Understanding, using, and promoting biological control: from commercial walnut orchards to school gardens

In this dissertation I used walnuts to address the effect of increased nitrogen availability to the host plant on walnut aphid Chromaphis juglandicola and on parasitism by the specialist parasitoid Trioxys pallidus. From laboratory experiments and field sampling, nitrogen content of foliage did not change aphid population growth rate or aphid size. However, in laboratory experiments added nitrogen decreased the number of mummies produced by female parasitoids over a 24 h period, but increased the proportion and the size of female offspring. Field sampling of walnut orchards showed no relationship between the percent parasitism of walnut aphids by T. pallidus and nitrogen content of foliage. Although nitrogen fertilizer and plant quality can affect biological control in other crops, it did not appear to be a problem for biological control of walnut aphids.;Biological control in action is often hard to visualize and even more difficult to quantify in the context of pest population management. Readily measured metrics are needed to accurately predict the effectiveness of biological control services: this would then allow managers to say, given this set of measurements, control of an insect pest can be expected today or at some point in the near future. Using walnuts and walnut aphids as a model system, I investigated whether activity measurements and diversity indices for the natural enemy assemblages present in walnut orchards would be good indicators of current and future biological control. While percent parasitism, predator:prey ratio, and natural enemy evenness were good indicators of current biological control, there were no good indicators of future biological control. An increase in natural enemy units, a measure of abundance weighted by the feeding or parasitism capacities of individual species, was correlated with a decrease in aphids over the season, but, as there was also a strong effect of within-season density dependence in the walnut aphid populations, the correlation is more likely to be an artifact rather than a causal relationship.;The composition of a local natural enemy assemblage depends on both the local environment, which acts as a filter, and the surrounding landscape, which serves as a regional pool of natural enemy species. When the surrounding landscape is the dominant factor, measures of landscape complexity can be used to predict the composition of the local natural enemy community. For more urbanized environments, however, local factors can be as important as landscape factors. Urban gardens are an ideal study system to address such questions as they are often spread along an urbanization gradient and yet differ considerably in local composition. For home and school gardens in the South Peninsula of the San Francisco Bay, urbanization had a non-linear effect on natural enemy richness with greater richness in both the most urban and most rural gardens. Moreover, urbanization was also positively related to natural enemy abundance on tomato plants, a key urban garden crop. In addition, the use of natural enemy exclosures showed that natural enemy richness was significantly related to the degree of suppression of aphids on fava bean plants, suggesting that increasing natural enemy richness could improve biological control.;The best indicators and economic incentives do not ensure that farmers and gardeners will use biological control instead of pesticides if they are not comfortable with the idea. In this context, Cooperative Extension (CE) provides an effective means to accomplish the successful transfer of new research information on biological control to the farmers and gardeners who could put it into practice. While outreach from CE to large agricultural industries has been well established, urban gardeners represent a relatively new audience for the transfer of research knowledge. As many gardeners in urban and school gardens include children, education about insects and natural enemies could start with very young audiences. A combined scientific and outreach event put on by San Mateo Cooperative Extension demonstrated the significance of early education on this topic. Using pre and post-participation surveys I was able to show that children felt more positively about insects and were even more likely to eat insects after the event. Making research knowledge available and accessible to all ages involved in food production may help to increase the level of adoption of biological control in the future. (Abstract shortened by ProQuest.).