Summary
Determing habitat requirements for natural enemies of farm pests
Coordinator: Rebecca Chaplin-Kramer, University of California, Berkeley
Maintaining a healthy population of beneficial insects that serve as natural enemies to crop pests is an important component of pest management for organic farmers. To do so, we must understand the habitat requirements of these insects.
In this project we worked with growers in the Salinas Valley and surrounding areas to understand how landscape factors around the farm contribute to natural pest control. For the study, we selected 18 organic broccoli farms representing a gradient of landscapes ranging from less than 5% to more than 80% natural habitat within a 3 km radius of the farm.
Cabbage aphids are a major pest of broccoli and can be controlled by natural enemies if not disrupted by pesticides. There are many important natural enemies of aphids. In this study we focused on syrphid flies because larvae of flies in the family Syrphidae are by far the most abundant aphid predator in broccoli.
Adult syrphid flies are extremely mobile and search in many different habitats for aphid colonies on which to lay their eggs. This makes the surrounding habitat an important variable in their distribution on farms. Syrphid flies are especially vulnerable to pesticides and cannot be acquired commercially; they must migrate into the fields from surrounding areas. Syrphid flies are therefore an excellent study species to use to explore the effects of landscape on pest control.
Our study was composed of two main components: habitat mapping using ArcGIS software and weekly insect surveys at each of the farms over the growing season in three separate years (2006, 2007, and 2008).
Two additional components emerged out of questions raised by the insect surveys: a field experiment to measure pest control function at each end of the landscape gradient and laboratory experiments focusing on the physiological impacts of a species of wild mustard which serves as an alternate host plant for aphids. We investigated whether cabbage aphids feeding on these mustards become more toxic to predators as a result of sequestering chemicals that essentially constitute a “mustard oil bomb.”
GIS habitat mapping
From aerial photographs we used Geographic Information System (GIS) methods to create maps that were classified into the following land-use categories: annual agriculture, perennial agriculture, fallow agriculture, industrial, residential, road, bare, water, and natural habitat. We defined a “landscape complexity” gradient in terms of the amount of natural habitat around the farm. Areas with lower proportions of natural habitat (usually corresponding to higher proportions of agricultural habitat) were considered “simple” while areas with higher proportions of natural habitat (or lower proportions of agricultural habitat) were considered “complex.”