Multitrophic interactions involving the giant willow aphid, Tuberolachnus salignus (Gmelin) : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Ecology at Massey University, Manawatu, New Zealand

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Massey University
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The giant willow aphid Tuberolachnus salignus, Gmelin, 1790 has become an important pest of willows, causing negative impact on host plant physiology and growth, and indirectly posing a unique problem in the apiculture industry. As it is a new invasive species to New Zealand, aphid-host interactions, extent of damage and ecological impacts of aphid infestation are not yet known. Aphid interactions with host plants, associated insect species and soil microbes were addressed in this thesis to fill knowledge gaps in formulating sustainable aphid management. Aphid population numbers and the extent of plant damage were cultivar-specific, with wide variations between resistant and susceptible cultivars in Chapter 2. Aphid infestation delayed the flowering time, extended the duration of flowering, and decreased the catkin length in susceptible cultivars. Interestingly, aphid infestation was found to increase the total floral output of some willow cultivars. Aphid infestation had no measurable effect on the number of shoots of willow cultivars, but reduced the survival, height, and shoot diameter of the plants by the end of the second growth season. In Chapter 3, the VOC emissions were cultivar-specific and varied with plant type (tree vs. shrub willows). The results also showed that resistant cultivars appear to emit more green leaf volatiles than other cultivars, suggesting that there can be a link between T. salignus resistance and VOC emission in willows, which deserves further exploration. However, most cultivars did not experience significant changes in their VOC emissions after aphid attack, while few have reduced emissions. It was proven in Chapter 4 that melezitose concentration in T. salignus honeydew did not vary with willow cultivar or plant age, but concentrations of other sugars (such as fructose) did. There was no obvious link between willow susceptibility to T. salignus and melezitose content, however, total honeydew sugar concentration was lower while fructose content was higher in highly susceptible cultivars identified in Chapter 1. Tuberolachnus salignus honeydew deposition has multiple ecological impacts. Copious amounts of honeydew fall on the understory vegetation or directly on the soil surface, resulting in irregular occurrence of black sooty mould areas under aphid-infested plants. This carbon-rich energy source is utilized by soil microorganisms (fungi, bacteria and yeasts), in turn increasing the abundance of fungivores and their predators in honeydew-receiving soil. This honeydew-mediated cascading effect was directly linked with honeydew availability and the level of input, with strongest effect in black sooty mould spots (Chapter 5). The results in Chapter 6 show that although this predator can feed on T. salignus, the aphid is not its preferred prey. H. axyridis that fed on immature T. salignus developed slower than on alternative prey, and preferentially selected other aphid prey species in dual choice tests, rejecting T. salignus. The results suggested that H. axyridis should not be promoted as a biocontrol agent for T. salignus. This work is a significant contribution to our understanding of the impacts of the giant willow aphid in New Zealand, and provides useful information towards the selection of resistant cultivars and biocontrol agents.
Aphids, Willows, Diseases and pests, New Zealand