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Different Laricobius nigrinus Fender release locations, numbers of predators, and timing of release were evaluated for biological control of the hemlock woolly adelgid (HWA), Adelges tsugae Annand (Hemiptera: Adelgidae). It established at 59% of the sites and location was the most important factor related with establishment and abundance, HWA density, and hemlock vigor index. Cold locations had poor establishment or low abundance, declines in HWA density, and increases in hemlock vigor over time. Paired release and control sites detected a predator impact on HWA density, but densities remained high and tree vigor declined. The phenology of L. nigrinus, L. rubidus LeConte, and HWA were studied at a field insectary and the species were highly synchronized. A cage exclusion study showed that HWA survival and density were lower and ovisac disturbance was higher when exposed to predation. To improve L. nigrinus monitoring, we compared beat sheets for adults or branch clipping for immatures, and the host searching behavior of L. nigrinus was studied to understand how it locates a tree and HWA. In the Appalachians, beat sheet sampling resulted in false negatives as larvae were collected by branch clipping. Adults orientated to a tree visually, fed when prey were present and flew when absent, and showed different search patterns on infested versus uninfested trees. In Seattle, both sampling methods detected L. nigrinus because the predator was common. Predator : prey ratios were high at heavily infested sites in Seattle and low in the eastern US, where is has been released recently. Partial life tables were constructed for HWA sistentes at four sites for 2 yr in Seattle. Unspecified causes of nymph and adult mortality were high and L. nigrinus was the dominant predator of ovisacs. Adult L. nigrinus abundance was positively related to HWA density and immature abundance was related to ovisac density, indicating an aggregation and numerical response to its prey. Laricobius nigrinus has not demonstrated complete biological control of HWA to date, but it may do so in the future and continued release is justified. / Ph. D.

Les cultures de Brassicacées sont très courantes en agriculture urbaine à Montréal. Elles comportent cependant leur lot de problèmes liés aux insectes ravageurs, tels que la piéride du chou, Pieris rapae. L’utilisation de méthodes de lutte classique contre les insectes ravageurs, tels les insecticides, engendrent de sérieux dommages environnementaux, incluant la contamination de l’eau et du sol ainsi que la toxicité pour les plantes environnantes, les insectes auxiliaires et les microorganismes du sol. Le compagnonnage végétal, une pratique agroécologique alternative, est connue en tant que stratégie d’IPM (lutte intégrée contre les insectes ravageurs). L'objectif général de l'étude était d'évaluer in situ l'importance de diverses méthodes de compagnonnage sur l'infestation de P. rapae au sein du chou chinois, Brassica rapa. Les plantes utilisées dans les systèmes de compagnonnage étaient des cultures-pièges (Eruca sativa et Brassica carinata), des plantes compagnes principales qui étaient des plantes insectaires et répulsives pour les insectes ravageurs (Tagetes erecta, Amaranthus cruentus et Ocimum grattissimum) ainsi que des plantes compagnes secondaires (Solanum aethiopicum, Ocimum basilicum et Hibiscus sabdariffa). Les paramètres de physiologie végétale ainsi que les structures des communautés microbiennes et d’insectes ont été suivis de près tout au long de la saison croissance où l’expérience a eu lieu. La taille des larves était significativement plus importante au sein de la monoculture (contrôle) et les taux de concentration de glucosinolates dans les feuilles de Brassica rapa étaient deux fois plus élevés dans les contrôles comparativement aux systèmes de cultures plus diversifiés. Une PERMANOVA a confirmé une différence significative entre les méthodes de cultures concernant la composition des communautés d’insectes bénéfiques. Les communautés bactériennes du sol ont été améliorées par l'agriculture durable par rapport au sol d'origine et ont été bonifiées dans les systèmes de compagnonnage végétal (plus diversifiés). L'étude a mis en évidence les avantages de l'agroécologie, y compris le compagnonnage végétal, en termes de lutte intégrée contre les insectes ravageurs et d'autres composantes de l'agroécosystème. / Brassica crops are very common culture for urban farmers in Montreal where insect pests like the cabbage whitefly, Pieris rapae, are a real concern. The use of conventional insect pest control methods, such as insecticides, causes serious damages to a highly anthropized and therefore already constrained environment. These include air, water and soil contamination, as well as toxicity to surrounding beneficial insects, soil microorganisms, plants and the entire food chain. Companion planting, an agroecological practice, is an alternative strategy for insect pest management. The general objective of the study was to evaluate in situ the relevance of various traditional methods of companion planting on infestation of Chinese cabbage, Brassica rapa, by P. rapae. The plants used in the companion systems were two trap crop species: Eruca sativa and Brassica carinata; Tagetes erecta, Amaranthus cruentus and Ocimum grattissimum were used like companion plants because of their properties as insectary plants and repellent species against the targeted pest; Solanum aethiopicum, Ocimum basilicum and Hibiscus sabdariffa were also considered (secondary) companion plant species. Plant physiological parameters as well as microbial and insect community structure were carefully monitored over the growing season where this experiment took place. Larval size was significantly greater in the monoculture and glucosinolate concentrations in leaves of Brassica rapa was two-times higher in control than in more diversified cultivation systems. With larger yield and the absence of pest in the field, ‘trap cropping system’ was overall the most efficient albeit the 'mix of companion planting’ system had the lowest leaf area damage. A PERMANOVA confirmed a significant difference between the cultivation methods regarding beneficial insect communities’ composition. Belowground, soil bacterial communities were readily modified by sustainable agriculture practice, even more so in biodiversified systems. The study highlighted benefits of agroecology, including companion planting, in terms of integrated pest management and other components of the agroecosystem.