4 pp. / The alfalfa aphid discussed in this publication includes blue alfalfa aphid, pea aphid, and the spotted alfalfa aphid. This publication discusses the biology of these alfalfa aphids, the damages they cause, the resistant varieties and biological control, and their monitoring and treatments.

Published Article / Direct feeding damage to head lettuce (Lactuca sativa L.) by varying aphid populations in two differently constructed shadehouse structures (fully- and partially covered) was examined. Fresh lettuce head weight, the number of lettuce leaves formed, and the number of lettuce leaves infested with aphids were compared between the two structures. Warmer months showed a significant lower fresh lettuce head weight in the fully covered structure with more aphid-infested leaves. During June/September, the mean number of aphid-infested leaves and aphid infestation levels were significantly higher in the partially covered structure. Visible feeding damage to the lettuce crop was restricted, but asymptomatic damage in terms of a decrease in head weight did occur under severe infestation levels.

Insects have become increasingly resistant to chemical control methods, while at the same time public awareness of the harmful effects of synthetic pesticides has increased. The search for more environmentally 'friendly' means of pest suppression is gaining momentum and biological control (the use of natural enemies to reduce populations of noxious organisms) has become an increasingly sought-after option. Despite an increase in the establishment rate of insect natural enemies, classical biological control of arthropods is currently no more successful than it was one hundred years ago. Ecological theory relevant to population biology, food webs and diversity provides insight into how biological control agents can be made more effective, yet this theory has often been absent from the biological control literature. Examples of the use of ecological concepts (including intraguild predation, life-history omnivory and resource subsidies) in practical biological control are reviewed, and aspects of theory not yet considered in this context are discussed. Cereals are important as primary food crops, globally and within New Zealand. Possibly the greatest amount of damage sustained by cereal crops in New Zealand is caused by aphids and chemical control of these pests is very expensive relative to biological control. This thesis examines how biological control of the rose-grain aphid Metopolophium dirhodum (Walker) (Hemiptera: Aphidiidae) by the koinobiont, synovigenic endoparasitoid Aphidius rhopalosiphi De Stefani-Perez (Hymenoptera: Aphidiidae) can be enhanced by floral resource subsidies. The mechanisms underlying this enhancement were determined in a series of laboratory experiments and then tested in laboratory microcosms and in the field. Sugar resources significantly increased longevity and egg load in A. rhopalosiphi and another species of aphidiid, Diaeretiella rapae McIntosh. Pollen had no significant effect on longevity or egg load in these species. These results are discussed in terms of the effects of resource subsidies on egg- versus time-limitation. Laboratory microcosm experiments tested whether the mechanisms of increased potential fecundity via enhanced egg load and longevity translate into increased rates of parasitism (i.e., realised fecundity). Only treatments receiving sugar showed increased reproductive success. The presence of flowering buckwheat Fagopyrum esculentum Moench (c.v. Kitawase) (Polygonaceae) plants caused a slight, non-significant increase in rates of parasitism. Field surveys of natural aphid populations in a wheat Triticum aestivum (L.) (c.v. Otane) (Gramineae) field showed that proximity to floral buckwheat patches, distance to the nearest edge or the leeward end of the field were not significantly correlated with rates of parasitism. These variables were significantly correlated with aphid density in some surveys. Rates of parasitism were not correlated with aphid density. When aphid population density was controlled by experimental placement of aphids, proximity to floral resource patches significantly affected rates of parasitism. Parasitism rates were highest at the edges of buckwheat patches and declined exponentially with distance, eventually reaching zero beyond 14 m. Lower levels of parasitism were observed within the floral patches than at their edges. This phenomenon is compatible with the concept of spatio-temporal partitioning between parasitoid feeding and host-searching behaviours. Potential costs (increased predation risk, opportunity costs) and benefits (increased fecundity and longevity) associated with floral feeding by parasitoids, and temporal variation in these factors, are discussed. It is concluded that resource subsidies are not only effective in the practical enhancement of the efficacy of a specific biological control agent, but their use is based on a sound foundation in ecological theory that allows extension of these principles across taxa.

No description available.

Hereditary bacterial symbiosis is a common mechanism by which eukaryotic hosts can acquire traits beneficial for their fitness. Many insects have symbiotic associations with bacteria that trace back millions of years, whose function and evolution are well characterized. Insects can also possess more recently derived symbionts that are closely related to free-living bacteria, and often play a role in host defense. Serratia symbiotica is a recently derived symbiont that infects aphids and provides protection against heat stress, and possibly also plays a nutritional role. Many aspects of the biology of recent symbionts are less well studied, including the diversity of functional roles and evolution among hosts for single lineages of symbionts, the molecular mechanisms that contribute to defense, the early stages of symbiont genome evolution, and interactions with hosts. This dissertation focuses upon S. symbiotica to contribute research addressing each of these themes. Functional studies revealed that S. symbiotica lysis during heat-shock is correlated with protection of the nutritional symbiont Buchnera , and that S. symbiotica has a large effect upon aphid metabolite pools. Despite this large metabolic effect, S. symbiotica does not seem to dramatically influence expression of aphid genes, including those involved in immunity. Analysis of the evolution of S. symbiotica lineages in different aphid hosts revealed this symbiont is common in the aphid subfamily Lachninae, but did not support the obligate nutritional role hypothesized in the literature for this group. Finally, comparison of the S. symbiotica genome to close free-living relatives revealed a genome undergoing massive decay, and provided a rare opportunity to examine the evolution of a recently acquired symbiont.

No description available.

No description available.

No description available.

The activity of plant 9-lipoxygenases (LOXs) influences the outcome of Arabidopsis thaliana interaction with pathogen and insects. Evidence provided here indicates that in Arabidopsis, 9-LOXs facilitate infestation by Myzus persicae, commonly known as the green peach aphid (GPA), a sap-sucking insect, and infection by the fungal pathogen Fusarium graminearum. in comparison to the wild-type plant, lox5 mutants, which are deficient in a 9-lipoxygenase, GPA population was smaller and the insect spent less time feeding from sieve elements and xylem, thus resulting in reduced water content and fecundity of GPA. LOX5 expression is induced rapidly in roots of GPA-infested plants. This increase in LOX5 expression is paralleled by an increase in LOX5-synthesized oxylipins in the root and petiole exudates of GPA-infested plants. Micrografting experiments demonstrated that GPA population size was smaller on plants in which the roots were of the lox5 mutant genotype. Exogenous treatment of lox5 mutant roots with 9-hydroxyoctadecanoic acid restored water content and population size of GPA on lox5 mutants. Together, these results suggest that LOX5 genotype in roots is critical for facilitating insect infestation of Arabidopsis. in Arabidopsis, 9-LOX function is also required for facilitating infection by F. graminearum, which is a leading cause of Fusarium head blight (FHB) disease in wheat and other small grain crops. Loss of LOX1 and LOX5 function resulted in enhanced resistance to F. graminearum infection. Similarly in wheat, RNA interference mediated silencing of the 9-LOX homolog TaLpx1, resulted in enhanced resistance to F. graminearum. Experiments in Arabidopsis indicate that 9-LOXs promote susceptibility to this fungus by suppressing the activation of salicylic acid-mediated defense responses that are important for basal resistance to this fungus. the lox1 and lox5 mutants were also compromised for systemic acquired resistance (SAR), an inducible defense mechanism that is systemically activated throughout a plant in response to a localized infection. the lox1 and lox5 mutants exhibited reduced cell death and delayed hypersensitive response when challenged with an avirulent strain of the bacterial pathogen Pseudomonas syringae pv tomato. LOX1 and LOX5 functions were further required for the synthesis as well as perception of a SAR-inducing activity present in petiole exudates collected from wild-type avirulent pathogen-challenged leaves. Taken together, results presented here demonstrate that 9-LOX contribute to host susceptibility as well as defense against different biotic stressors.

Bibliography : leaves [70]-79. / ix, 80 leaves, [6] leaves of plates : ill. (some col) ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.)--University of Adelaide, Dept. of Crop Protection, 1994