Bibliography : leaves [70]-79.

<p>Plant resistance hypothesis says that under a period of time when a plant is exposed to powerful herbivore attack it will prioritise defence as a major metabolic function. In theory, induced plant defence (resistance) will provide opportunities for this organism to “invest” in other functions, in example growth when attackers are absent.</p><p>One of the compounds taking part in plant defence is callose. This β-1,3-glucan is synthesised by callose synthase and broken down by β-1,3-glucanase. Deposition of callose occurs as a reaction to aphid attack an varies, depending on cultivars, and aphid species. In this experiment barley (Hordeum vulgare) cultivar Clipper is being infested with two types of aphids: Russian wheat aphid (RWA, Diuraphis noxia) and bird cherry-oat aphid (BCA, Rhopalosiphium padi) over a time period. Infestation by those two insects results in different callose formation and deposition level.</p><p>Six sequences encoding for putative callose synthase genes and nine sequences encoding for β-1,3-glucanase were examined by RT-PCR and Real – Time PCR methods for different expression patterns.</p><p>The results did not show any significant regulation of gene expression during RWA and BCA attack for any of these genes. Thus the pathway regulating aphid – induced callose deposition in barley reminds unresolved.</p>

Plant resistance hypothesis says that under a period of time when a plant is exposed to powerful herbivore attack it will prioritise defence as a major metabolic function. In theory, induced plant defence (resistance) will provide opportunities for this organism to “invest” in other functions, in example growth when attackers are absent. One of the compounds taking part in plant defence is callose. This β-1,3-glucan is synthesised by callose synthase and broken down by β-1,3-glucanase. Deposition of callose occurs as a reaction to aphid attack an varies, depending on cultivars, and aphid species. In this experiment barley (Hordeum vulgare) cultivar Clipper is being infested with two types of aphids: Russian wheat aphid (RWA, Diuraphis noxia) and bird cherry-oat aphid (BCA, Rhopalosiphium padi) over a time period. Infestation by those two insects results in different callose formation and deposition level. Six sequences encoding for putative callose synthase genes and nine sequences encoding for β-1,3-glucanase were examined by RT-PCR and Real – Time PCR methods for different expression patterns. The results did not show any significant regulation of gene expression during RWA and BCA attack for any of these genes. Thus the pathway regulating aphid – induced callose deposition in barley reminds unresolved.

No description available.

No description available.

3 pp. / A description of aphids, the damage they cause, their lifecycle, and management recommendations.

Symbiotic associations between animals and microbes are widespread in nature, yet the factors controlling the abundance and distributions of particular symbionts are mostly unknown. Vertically transmitted (VT) symbionts can invade host populations by providing net benefits to hosts. While most beneficial symbiotic interactions that have been characterized are nutritional, other major pathways likely exist that facilitate symbiont transmission within host populations. Natural enemies, for example, are important selective forces in shaping the life history of many invertebrates and hosts may benefit from collaborations with microbes to aid in their defense.In this dissertation I have addressed the role of VT, facultative (= secondary) symbionts (SS) of Acyrthosiphon pisum in mediating interactions with an important natural enemy, the parasitic wasp, Aphidius ervi. I found that, in a common genetic background, two A. pisum SS (R- and T-type SS) confer resistance to A. ervi, by causing mortality to developing wasp larvae. Defensive mutualisms with microbes provide a mechanism for the spread and persistence of VT symbionts.A. pisum superinfected with both R- and T-type SS were found to be more resistant to parasitism that those singly infected with either SS. Despite this added benefit to resistance, R + T-type superinfections were rare in a survey of A. pisum symbionts, likely attributable to severe fecundity costs. R-type densities increased dramatically in superinfected hosts and over-proliferation of SS may result in poor aphid performance. Thus, interactions among the symbionts themselves also likely play a critical role in determining the distributions of symbionts in nature.I also found that four T-type isolates from A. pisum, and one from another aphid species, all conferred resistance to parasitism in the same A. pisum host background. The levels of resistance varied greatly among isolates, including one that conferred nearly complete resistance. A single T-type isolate was also found to confer similar levels of resistance in five A. pisum backgrounds. These results indicate that SS-mediated resistance is a general phenomenon in A. pisum and that the SS isolate is more important in determining the level of resistance than is the aphid genotype or interaction between isolate and aphid genotype.

Threshold and non-threshold linear models that govern toxicology are challenged by an alternative model, hormesis. It is defined as low-dose stimulation and high-dose inhibition from a stressor. Insecticide-induced hormesis has been studied in a plethora of insect-insecticide models at biochemical, individual and population levels. This research focuses on the effects of sublethal concentrations of insecticides on reproductive responses of green peach aphid, Myzus persicae (Sulzer), at individual and population level besides regulation of stress, dispersal and developmental genes during hormesis. In laboratory studies, irrespective of the duration and route of exposure, sublethal concentrations of imidacloprid induced stimulations in fecundity of M. persicae but the nature of response differed intra- and trans-generationally. Fitness tradeoffs could be rendered due to declined fecundity in successive generations. However, continuous exposure to sublethal concentrations does not compromise overall fitness trans-generationally, considering recovered levels of fecundity as controls in successive generations and the total reproduction after four generations. Greenhouse experiments affirmed uncompromised fitness where reproductive stimulations were noted in aphids exposed to imidacloprid treated potato plants. Up- and down- regulation of stress, dispersal and developmental genes was noted during imidacloprid-induced hormesis in M. persicae that mirrored the reproductive responses in few instances. Global DNA methylation results emphasized the heritability of adapted traits trans-generationally via hypermethylation. Dispersal related genes (OSD, TOL and ANT) that are predominantly expressed in alates (about 2- to 5-fold) were affected in apterous aphids continuously exposed to sublethal concentrations of imidacloprid. No direct relation with the previously noted fecundity was established implying adaptive cellular stress response pathways might be triggered rather than normal regulatory processes due to low-dose imidacloprid exposure. At a biochemical level, a study noted that imidacloprid-induced hormesis concurrently stimulated juvenile hormone III (JH) production and fecundity in M. persicae. Precocene, an anti-JH, at sublethal concentrations induced reproductive stimulations in M. persicae. Gene regulation during precocene-induced hormesis mirrored imidacloprid results for few genes including FPPS, a JH precursor gene, with a higher magnitude of regulation. Considering these stimulatory effects that insecticide-induced hormesis at various biological hierarchies, causes for pest resurgence, hormesis could have ramifications from declines in natural enemy population.

Thesis (M.S.)--University of Hawaii at Manoa, 2007. / Includes bibliographical references (leaves 48-50). / vi, 50 leaves, bound ill. 29 cm

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.