial protection by ants is less costly for the plant than its own biochemical defence mechanisms. Alternatively, plant pathogens, which express multiple effectors in order to suppress their host resistance strategies,¹ might be less capable to cope with ant-derived resistance mechanisms. Every attempt of an explanation, however, remains speculative as long as we do not understand how the ants can protect the leaves of their host plant from infection. The chemical mechanisms remain to be analyzed and may comprise both an ant-mediated resistance induction in the plant and directly ant-derived antimicrobial compounds. Most importantly, the joined efforts of both plant host and ant inhabitant are required to keep leaves, nectaries and nectar free of microbial infections and microbial pathogens have been identified as a further target of the indirect defence, which plants can obtain when they establish an obligate mutualism with ants.     

A comparison of mandibular gland volatiles from ants of the bull horn acacia,Acacia collinsii

GC–MS analyses of dichloromethane extracts of the mandibular glands from three obligate symbiotic Psuedomyrmex ant species of Acacia collinsii from Costa Rica: Pseudomyrmex flavicornis (synonym Pseudomyrmex belti), Psuedomyrmex spinicola, and Psuedomyrmex nigrocincta, showed distinct differences in the 16 ketones, 15 alcohols, 2 aldehydes and 2 carboxylic acids that were identified. Different compounds were the major component from each species: P. flavicornis, 3-octanone; P. spinicola, 4-methyl-3-heptanone; and P. nigrocincta, 3-methyl-2-hexanol. The secretion of P. flavicornis contained 10 compounds not found in the other species, including the two terpene alcohols, citronellol and geraniol. The secretions of P. spinicola and P. nigrocincta had 12 compounds in common, that were not found in P. flavicornis' secretion. The similarity of the mandibular gland secretion of P. spinicola and P. nigrocincta may indicate that they are more closely related to each other than either is to P. flavicornis. The components from the mandibular gland of Crematogaster rochai, another ant associated with this acacia, are 2- and 3-methylbutanoic acid, 3-octanone, 3-octanol, 6-methyl-3-octanol and 3-nonanone.     

The influence of ants on the guild structure of Acacia insect communities in Mkomazi Game Reserve, north-east Tanzania

A total of 41,099 insect specimens of 133 families and 492 morphospecies were collected from 31 trees of six species of Acacia in north-east Tanzania, representing one of the largest insect samples ever analysed from a tropical savannah habitat. Herbivores (sapsuckers and chewers) and parasitoids had the highest diversity shares, whereas the highest biomass shares were obtained by phytophagous chewers, ants and predators. The percentage biomass of ants was correlated positively with the diversity share of sapsuckers and negatively with the diversity share of tourists. A positive correlation was found with the residual biomass share of phytophagous sapsuckers, indicating a protective function of ants for this guild. Diversity and abundance share was much higher in egg and coccoid parasitoids compared to larval parasitoids, probably due to predation by ants on larval parasitoids. Their low diversity supports the hypothesis of a decline towards the equator in ichneumonid diversity.     

Isolation of 15 polymorphic microsatellite loci in Acacia hybrid (Acacia mangium×Acacia auriculiformis)

This study reports the isolation of 15 microsatellites in Acacia hybrid (Acacia mangium×Acacia auriculiformis) based on the 5′ anchored polymerase chain reaction technique. Polymorphism of these loci was evaluated in a sample of 24 hybrid individuals. The level of polymorphism ranged from two to eight alleles with an observed heterozygosity ranging from 0.083 to 0.875. These loci were also characterized in both the parental species. The number of alleles ranged from two to six for both A. mangium and A. auriculiformis with the observed heterozygosity values ranging from 0.167 to 0.625 and 0.042 to 0.458 respectively. Five of these loci demonstrated Mendelian inheritance in a segregating F₁ population; four presented a distorted ratio and the remaining six did not segregate in progenies as they were homozygous in both parents.     

Trap-mulching Argentine ants

Argentine ant, Linepithema humile (Mayr), management is constrained, in large part, by polydomy where nestmates are distributed extensively across urban landscapes, particularly within mulch. Management with trap-mulching is a novel approach derived from trap-cropping where ants are repelled from a broad domain of nest sites to smaller defined areas, which are subsequently treated with insecticide. This concept was field-tested with mulch surrounding ornamental trees replaced with a narrow band of pine (Pinus spp.) needle mulch (trap) within a much larger patch of repellent aromatic cedar (Juniperus spp.) mulch. After ants reestablished around the trees, the pine needle mulch band was treated with 0.06% fipronil (Termidor). Poor results were obtained when the trap extended from the tree trunk to the edge of the mulched area. When the trap was applied as a circular band around the tree trunk reductions in the number of foraging ants were recorded through 14 d compared with an untreated mulch control, but not for longer periods. Reductions in the number of ant nests within mulch were no different between the trap mulch and any of the other treatments. We conclude that trap-mulching offers limited benefits, and that successful management of Argentine ants will require implementation of complementary or perhaps alternative strategies.     

Introduced fire ants can exclude native ants from critical mutualist-provided resources

After over 30 years of research, it was recently shown that nectar amino acids increase female butterfly fecundity. However, little attention has been paid to the effect of nectar amino acids on male butterfly reproduction. Here, we show that larval food conditions (nitrogen-rich vs. nitrogen-poor host plants) and adult diet quality (nectar with or without amino acids) affected the amount of consumed nectar in Coenonympha pamphilus males. Furthermore, amino acids in the nectar diet of males increased progeny’s larval hatching mass, irrespective of paternal larval reserves. Our study takes the whole reproductive cycle of male butterflies into account, and also considers the role of females in passing male nutrients to offspring, as males’ realized reproduction was examined indirectly via nuptial gifts, by female performance. With this comprehensive approach, we demonstrate for the first time that nectar amino acids can improve male butterfly reproduction, supporting the old postulate that nectar amino acids generally enhance butterfly fitness.     

Long-term impact of exotic ants on the native ants of Madeira

Abstract.  1. The earliest exotic records for two notorious invasive ants, the big-headed ant ( Pheidole megacephala ) and the Argentine ant ( Linepithema humile ), both come from the Atlantic islands of Madeira, where the two species underwent population explosions in the 1850s and 1890s respectively. Researchers have long assumed that these invaders spread across all of Madeira and exterminated most or all native ants, despite no research actually documenting such impact.
2. Re-examination of first-hand nineteenth century accounts suggest that P. megacephala and L. humile may never have spread beyond coastal lowland areas, representing < 10% of Madeira's land area. In 2002, native ants dominated most of Madeira; P. megacephala and L. humile were restricted to ≈ 0.3% and ≈ 6% of Madeira's land area respectively.
3. Of the 10 native ant species known from Madeira, only one ( Temnothorax wollastoni ) was not present in 1999–2002 surveys. Although exotic ants may have exterminated T. wollastoni , it seems likely that this species still survives.
4. Thus, even after 150 or more years of residence, P. megacephala and L. humile have come to occupy only a small part of Madeira, and appear to have had little impact.
5. Most of Madeira may be too cool for P. megacephala and perhaps too moist for L. humile to dominate. Also, Madeira's vast natural areas may generally lack weedy vegetation that can support high densities of plant-feeding Hemiptera critical for the ecological dominance of invasive ants. Finally, a dominant native ant, Lasius grandis , inhabiting ≈ 84% of Madeira, may actively exclude P. megacephala and L. humile .     

Cyanogenesis in Acacia sutherlandii

Two cyano-glucosides have been isolated from leaves of Acacia sutherlandii. One is the previously described cyanogenic glucoside proacacipetalin and the other is the novel, non-cyanogenic, glycoside 1-cyano-2-β-D-glucopyranosyloxymethyl-(Z)-prop-1-en-3-ol which has been given the trivial name sutherlandin.     

Acacia Farnesiana W.

Ohne Zusammenfassung     

Cyanogenesis in Acacia farnesiana

Linamarin and lotaustralin are the major cyanogens of Acacia farnesiana; a third unidentified cyanogen is also present. The amount of cyanide produced by plants within one population of the taxon varies from below the level of detection with picrate paper to approximately 5 μmol per g (1.4%) dried plant material.