Field observation, field cages, and laboratory arenas were compared as methods to estimate daily per capita consumption for larvae of Harmonia axyridis Pallas (Coleoptera: Coccinellidae) and Chrysopa nigricornis Burmeister (Neuroptera: Chrysopidae), the two most abundant predators of Hyalopterus pruni Geoffroy (Hemiptera: Aphididae) in prune orchards in California's Central Valley. Daily per capita consumption increased with larval size, and the highest estimates were obtained from the field observation method and the lowest from the laboratory arena method for both predator species. Possible explanations for differences between estimates for each method are explored in detail, the most important of which is the need to measure both biomass killed and biomass consumed, because H. axyridis consumed nearly all of each prey item, whereas C. nigricornis always killed more biomass than they consumed. This study suggests that the laboratory arena method can lead to underestimation of daily consumption and that field cages may be more appropriate for quantifying daily consumption when prey are colonial and predators are relatively immobile. For highly mobile predators or predators of dispersed prey, the field observation method, combined with observations of both the duration and pattern of feeding activity throughout the day, is the best option for quantifying daily per capita consumption.
1. Generalist insect predators provide an important ecosystem service in the regulation of agricultural pests. However, the role of predation in the dynamics of aphid populations has been difficult to quantify as any change in aphid density results from a balance between two opposing processes, aphid recruitment through reproduction and aphid mortality through predation. 2. Our goal in this study was to determine the degree to which aphid consumption by naturally occurring densities of aphidophagous predators is responsible for observed seasonal changes in aphid densities. We quantified consumptive predation using a mechanistic approach that incorporates the balance between aphid recruitment and mortality. We used the mealy plum aphid Hyal-opterus pruni in Prunus domestica plum orchards as a model system. 3. Our approach was to compare two quantities: a demographic-based estimate of predation and an observation-based estimate of predation. The demographic-based estimate used a simple model of aphid population growth and predation. It estimates the mortality that must have occurred to give rise to observed changes in aphid densities between sample dates. The model uses independent estimates of aphid population growth rate in the absence of predation. The observation-based estimate combined field data on predator densities in plum orchards with stage-and species-specific per capita predation rates estimated from field observation. 4. Predation may have reduced the rate at which mealy plum aphid populations increased early in the season and rebounded from mid-season declines, but was not sufficient to prevent high aphid densities or to accelerate rates of decline later in the season. Moreover, predation at naturally occurring densities did not appear sufficient to drive the directional changes in mealy plum aphid densities at any point through the season. 5. Synthesis and applications. Our study shows that natural densities of generalist predators can be too low to provide effective biological control services for an aphid pest. Generalist aphid predators could provide more effective control if early-season densities were increased through augmentative releases or encouragement of crop colonization, but the commercial viability of these options remains to be tested.
Aphidius transcaspicus Telenga (Hymenoptera: Braconidae: Aphidiinae) has been selected for use as a biological control agent against Hyalopterus pruni Geoffroy (Hemiptera: Aphididae) in prune orchards in California. In this study we focus on the host instar preference and functional response of A. transcaspicus. Host instar preference was determined using laboratory arenas by allowing female parasitoids to attack aphids on field-collected leaves of Prunus domestica L. supporting a natural range of H. pruni instars. A relative preference index indicated that A. transcaspicus was 1.4 times more likely to attack intermediate instars (2-4) than 1st or adult instars. The functional response of A. transcaspicus to H. pruni density was determined on enclosed P. domestica shoots in a prune orchard. The functional response to host density was linear (Type I) at the lowest aphid densities, and the asymptotic number of aphids attacked at higher aphid densities was 64.3 ± 6.7 aphids day -1 .
The mealy plum aphid, Hyalopterus pruni (Geoffroy) (Hemiptera: Aphididae) is a pest of prune trees in California. The impact of aphids as pests is well characterized by their population growth rate, a parameter integrating their age-specific development, survivorship, and fecundity. These population parameters were measured at five constants temperatures on potted prune trees. Development rates increased with temperature up to an optimum. The relationship between development rate and temperature was described by linear and nonlinear models. Developmental threshold temperature was greater for the nonlinear model than for the linear model. Thermal requirement for development and maximum lethal temperature determined by these models were similar to those for other aphids. The greatest proportional survivorship of nymphs occurred at 26 degrees C. Mean daily fecundity was lowest at 14 degrees C and highest at 22 degrees C. Adult longevity decreased with temperature. Population growth rates for H. pruni were estimated from measurements of fecundity and development time and were highest at 22 degrees C. This is the first study to document the temperature dependence of the life history parameters for H. pruni and the first to generate a degree-day model for the prediction of phenological events.
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