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Associational resistance
- 1. Associational resistance mediated by naturalenemies Safeena Majeed, A A PA1TAH082
- 3. Plant resistance Relative amountof heritable qualities that influence the ultimately degree of damage done by the insect Painter (1951)
- 4. Plant v/s insect Attackrates of plants by insect herbivores are affected by • Host plant defenses • Host plant nutrition • Community composition- most plant species exist in communities Stilling et al., 2003
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- 6. Repellent Plant Hypothesis •Less palatable neighbours can repel herbivores from plant patches. • Sometimes, one plant species may emit volatile chemical stimuli that interfere with the host-finding ability of phytophagous insects searching for other species Stilling et al., 2003
- 7. Attractant Decoy Hypothesis •Finally, plants of higher palatability may attract herbivores away from other plant species • Trap crops are often used- to lure pests away from economically important crops. Stilling et al., 2003
- 8. Natural Enemies Hypothesis •Natural enemies are often more abundant in diverse plant communities • most enemies are generalists • better survive on the greater richness of herbivores there. • Generalists often suppress herbivore populations more in polycultures than in monocultures • Much less- effects of plant community composition on specialists enemies, particularly parasitoids- importance in biological control Russell, 1989
- 9. Associational Resistance • Resistancemediated by the third trophic level–the natural enemies of the herbivores. • Association of herbivore-susceptible plant species with herbivore-resistant plant species can reduce the herbivore density on the susceptible plant species. This is known as associational resistance Stilling et al., 2003
- 10. • Neighbours mayreduce damage to a focal plant by lowering specialist herbivore loads associational resistance hypothesis • Enhance damage by increasing generalist herbivore loads associational susceptibility hypothesis Plath et al., 2012
- 11. Factors contributing • Chemicaland physical interference of neighboring plant species • Greater abundances and diversity of natural enemies • Host density is predicted to have strong effects on specialist - resource concentration hypothesis Plath et al., 2012
- 12. Resource Concentration Hypothesis Specialist herbivoreloads increase with higher densities of the host plant in a given area unit, as specialist herbivores may locate their host plant more easily and stay longer in patches with high host plant densities. Plath et al., 2012
- 13. Associational Susceptibility Hypothesis • Plantsin diverse stands may suffer more from herbivore attack than plants in single-species stands • Occur when, • principal pests are generalist herbivores, benefit from the broader diet range of diverse plant communities • plant is a less-preferred host growing in close proximity to a highly preferred host, allows a spill-over of generalist herbivores after depletion of the favored host plant Plath et al., 2012
- 14. Associational resistance v/s susceptibility •Associational effects of tree diversity on herbivory patterns • Tropical focal tree Tabebuia rosea • Tree monocultures and mixed stands. • Chrysomelid Walterianella inscripta • Pyralid Eulepte gastralis • Tree diversity exerted opposite effects on tree infestation by the two herbivores Plath et al., 2011
- 15. • Resource concentrationeffects for the chrysomelid beetle - favored by tree monoculture • Resource dilution effects for the pyralid caterpillar favored by tree mixture Plath et al., 2011
- 16. • Herbivore damage washigher in mixed stands than in monocultures • Indicates that T. rosea did not benefit from associational resistance • Experienced associational susceptibility Plath et al., 2011
- 17. Odour-masking’ substances • Releaseof OMS into the air by non-host plant species is considered to confer some protection to the associated host plants. • The possibility that the odour of the host plant could be ‘masked’ by that of the non-host plant • now seems much less likely, though not impossible Finch & Collier, 2000
- 18. Host plant selectionby the cabbage root fly • was disrupted when its host plants were surrounded by a range of different plants • weeds spurrey (Spergula arvensis) • peas (Pisum sativum) • rye-grass (Lolium perenne) • clover • each of these non-host plants has a different odour profile • capable of preventing an adapted specialist insect from finding its host plants Finch & Collier, 2000
- 19. AR mediated byNE • Examined parasitism levels of hosts in simple and diverse plant communities • Higher rates of parasitism in plant monocultures, • ease of searching • lack of disruption of chemical cues • Addition of plant species to a community can increase the effectiveness of some specialist NEs • As a result of spilling over of NEs from herbivores on one host plant species to herbivores on another plant species. • AR- the herbivore population decreases on one host plant species by adding another species- mediated by NEs
- 21. AR in Asphondyliaborrichiae • The gall midge, Asphondylia borrichiae Rossi and Strong (Diptera: Cecidomyiidae) • Attacks the terminals of both Borrichia and Iva • Borrichia suffering higher rates of galling. • Host-plant choice experiments & recent electrophoretic studies • each host plant supports its own race of A. borrichiae • Both races share the same specialist parasitoids. Stilling et al., 2003
- 22. Hymenopteran parasitoids • Rileyacecidomyiae Ashmead (Eurytomidae) • Tenuipetiolus teredon (Walker) (Eurytomidae) • Torymus umbilicatus (Gahan) (Torymidae) • Galeopsomyia haemon (Walker) (Eulophidae) • Most parasitoid- similar size, with similar length ovipositors & attack galls of similar size. • T. umbilicatus, a large torymid • ovipositor twice as long as those of the other species • more common in large galls since it can oviposit last • feed on either fly larvae or other parasitoid larvae hyperparasitism. Endoparasitic Ecto & facultative hyperparasite Stilling et al., 2003
- 23. • Bagging experimentsconfirmed that T. umbilicatus - attacks galls after the other three species • Both parasitoids and gall flies are multivoltine with overlapping generations. Stilling et al., 2003
- 24. (a) Avg. galldensity/1000 stems (b) % parasitism of flies inside galls Stilling et al., 2003
- 25. Changes in parasitoidcommunity Stilling et al., 2003
- 26. Changes in (a)Gall density (b) % parasitism Stilling et al., 2003
- 27. • In thiscase, the ARs are strongly asymmetrical. • Asphondylia galls on Borrichia are much larger than those on the Iva with larger Torymus umbilicatus parasitoid • When Borrichia is present- source of Torymus parasitoids - easily parasitisation of flies in the small galls of the Iva sp. • Parasitism levels on Iva flies are significantly higher in the presence of Borrichia than on pure patches • Parasitoids that attack the Iva flies are small - cannot parasitise the fly larvae within bigger Borrichia galls and hence parasitism on Borrichiais unchanged in the presence of Iva. These results are consistent
- 28. Conclusion • Mixed plantingnot always benefits- AS • Have implications for the design of new planted forests, which are mostly managed as monocultures • Identity and biology of herbivore species has to be taken into account when attempting to predict damage patterns • AR – mediated by NEs
- 29. Future line ofwork • Relative strength & frequency of AR mediated by NEs • Relative patch sizes of host plants • Relative distances between host patches • Host choice of natural enemies • Relative dispersal distances of both herbivores and their natural enemies
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