Ecological windows for stable tritrophic interactions in agro-ecosystems.
Creators: name: Lorenzetti, François.
Publisher: name: Université d'Ottawa / University of Ottawa
Year: 1998
Subjects: subject: Biology, Botany.

description: This thesis addresses the question of how plant resistance, by mediating herbivore number, affects the surivorship of the immature stages of a natural enemy. It was hypothesized that survivorship would initially increase and then saturate with decreased resistance and, thus, increased herbivore number. This hypothesis was tested in a maize agro-ecosystem composed of differentially resistant maize genotypes (DIMBOA-based resistance), the corn leaf aphid Rhopalosiphum maidis (Homoptera: Aphididae), and ladybird beetles (Coleoptera: Coccinellidae). The effect of plant stress on the survivorship of ladybird larvae was also investigated. Stress was imposed by either increasing plant density or by varying nitrogen supply. A preliminary laboratory experiment indicated that DIMBOA did not influence the development of larvae of the ladybird Propylea quatuordecimpunctata fed aphids from a high DIMBOA genotype, but the combination of plant stress and resistance significantly increased pupal development time by about half a day (Chapter III). Mature ladybirds showed a significant preference for non-infested stressed greenhouse grown plants transferred to the field (Chapter IV). However, in the presence of aphids, the reproductive response of ladybirds in the field was related to peak aphid number, not to plant appearance per se when plant were nitrogen stressed (Chapter V). In the greenhouse, a 2-fold increase in DIMBOA concentration was observed in stressed plants of a high DIMBOA genotype, whereas no significant variation was found between non-stressed and stressed plants of both intermediate and low DIMBOA genotypes (Chapter II). The increase in DIMBOA concentration in stressed plants of the high DIMBOA genotype did not result in increased resistance to aphids in the laboratory because this genotype was already highly resistant. However, plant stress significantly decreased the growth of aphid colonies on the low DIMBOA genotype. In the field, variations in peak aphid number, with respect to maize genotype and to plant stress, were consistent with laboratory results. (Chapter V). Thus, the deployment of intermediate or high DIMBOA genotypes can be considered to be stable strategies against aphids in this system. In the field, the reproductive response of ladybirds saturated rapidly with respect to peak aphid number. However, the survivorship of immature ladybirds did not saturate, as predicted. Although survivorship initially increased with peak aphid number, as predicted, it decreased rapidly with further increase in peak aphid number. Two laboratory experiments were performed to isolate factors thought to be important in shaping the survivorship of ladybird larvae. The first experiment (Chapter VI), in which encounter rate among larvae and alternative food source were controlled for, indicated that cannibalism occurred readily and was density-dependent. In the second experiment (Chapter VII), small cages containing one sorghum (Sorghum vulgare) plant, aphid colonies of different size, and an increasing number of ladybird larvae were used. The pattern observed in the field was confirmed in this fully factorial experiment. The results suggest that an intermediate resistance strategy opens a window of stability in agro-ecosystems in which aphids are the consumers and ladybird beetles are the predators. (Abstract shortened by UMI.)