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While many field studies have estimated the numerical response of predator populations to their prey, little is known about the numerical response of omnivores, especially at different spatial scales. We sampled populations of omnivorous Orius bugs and their thrips prey in Verbesina encelioides flowers at three hierarchical spatial scales; a flower, a small patch of plants (2 m diameter), and a large patch of plants (6 m diameter). Omnivore and prey densities were correlated using hierarchical statistic models. Data show that while prey was aggregated at the flower scale, adult omnivores were distributed uniformly at that level. These results suggest that antagonistic intraguild interactions may be shaping omnivore distribution at the smallest scale studied. The correlation between omnivore and prey densities was scale-dependent. Density of Orius nymphs was positively correlated with prey density at all spatial scales, but the density of adult bugs was positively correlated with prey density only at the largest scale. Results obtained here and from a companion study suggest that female bugs aggregate and lay more eggs where prey is more abundant. Differential mobility of Orius adults and nymphs may therefore underlie observed differences in omnivore-prey association at different spatial scales. Future studies should explore effects of plant materials on the aggregative response of omnivores to variation in prey density.
The pear psylla, Cacopsylla bidens (Sulc), is one of the most damaging pests of commercial pear orchards in Israel. However, growers today have only two pesticides left to control the pear psylla, therefore alternative control methods are needed. Our goal was to find ways to control the psylla population through rational use of cultural control methods: manipulating the levels of nitrogen fertilization, and using growth regulators. Our objectives were to determine the impact of nitrogen fertilization level on pear psylla populations, the impact of application of plant growth regulators (clormequat chloride and prohexadion-calcium) on psylla populations, and whether there is an interaction between the two factors in a semi-field-scale and in field-scale experiments. Higher oviposition and nymph development rates were found in trees that had higher leaf-nitrogen contents. Lower oviposition, nymph survival, and nymph development rates were found in trees that were treated with growth regulators. The suppressive impact of growth regulators was expressed even in trees with high nitrogen levels. We concluded that psylla population levels in pear trees can be reduced by using lower levels of nitrogen fertilization and that growth regulators impair psylla growth and development, and may possibly be used to reduce psylla populations.
Abstract
Animals of a wide range of taxonomic groups mix various food sources to achieve a nutritionally balanced diet. The strategies they adopt to balance multiple nutrients depend on their availability in the environment. Behavioural and physiological adaptations to forage for nutrient-differing food sources have rarely been investigated in respect to nutrient availability in the environment. We developed a simulation model to explore the strategy consumers should adopt in response to the abundance of two nutritionally complementary food types. Results show that (1) consumers should invest more effort in detecting the scarce resource; (2) there is an optimized negative relationship between effort foragers should allocate to find the two types of food; (3) consumers should exhibit higher selectivity when the proportion of food types in the habitat deviates from their optimal ratio in the diet. These findings have important implications for pest control using predators that benefit from plant-based food supplements.