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Some plant features may directly shelter natural enemies.
As predicted by the TTI, there was a three-way interaction among the effects of host-plant quality, herbivore diet breadth and natural enemies. That the generalist A. gossypii but not the specialist U. macolai varied in performance between plant sexes had tri-trophic consequences, as plant sex mediated natural enemy effects for the former but not the latter (). This result demonstrates the inadequacy of considering only the pairwise effects of these factors, and underscores the need for integrative theory.
This might explain why phytoseiids evolve resistance to pesticides more readily than other natural enemies, but it does not support the idea that the genetic systems of natural enemies retard their resistance development.
Responses of arthropod natural enemies to insecticides.
If resistance is sufficiently severe to prevent control of the pest, chemical control may be abandoned and management systems based on biological control, including the conservation of native natural enemies, may finally be implemented instead.
On the other hand, when natural enemies develop resistance to pesticides commonly used on a crop, this resistance may make it possible to conserve such natural enemies as important mortality agents contributing to the control of pests in crops even with continued pesticide use.
Recent advances on pesticide resistance in natural enemies.
Biological invasions—the establishment and spread of species outside their historical native ranges—has implications for basic ecology as well as conservation and human well-being. As such, identifying the mechanisms that promote invasions is crucial for both applied and basic ecology. While most major invasion hypotheses focus on a single causal mechanism (e.g., nutrient availability, traits of the invasive species), my research examines whether trade-offs between resource allocation to growth of new tissue and defense of tissue against disease and herbivory can explain why some non-native species become invasive in their new range and others do not. Specifically, I tested whether exotic species benefit more from enemy release relative to native competitors in high resource environments. To that end, I conducted a series of field experiments at the level of individual plants and plant communities. This research represents the first thorough test of the assumptions and key predictions of a hypothesis which integrates information about invasive species, invaded communities, and the environment in which invasion occurs to explain invasion success more broadly than previously possible (the Resource-Enemy Release Hypothesis, R-ERH). I tested this hypothesis in grassland communities and with individuals of several grass species. At the community level, exotics were less damaged than natives, especially in fertilized communities. Moreover, fertilization increased foliar damage on native species. Finally, fertilization increased exotic dominance only in communities exposed to vertebrate herbivores, and excluding insect herbivores and fungal pathogens reduced exotic dominance regardless of fertilization. At the individual level, species benefitting most from fertilization also benefitted most from exclusion of fungal pathogens and insect herbivores; this relationship was similar for natives and exotics. Within assembled native communities, fertilization increased, and enemy exclusion reduced, exotic dominance. Furthermore, fertilization and enemy exclusion each reduced native colonization of exotic-dominated communities. Together, these results provided partial support for R-ERH. Importantly, they also show that invasions can be driven by multiple independent, not interacting, factors.
Several influential hypotheses in plant-herbivore and herbivore-predator interactions consider the interactive effects of plant quality, herbivore diet breadth, and predation on herbivore performance. Yet individually and collectively, these hypotheses fail to address the simultaneous influence of all three factors. Here we review existing hypotheses, and propose the tri-trophic interactions (TTI) hypothesis to consolidate and integrate their predictions. The TTI hypothesis predicts that dietary specialist herbivores (as compared to generalists) should escape predators and be competitively dominant due to faster growth rates, and that such differences should be greater on low quality (as compared to high quality) host plants. To provide a preliminary test of these predictions, we conducted an empirical study comparing the effects of plant (Baccharis salicifolia) quality and predators between a specialist (Uroleucon macolai) and a generalist (Aphis gossypii) aphid herbivore. Consistent with predictions, these three factors interactively determine herbivore performance in ways not addressed by existing hypotheses. Compared to the specialist, the generalist was less fecund, competitively inferior, and more sensitive to low plant quality. Correspondingly, predator effects were contingent upon plant quality only for the generalist. Contrary to predictions, predator effects were weaker for the generalist and on low-quality plants, likely due to density-dependent benefits provided to the generalist by mutualist ants. Because the TTI hypothesis predicts the superior performance of specialists, mutualist ants may be critical to A. gossypii persistence under competition from U. macolai. In summary, the integrative nature of the TTI hypothesis offers novel insight into the determinants of plant-herbivore and herbivore-predator interactions and the coexistence of specialist and generalist herbivores.
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Resistance of aphid natural enemies to insecticides.
If ability to survive field rates of pesticide is a criterion for resistance, then a natural enemy with lower intrinsic tolerance would have to increase its tolerance more substantially to be considered resistant.
: Aphids, Their Biology, Natural Enemies and Control.
They note that LC/LD50 comparisons between pest/natural enemy pairs are available for only a small subset of the studies of pesticide impact on natural enemies.
Evolution of pesticide resistance in natural enemies.
Simulations based on a one locus model showed that 10 or 100-fold reduction in the LC50 of homozygous susceptible (SS) individuals had little impact on projected times for resistance development in natural enemies of apple pests.
Pesticide effects on arthropod natural enemies: a database summary.
Reduced intrinsic tolerance could retard evolution of resistance in some natural enemies if resistance is defined as the ability to survive field concentrations of a pesticide or if resistance alleles confer a fixed multiple of increased tolerance relative to susceptible individuals.
Evidence for the natural enemies and biotic resistance hypotheses.
Furthermore, for 10 of 12 families of natural enemies, including Braconidae and Aphelinidae, the average ratio for the family showed that the natural enemy was more tolerant than its prey or host.
Natural enemies and host plant use Page 1 - …
The finding that Phytoseiidae, which are known for their ability to evolve resistance, had low selectivity ratios compared to other natural enemies (Theiling & Croft 1988) suggests that low intrinsic tolerance relative to pests is not a major impediment to evolution of resistance in natural enemies.
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