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

  • Author(s): Mace-Hill, Kevi Claire
  • Advisor(s): Mills, Nicholas J
  • et al.
Abstract

Growing populations and changing diets have put great pressure on food systems throughout the world and have lead to increasing agricultural intensification including the greater use of pesticides and fertilizers. Given the associated human and environmental health costs of intensification, the development of more sustainable practices is imperative. One such avenue is to make better use of natural ecosystem services, such as the use of conservation biological control to reduce reliance on pesticide. However, there are logistical and educational impediments to the more effective use of resident natural enemies through conservation biological control that need to be addressed. These include how other agricultural management practices impact biological control, and how managers can tell if it is working or when it will work.

Along with pesticides, fertilizers are a defining aspect of modern agriculture, but their effect on biological control services has seldom been evaluated. The effects of plant quality on natural enemies are often overlooked developing biological control programs for insect pests in agriculture. An enhanced nutritional status of plants can fuel insect population growth because nitrogen is an important component of proteins that are known to be limiting for phytophagous insects. Additionally, nitrogen fertilizers directly and indirectly affect plant defenses.

In this dissertation I used walnuts to address the effect of increased nitrogen availability to the host plant on walnut aphid \textit{Chromaphis juglandicola} and on parasitism by the specialist parasitoid \textit{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 \textit{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. However, I found no clear elements of the local composition of gardens that could be modified to increase natural enemy richness.

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.

Biological control is a valuable ecosystem service that could help reduce the negative impacts of the intensification of agriculture. However, more effective ways to visualize, quantify and predict the action of natural enemies are needed to make conservation biological control an essential part of the pest management tool kit for farmers and gardeners.

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