Caterpillars and fungal pathogens: two co-occurring parasites of an ant-plant mutualism

In mutualisms, each interacting species obtains resources from its partner that it would obtain less efficiently if alone, and so derives a net fitness benefit. In exchange for shelter (domatia) and food, mutualistic plant-ants protect their host myrmecophytes from herbivores, encroaching vines and fungal pathogens. Although selective filters enable myrmecophytes to host those ant species most favorable to their fitness, some insects can by-pass these filters, exploiting the rewards supplied whilst providing nothing in return. This is the case in French Guiana for Cecropia obtusa (Cecropiaceae) as Pseudocabima guianalis caterpillars (Lepidoptera, Pyralidae) can colonize saplings before the installation of their mutualistic Azteca ants. The caterpillars shelter in the domatia and feed on food bodies (FBs) whose production increases as a result. They delay colonization by ants by weaving a silk shield above the youngest trichilium, where the FBs are produced, blocking access to them. This probable temporal priority effect also allows female moths to lay new eggs on trees that already shelter caterpillars, and so to occupy the niche longer and exploit Cecropia resources before colonization by ants. However, once incipient ant colonies are able to develop, they prevent further colonization by the caterpillars. Although no higher herbivory rates were noted, these caterpillars are ineffective in protecting their host trees from a pathogenic fungus, Fusarium moniliforme (Deuteromycetes), that develops on the trichilium in the absence of mutualistic ants. Therefore, the Cecropia treelets can be parasitized by two often overlooked species: the caterpillars that shelter in the domatia and feed on FBs, delaying colonization by mutualistic ants, and the fungal pathogen that develops on old trichilia. The cost of greater FB production plus the presence of the pathogenic fungus likely affect tree growth

Strategies of a parasite of the ant–Acacia mutualism

Mutualisms can be exploited by parasites—species that obtain resources from a partner but provide no services. Though the stability of mutualisms in the presence of such parasites is under intensive investigation, we have little information on life history traits that allow a species to be a successful mutualist or rather a parasite, particularly in cases where both are closely related. We studied the exploitation of Acacia myrmecophytes by the ant, Pseudomyrmex gracilis, contrasting with the mutualistic ant Pseudomyrmex ferrugineus. P. gracilis showed no host-defending behavior and had a negative effect on plant growth. By preventing the mutualist from colonization, P. gracilis imposes opportunity costs on the host plant. P. gracilis produced smaller colonies with a higher proportion of alates than did the mutualist and thus showed an “r-like” strategy. This appears to be possible because P. gracilis relies less on host-derived food resources than does the mutualist, as shown by behavioral and stable isotope studies. We discuss how this system allows the identification of strategies that characterize parasites of mutualisms

Costs and benefits of induced resistance in clonal plant network

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Inelastic buckling of tubular section beams

104 σ.Στη μηχανική με τον όρο “λυγισμό”, νοείται αστοχία λόγω απώλειας της ευστάθειας ενός φορέα που υπόκειται μόνο σε αυστηρά κεντρικό αξονικό φορτίο. Η ύπαρξη, ωστόσο, αρχικών ατελειών ή εκκεντρότητας του φορτίου, οδηγεί στην καμπτοθλιπτική καταπόνηση του μέλους. Έτσι, η κατανομή των τάσεων καθ’ ύψος της διατομής δεν είναι ομοιόμορφη, και η τάση στη δυσμενέστερη ίνα του μέλους, που είναι η ακραία θλιβόμενη της μεσαίας διατομής, μπορεί να φτάσει την τιμή της τάσης διαρροής πριν φτάσει η φόρτιση το κρίσιμο φορτίο λυγισμού, οπότε και έχουμε πλαστικό λυγισμό. Το φαινόμενο του πλαστικού λυγισμού εμφανίζεται περισσότερο σε ένα εύρος τιμών της λυγηρότητας των υποστυλωμάτων. Στη συγκεκριμένη εργασία επιλέξαμε διάφορες διατομές κλιμακωτά αυξανόμενης διαμέτρου ,τις οποίες ελέγξαμε στο φαινόμενο του πλαστικού λυγισμού, αφού θεωρήσαμε σε αυτές διάφορες τιμές αρχικών βελών, επί του ισχυρού άξονα. Για να το κάνουμε αυτό, θέσαμε κάποιες παραδοχές και εν συνεχεία προσεγγίσαμε θεωρητικά την εκδήλωση του φαινομένου. Το επόμενο βήμα είναι η σύγκριση των αποτελεσμάτων που προέκυψαν από την προσέγγισή μας, με τις αντίστοιχες κρίσιμες τιμές που προκύπτουν από τον Ευρωπαικό Κανονισμό (EC3) και η επαλήθευση των αποτελεσμάτων από πρόγραμμα Η/Υ με τη μέθοδο των πεπερασμένων στοιχείων (Abaqus). Τέλος, παρουσιάζονται διάφορες καμπύλες για τις υπό μελέτη διατομές, όπου φαίνεται η επίδραση της αλλαγής των γεωμετρικών τους στοιχείων,η ποιότητα χάλυβα,οι αρχικές ατέλειες κτλ,στην εκδήλωση του πλαστικού λυγισμού. Το φαινόμενο του πλαστικού λυγισμού, χαρακτηρίζεται από μικρές τιμές του ποσοστού της διαφοράς ανάμεσα στο κρίσιμο φορτίο (πλαστικό) και του ελαστικού φορτίου (φορτίο πρώτης διαρροής) ως προς το δεύτερο. Οι τιμές αυτές της πλαστικής “υπεραντοχής” εξαρτώνται από τη διατομή και την ποιότητα του χάλυβα, αλλά ακόμη περισσότερο από τη λυγηρότητα του κάθε υποστυλώματος. Επίσης, είδαμε ότι όσο πιο μεγάλη είναι η αρχική ατέλεια (αρχικό βέλος κάμψης), τόσο μικρότερο είναι το κρίσιμο φορτίο και τόσο πιο μεγάλη η τελική παραμόρφωση του υποστυλώματος.In mechanics, the term "buckling" means failure due to loss of stability of a member, that is subject of strictly central axial load. The presence, however, of initial imperfections or load eccecintricity leads to compressive and flexural loading of the member. Thus, the stress distribution on height of the section is not uniform., and the stress of the least favourable fiber of the member, which is the farther fiber under compression at the middle section of the member, can reach the value of the characteristic yield stress of the steel, before the load reaches the critical buckling load, so it appears the effect of inelastic buckling. This effect appears more in a value range of the slenderness of the columns.In this particular thesis we chose various sections of increasing diameter ,which we had them tested on the effect of inelastic buckling, since we considered different values of initial bending displacement, on the strong axis. To do that, we have some assumptions and then we approached theoritically the progress of the effect. The next step is the comparison of the theoritical results of our approach, with the corresponding critical values result-ing by the Eurocodes. (EC3) and the verification of the results with the use of a PC programm with finite elements analysis (Abaqus). Finally, are being presented some curves for the studied sections, where we can see the effects of the geometrical changes,steel quality,initial imperfections etc. at inelastic buckling. The effect of inelastic buckling, characterized by small values of the percentage of the difference between the critical load (inelastic) and the elastic load (first failure) divided by the second. The values of this inelastic “overstrength”, depend on the section and the quality of the steel, but even more on the slenderness of each column. We also noticed, that the bigger the initial imperfection (initial bending displacement), the less the critical load, and the bigger the final displacement of the column.Αντώνιος Χ. Ανουσάκη

Economic Rationality and the Areeda–Turner Rule

The Areeda-Turner rule in U.S. antitrust jurisprudence limits successful predatory pricing cases to circumstances where prices can be shown to have been set below marginal costs. While not cast so, the rule reflects the view that predatory pricing is rarely attempted; and even where attempted is rarely successful; and even where attempted and successful, is difficult to identify. In this paper, we examine the theoretical and empirical foundations of this rule, and conclude that it is time to demote the Areeda-Turner analysis from the status of a rule to that of a potentially useful form of inquiry in predatory pricing litigation, but one which is neither necessary nor dispositive

Unravelling mycorrhiza-induced wheat susceptibility to the English grain aphid Sitobion avenae

Arbuscular mycorrhizal (AM) fungi are root symbionts that can increase or decrease aphid growth rates and reproduction, but the reason by which this happens is unknown. To investigate the underlying mechanisms of this interaction, we examined the effect of AM fungi on the English Grain aphid (Sitobion avenae) development, reproduction, attraction, settlement and feeding behaviour on two naturally susceptible varieties Triticum aestivum (L.) variety Solstice and T. monococcum MDR037, and two naturally resistant lines, T. monococcum MDR045 and MDR049. Mycorrhizal colonisation increased the attractiveness of T. aestivum var. Solstice to aphids, but there was no effect on aphid development on this variety. Using the Electrical Penetration Graph (EPG) technique, we found that mycorrhizal colonisation increased aphid phloem feeding on T. monococcum MDR037 and MDR045, colonisation also increased growth rate and reproductive success of S. avenae on these varieties. Mycorrhizas increased vascular bundle size, demonstrating that these fungi can influence plant anatomy. We discuss if and how this could be related to an enhanced success rate in phloem feeding in two varieties. Overall, we present and discuss how mycorrhizal fungi can affect the feeding behaviour of S. avenae in wheat, inducing susceptibility in a resistant variety