No description available.

No description available.

Neighbouring plants in the locale of an individual plant may help or hinder it in the task of defence against herbivores, depending on their levels of defence (chemical or physical), and their interactions with potential herbivores. Such ‘neighborhood effects’ are part of the complex network of species interactions that structure ecological communities. This thesis sets out to test whether there are neighbourhood effects on insect herbivory among the plants of the Al Jabal Al Akhdar region of Libya. Having chosen to concentrate on the two main species of three study sites, Juniperus phoenicea and Pistacia atlantica, nine plots were mapped in detail and the insect herbivores sampled from focal plants, and then from all plants. Leaves were sampled for chemical analysis of their phenol (tannin) content. The set of insect herbivores collected from plants in the plots were identified to species using the expertise of the staff of the Natural History Museum in London. Some insects recorded are new to Libya, and there are several species not previously recorded as feeding on either of the two plant species studied. Tannin levels were much higher in Pistacia than in Juniperus, and there were effects of elevation as well: plants from middle elevation plots had the highest levels, while those from the lowest elevation at the coast had the lowest levels of tannins. There were clear effects of neighbouring plants on the insects of individual plants, in both Pistacia and Juniperus. These were much more complex effects in Pistacia, but both sets of predictors of insect herbivore density or species richness contained clear signs of neighbourhood effects, where the existence of close neighbours reduced the herbivore load on individual plants. There were no signs of any protective neighbourhood effects of tannin levels. Thus in the plant communities of Al Jabal Al Akhdar, associational avoidance appears to be the major mechanism of neighbourhood effects, rather than associational resistance.

Bibliography: leaves 121-151. / xix, 151 leaves : ill. (some col.) ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Examines seasonal variation in population numbers of Diamondback moths, and estimates mortality, due to parasitoids, of larvae at different stages of development. / Thesis (Ph.D.)--University of Adelaide, Dept. of Crop Protection, 1996

Dans un contexte de réduction de la pression phytosanitaire sur les cultures il est importantde recourir à des méthodes permettant de réduire l’usage des pesticides pour lutter contreles ravageurs. Un des moyens consiste à réguler les ravageurs par leurs ennemis naturels(Lutte biologique par conservation). Le carpocapse du pommier, Cydia pomonella, est unravageur majeur des vergers de pommiers au Sud Est de la France. L’objectif de cette thèseétait de définir les déterminants du parasitisme larvaire de ce ravageur. Nous avonscaractérisé la composition de la communauté des parasitoïdes sur les larves diapausantes etde saison du carpocapse. Cette communauté est majoritairement représentée par troisespèces d’hyménoptères : Ascogaster quadridentata, Pristomerus vulnerator (parasitoïdesprimaires) et Perilampus trisits (parasitoïde secondaire d’A. quadridentata et de P.vulnerator) dans les zones étudiées. Nous avons déterminé les caractéristiques des pratiquesagricoles et des éléments semi-naturels au niveau du verger et du paysage qui affectent letaux de parasitisme et la composition de la communauté de parasitoïdes pour des larvesdiapausantes du carpocapse. Les haies brise-vent et les haies composites autour du vergersemblent jouer sur la composition de la communauté en favorisant par leur présence lesparasitoïdes primaires A. quadridentata et P. vulnerator par rapport au parasitoïdesecondaire P. tristis. La protection phytosanitaire au niveau du verger et du paysageenvironnant a un effet sur le taux de parasitisme. Le taux de parasitisme est plus élevé dansles vergers en agriculture biologique que dans les vergers conventionnels ainsi que dans lesvergers entourés d’une faible proportion de vergers conventionnels dans un voisinage de250 m. Nous avons étudié les dynamiques temporelles de la communauté des parasitoïdesdu carpocapse. Le taux de parasitisme a globalement augmenté au cours de la saison dedéveloppement du carpocapse et il est plus élevé lorsque l’on piège de jeunes larves dans lesfruits que des larves âgées dans les bandes cartonnées enroulées autour des troncs. Lacomposition de la communauté varie au cours du temps. La proportion relative duparasitoïde secondaire P. tristis augmente au cours de la saison en parallèle d’unediminution de la proportion relative d’A. quadridentata. Les émergences des adultes A.quadridentata sont de plus synchronisées avec celles des adultes carpocapses. Enfin, nousavons développé et testé une méthode de PCR-RFLP et des marqueurs ADN spécifiques pourdétecter et identifier les parasitoïdes du carpocapse. La PCR-RFLP permet d’identifier lesparasitoïdes adultes et leurs hôtes. Les marqueurs spécifiques permettent, en outre, ladétection de parasitoïdes dans les oeufs et les jeunes larves de carpocapse. Ces approchesmoléculaires ont également permis d’évaluer le parasitisme dans des populations naturellesde carpocapse et d’estimer les interactions trophiques au sein de la communauté desparasitoïdes / In the context of a more environment-friendly agriculture, it is important to design methodsthat enable us to reduce the use of pesticides to fight pests. One possible way consists inincreasing pest regulation by their natural enemies (Conservation biological control). Thecodling moth, Cydia pomonella, is a major insect pest of apple orchards in SoutheasternFrance. The aim of this thesis was to identify determinants of the larval parasitism of thispest. We characterized the composition of the parasitoid community on diapausing and nondiapausing codling moth larvae. This community is mainly represented by threeHymenoptera species: Ascogaster quadridentata, Pristomerus vulnerator (two primaryparasitoids) and Perilampus trisits (a secondary parasitoid of A. quadridentata and P.vulnerator) in the study sites. We determined the characteristics of agricultural practices andsemi- natural habitats at the orchard and landscape level that affect the parasitism rate andthe composition of the parasitoid community of diapausing codling moth larvae. Thewindbreak and spontaneous hedgerows around the orchard seemed to impact theparasitoid community composition by promoting, when present, the primary parasitoids A.quadridentata and P. vulnerator versus the secondary parasitoid P. tristis. Crop protectionpractices at the orchard and surrounding landscape levels affected the parasitism rate.Parasitism rate was higher in organic orchards than in conventional orchards as well as inorchards surrounded by a low proportion of conventional orchards in a 250 m vicinity. Wefurther studied the within-season temporal dynamics of the codling moth parasitoidcommunity. The parasitism rates globally increased along the season among cohorts ofmature codling moth larvae and were higher in young larvae trapped in fruits than in maturelarvae trapped in band traps around the tree trunks. The community composition variedalong the season. The relative proportion of the secondary parasitoid P. tristis increasedamong the codling moth cohorts whereas the proportion of A. quadridentata decreased.Furthermore, the emergences of adult A. quadridentata were synchronized with theemergences of the adult codling moths. Finally, we developed and tested a PCR -RFLPmethod and specific DNA markers to detect and identify parasitoids of the codling moth. ThePCR -RFLP method was powerful to identify adult parasitoids and their hosts. Specificprimers allowed detection of parasitoids in the eggs and young larvae of codling moth. TheseDNA-based techniques allowed molecular evaluation of parasitism in C. pomonella naturalpopulation and reconstructing quantitative food web of the parasitoid community.