Insect-symbiont interactions have many bioecological consequences to the host. Their relationships expand through a complex network that includes other microorganisms, interactions with the environment and other trophic levels. An extensive literature has been produced on bacterial symbionts and aphids, especially for polyphagous aphids from North America and Europe, such as Acyrthosiphon pisum (Harris). They indicated symbionts influence host nutrition, heat tolerance, defense against natural enemies, virus transmission, host plant exploitation, among others. However, the outcome from host-symbiont interactions is context-dependent, with the expressed phenotype depending on intra and interspecific variations, symbiont strain, and biotic/abiotic stimuli. We explored the interaction between aphids and associated symbionts, aiming to contribute to this field by exploring new aphid systems, and focused on Aphis (Toxoptera) citricidus (Kirkaldy) and associated microbiota. We investigated the influence of the feeding habits on symbiont diversity in an oligophaogus, A. citricida, as compared to a polyphagous species, Aphis (Toxoptera) aurantii Boyer de Fonscolombe. We employed several approaches (biological, metagenomics, genomic, and proteomic) to investigate i) the impact of host plants on fitness traits and primary symbiont abundance in the oligophagous and polyphagous species, ii) differences in the draft genome of the primary aphid symbiont between A. citricidus and A. aurantii, iii) the influence of host plant in secondary symbiont distribution in A. citricidus, iv) secondary symbiont richness and abundance in both aphids, and investigate the effects of Spiroplasma infections v) in the fitness traits, vi) transcriptome and ii) proteome of A. citricidus when reared on two host plants (sweet orange and orange jasmine). Our data indicated that sweet orange is a better host plant than orange jasmine for both aphids, and that A. citricidus was more negatively affected by lower-quality host than A. aurantii. A. citricidus and A. aurantii had different strategies regarding Buchnera growth and the use of food in different stages of development. We observed differences in the draft genome of Buchnera associated to A. citricidus and A. aurantii. Host plant affected secondary symbiont abundance, but Spiroplasma was the most abundant symbiont in both aphids. Spiroplasma had neutral effects on A. citricidus biology, but affected host transcriptome and proteome. The host plant affected gene expression of A. citricidus, but the effect was dependent on Spiroplasma infection. Transcriptome analysis indicated Spiroplasma down-regulated aphid immune response genes on sweet orange, while regulating an entire different set of genes on orange jasmine, mainly chaperonins. Gene transcription of A. citricidus was strongly influenced by the host plant. But while a large number of transcripts were up-regulated in uninfected aphids in sweet orange as compared to orange jasmine, the same set of genes had an opposite pattern of expression in Spiroplasma-infected aphids. Comparative proteomic analysis of Spiroplasma-infected and uninfected aphids on sweet orange and orange jasmine demonstrated regulation of a larger number proteins on orange jasmine than on sweet orange. Spiroplasma down-regulated the immune response of aphids and up-regulated proteins related to nutritional processes when developing on a low-quality host plant, orange jasmine, while no such trend was observed on sweet orange. / A associação insetos - bactérias simbiontes resulta em várias implicações bioecológicas para o hospedeiro e se estende em uma rede de interações que inclue outros microrganismos, o ambiente e outros níveis tróficos. A interação bactérias simbiontes e pulgões tem sido amplamente estudada, especialmente em espécies polífagas do hemisfério norte, tal como o pulgão modelo Acyrthosiphon pisum (Harris). Essas pesquisas indicam que simbiontes influenciam vários aspectos ecológicos do hospedeiro, tais como: nutrição, resistência ao estresse térmico e inimigos naturais, capacidade de transmissão de fitovírus, uso de plantas hospedeiras, entre outros. Entretanto, o resultado da interação simbionte-pulgão é contexto dependente, no qual o fenótipo depende da variabilidade intra- e interespecífica, da linhagem do simbionte e de diversos fatores bióticos e abióticos. Assim, este trabalho aborda as interações do pulgão Aphis (Toxoptera) citricidus (Kirkaldy) e seus simbiontes, com o objetivo de contribuir com um sistema ecológico diferente. Foi investigada a influência da estratégia alimentar na diversidade da microbiota, comparando-se uma espécie oligófaga, A. citricidus, a outra polífaga, Aphis (Toxoptera) aurantii Boyer de Fonscolombe. Foram utilizadas diversas abordagens experimentais (biológica, metagenômica, genômica, transcritômica e proteômica) para atingir os objetivos de i) investigar o impacto das plantas hospedeiras na aptidão biológica de A. citricidus e A. aurantii e sua consequência ao simbionte primário, ii) explorar as diferenças no genoma do simbionte primário de A. citricidus e A. aurantii, iii) investigar a influência da planta hospedeira na distribuição de simbiontes secundários em A. citricidus, iv) pesquisar a riqueza e abundância de simbiontes secundários nas duas espécies de pulgão e indagar sobre a influência de Spiroplasma v) na aptidão biológica, vi) no transcritoma e vii) na proteômica de A. citricidus em duas plantas hospedeiras (laranja vs. murta). Os resultados indicaram ser laranja o melhor hospedeiro para ambos pulgões, sendo A. citricidus mais sensível à planta hospedeira que A. aurantii. As duas espécies de pulgão utilizam estratégias distintas para controlar o crescimento de Buchnera, assim como a utilização do alimento influenciou diferentemente cada fase do desenvolvimento dos pulgões estudados. A planta hospedeira também influenciou a abundância de simbiontes secundários; porém, Spiroplasma foi o simbionte secundário mais abundante em ambas espécies. Spiroplasma não afetou a biologia de A. citricidus, mas causou alterações no transcritoma e no proteoma do hospedeiro. A planta hospedeira também exerceu forte regulação na transcrição gênica de A. citricidus, mas esse efeito foi dependente da infecção do pulgão por Spiroplasma. Análises de transcritoma em pulgões infectados indicaram a regulação de transcritos relacionados à resposta imunológica quando em laranja, mas de chaperoninas em murta. A regulação gênica de A. citricidus foi fortemente influenciada pela planta hospedeira, mas enquanto insetos livres de Spiroplasma apresentaram superexpressão gênica em laranja quando comparada a murta, pulgões infectados com esse simbionte apresentaram padrão de expressão oposto para o mesmo conjunto de transcritos. Análises comparativas do proteoma de pulgões infectados ou não por Spiroplasma nas plantas hospedeiras estudadas indicaram diminuição dos mecanismos de defesa em favor de um aumento de proteínas ligadas à nutrição em insetos infectados quando se alimentando em murta.

Swede midge, Contarinia nasturtii Kieffer (Diptera: Cecidomyiidae), is an invasive pest causing marketable losses on Brassica crops in the Northeastern United States and throughout southern Canada. Heading brassicas, like cauliflower and broccoli, are particularly susceptible because larvae feed concealed inside meristematic tissues of the plant, where head formation occurs. Our work details the development of a sustainable, affordable pest management tactic for swede midge – plant derived repellents. First, it was necessary to establish both a damage and marketability threshold for swede midge, so we developed a technique to manipulate larval density of swede midge on cauliflower, We asked: (1) What is the swede midge damage threshold? (2) How many swede midge larvae can render cauliflower crowns unmarketable? and (3) Does the age of cauliflower at infestation influence the severity of damage? We found that even a single larva causes mild twisting and scarring rendering cauliflower unmarketable 52% of the time, with more larvae causing more severe damage and additional losses, regardless of cauliflower age at infestation. Repellency is an important management approach to consider for swede midge. Since the host range of specialist insects appears constrained by plant phylogeny, we hypothesized that odors from less phylogenetically related plants would be more repellent to swede midge. To test our hypothesis, we performed no-choice and choice biological assays, asking: (1) How do essential oils from different plant species influence midge densities on broccoli? (2) What is the relationship between phylogenetic distance of non-host odors and larval densities on broccoli? Biological assays identified multiple essential oils that reduced larval densities, and phylogenetic analyses showed that less related plants were more effective. In addition to the biological assays, we tested 15 essential oils for their ability to repel gravid females from broccoli tissue in y-tube olfactometer assays. While most of the essential oils reduced the frequency at which females chose host plant meristems, wintergreen, thyme, lemongrass, eucalyptus lemon, garlic, cinnamon, and star anise were most effective. Additionally, we used chemical fingerprints (physical/chemical properties) from PubChem to compare the essential oil volatile compounds and develop an index for their similarity. We found that physicochemical similarity was predictive of repellency. Finally, for repellency to be an effective, long-term strategy, it was important to consider how and whether the repellent response of midges changes over time or previous experience. In our final chapter, we performed electroantennography trials testing how previous experience with garlic or eucalyptus lemon odor for one or 10 s influences the neurophysiological response of swede midge to host (broccoli) or non-host (garlic or eucalyptus lemon) odors. We asked: (1) Does previous experience with garlic or eucalyptus lemon influence the physiological response of swede midge to host or non-host odors? (2) Does the time of previous exposure to non-host odors influence their physiological response to host or non-host odors? Our findings show that swede midge, after 10 s of exposure to either repellent, was more responsive to repellents than host compounds, suggesting that the effectiveness of repellents will not diminish over time.