This project comprises a large-scale investigation of the distribution and density patterns of agricultural pests and pest enemies in walnut orchards in relation to the distance to and area of riparian forest and restored riparian forest. The project includes an assessment of spatial distribution and density of insectivorous birds, agricultural pest birds, and two lepidopteran walnut pest species. The project also includes an analysis of foraging behavior of insectivorous birds on walnut orchards in relation to the proximity to and area of restored and remnant riparian forest.
The density and successful foraging of insectivorous birds increases significantly with the area of restored and remnant riparian forest surrounding the walnut orchard. The density of agricultural pest bird species decreases with increasing area of riparian habitat. Abundance of two major walnut pests, codling moth and navel orange worm, have no significant relationship to area of riparian habitat, although abundance of navel orange worm trended toward lower abundance with increased area of riparian habitat.
Research on sustainable agriculture has emphasized the biogeographic region as the most appropriate unit for pest-management research (Altieri 1994). However, there is little understanding of the effects of natural areas and landscape diversity on pests and pest predators. Some studies suggest that proximity to natural habitat could increase the numbers of generalist natural enemies of pest species through increased access to food and nesting resources, refugia, and winter habitat (Altieri 1994, Marino and Landis 1996). Movements of species between natural and agricultural areas could have cascading effects on all levels of the food webs in both habitats (Polis et al. 1997). These effects could include changes in the populations of natural enemies or pests of agriculture that could then have cascading effects on other trophic levels and thus on crop success. In order to support a diverse landscape and economic viability of producers, it is essential to understand how areas surrounding the farm affect the dynamics of natural biological control.
Integrated pest management usually focuses on natural enemies found on the farm. However, it is increasingly clear that assemblages or guilds of highly mobile predators found in adjacent habitat, such as generalist insectivorous birds, can also effectively suppress populations of pest species in agricultural settings (Symondon et al. 2002). For example, birds are mobile links between natural habitat and agricultural areas (Lundberg and Moberg 2003). Due to their mobility, they can move to high-density food patches, and are capable of complex prey-switching and specialization behaviors (McFarlane 1976, Kirk et al. 1996). Research has demonstrated that birds can suppress insect populations (Holmes et al. 1979), have top-down effects on pests in agricultural systems, and consequently increase plant growth (Marquis and Whelan 1994, Hooks et al. 2003). Research has also demonstrated that birds can suppress insect pests that are not affected by other natural enemies, such as parasitoids (Jones et al. 2005). A better understanding of the dynamics of generalist bird natural enemies will help producers understand some of the services from natural areas in order to make informed habitat management decisions on a landscape scale
Altieri, M.A. 1994. Biodiversity and Pest Management in Agroecosystems. Food Products Press, New York.
Holmes, R.T., J.C. Schultz, P. Nothnagle. 1979. Bird predation on forest insects: an exclosure experiment. Science 206:462-463.
Hooks, C.R., R.R. Pandey, and M. W. Johnson. 2003. Impact of avian and arthropod predation on lepidopteran caterpillar densities and plant productivity in an ephemeral agroecosystem. Ecological Entomology 28:522-532.
Jones, G. A., K. E. Sieving, M. L. Avery, R. L. Meagher. 2005. Parasitized and non-parasitized prey selectivity by an insectivorous bird. Crop Protection 24: 185-189.
Kirk, D.A., M.D. Evenden, et al. 1996. Past and current attempts to evaluate the role of birds as predators of insect pests in temperate agriculture. Current Ornithology 13:175-269.
Lundberg, J. and F. Moberg. 2003. Mobile link organisms and ecosystem functioning: implications for ecosystem resilience and management. Ecosystems 6:87-98.
Marino, P.C., and D.A. Landis. 1996. Effect of landscape structure on parasitoid diversity and parasitism in agroecosystems. Ecological Applications 6:276-284.
Marquis, R.J. and Whelan, C.J. 1994. Insectivorous birds increase growth of white oaks through consumption of leaf-chewing insects. Ecology 75:2007-2014.
McFarlane,R.W. 1976. Birds as agents of biological control. Biologist 58,123 –140.
Polis, GA, WB Anderson, RD Holt. 1997. Toward an integration of landscape and food web ecology: The dynamics of spatially subsidized food webs. Ann Review of Ecology and Systematics 28: 289-316.
Symondson, W.O.C., K.D. Sunderland, and M.H. Greenstone. 2002. Can generalist predators be effective biocontrol agents? Annual Review of Entomology 47:561-594.
My objectives are to answer the following questions concerning the interactions between riparian areas and adjacent farmlands:
1) What are the quantities and distribution patterns of serious agricultural arthropod pests from natural areas into farmlands?
2) Does crop production benefit from elevated densities of pest enemies, including both insect and avian predators, that move from natural areas into nearby farmlands? If so, how far does this beneficial effect of natural habitat extend into farmlands?
Objective 1: What are the quantities and distribution patterns of serious agricultural arthropod pests from natural areas into farmlands?
To investigate this question, I conducted surveys of three agricultural bird species (Brewers Blackbird, American Crow, and European Starling) identified by the local farm community and two specific insect pests of walnuts, navel orange worm (Amyelois transitella) and codling moth (Cydia pomonella) on walnut orchards within the inner river zone of the Sacramento River where a large-scale restoration project is taking place. I collected two years of data on pest birds in orchards at different distances from restored and remnant riparian forest. I collected bird survey data at 828 points in the Sacramento River Conservation Area in 2007 and 2008. These data included over 16,000 observations of birds on the orchards and adjacent edge habitats. I also collected bird survey data at 60 points in restored habitat in 2007 and 2008. I conducted surveys of codling moth and navel orange worm at 9 sites, sampling 12 trees on each site.
Objective 2. Does crop production benefit from elevated densities of insectivorous birds that move from natural areas into nearby farmlands? If so, how far does this beneficial effect of natural habitat extend into farmlands?
To investigate this question, I collected two years of data on insectivorous birds in orchards at different distances from restored and remnant riparian forest. I collected bird survey data at 828 points in the Sacramento River Conservation Area in 2007 and 2008. These data included over 16,000 observations of birds on the orchards and adjacent edge habitats. I also collected bird survey data at 60 points in restored habitat in 2007. I collected foraging behavior data on 15 farms in 2007 and 2008, spending over 150 hours observing birds each year and collecting over 600 observations of insectivorous birds.
In 2007, I found that density of agricultural pest birds on walnut farms significantly decreased with an increasing area of riparian habitat. In terms of insect pests, I found that there was no significant effect of area of riparian habitat on codling moth or navel orange worm, although navel orange worm had a trend toward decreasing abundance with increasing area of riparian habitat.
I raised and set out overwintering codling moth larvae on three farms with caged and uncaged larvae at 0-m and 400-m from the edge of restored, remnant or agricultural habitat. I had bird predation at only one site, at the edge of restored habitat.
In 2007 I found that insectivorous bird density increased on walnut farms with increasing area of riparian habitat. After analyzing several other possible variables that could affect insectivorous bird density (tree height, understory, distance to water), I found that area of riparian habitat best explained density of insectivorous birds. I also found that insectivorous bird foraging on walnut farms, and successful foraging on walnut farms increased with increased area of riparian habitat.
Educational & Outreach Activities
I have presented results to the community through the Sacramento River Conservation Area Symposium in April 2007 and the Sacramento River Conservation Area, Technical Advisory Committee in 2008. I also summarized this information for use in a report by The Nature Conservancy on pests and pest control to be used to reduce conflict regarding pest issues on the Sacramento River associated with habitat restoration. I also presented the research to the Point Reyes Bird Observatory. I presented the work as well to the international ecological community at the Ecological Society of America meeting, the National Conference for Agriculture and the Environment, and the Society for Conservation Biology.
Langridge, S.M., M. Buckley, K.D. Holl. 2007. Strategies for overcoming obstacles to restoring natural capital: Large-scale restoration on the Sacramento River. In Restoring Natural Capital, Island Press.
Langridge, S. (In revision) Limited effects of large-scale restoration on seed banks in agriculture. Restoration Ecology.
Langridge, S. (In review) Distribution and behavior of bird pests and pest-predators at the interface of agriculture and restored riparian forest. Ecological Applications.
Langridge, S. (In prep). Fish, farms, water and words: spatial differences in support for large-scale restoration. To be submitted to Society and Natural Resources
Langridge, S. and G. Golet. (In prep). Restoration and agricultural pests: what is the relationship? To be submitted to California Agriculture.
Culp, L. and Langridge, S. (In prep). Comparison of winter bird survey methods: point counts and area search. To be submitted to Journal of Field Ornithology.
Society for Conservation Biology Annual Meeting, Chatanooga, TN, 2008
Understanding landscape effects of ecological restoration of wildland on nearby farms
Calfed Science Conference, Sacramento, CA, 2008
Using science in collaborative restoration processes: Transboundary effects of riparian
habitat restoration on farms
University of California Santa Cruz, Environmental Studies Seminar Series, CA, 2008
Contested landscapes: can scientific information and collaborative processes lead to
success in large-scale restoration projects
Sacramento River Conservation Area Forum, Technical Advisory Committee, CA 2008
Invited talk: Distribution of agricultural pests and pest-predators on the Sacramento River
Bay Area Conservation Biology Symposium, Davis, CA, 2008
Large-scale restoration and ecosystem services
Ecological Society of America Annual Meeting, San Jose, CA, August 10, 2007.
Large-scale riparian restoration and agriculture: Ecosystem services and habitat buffers?
Invited talk for the Point Reyes Bird Observatory. Bolinas, CA, 2007.
Effects of large-scale restoration on the distribution of riparian birds in agriculture.
Sacramento River Conservation Area Conference, Chico, CA, 2007.
Effects of large-scale riparian restoration on bird pests and pest predators.
Results were presented to the Sacramento River Conservation Area Forum, Technical Advisory Committee, and used in the Colusa Subreach Restoration Project to address some of the concerns voiced by the agricultural community in regards to restoration of riparian habitat and pest issues. This information was considered by the Colusa Subreach Committee and should help result in restoring 400 acres of riparian habitat in the future with less conflict regarding pest issues.
The economic costs of restoring riparian habitat are highly variable depending on the area to be restored. However, since area of riparian habitat best explains insectivorous bird density and does not have a significant relationship to two major walnut pests, the returns for restoring riparian habitat appear to be greater than the risks. However, more research is necessary to determine the cascading effects of the insectivorous bird density of herbivory and crop yield and the economic implications of this research.
This research has been utilized in meetings of the Colusa Subreach Restoration Project, where concerns about restoring riparian habitat have led to less restoration occurring. There are not specific recommendations from this research on particular practices, but rather information that supports the restoration of riparian habitat near the farms.
Areas needing additional study
- Trophic cascade effects of increased abundance of insectivorous birds on crop herbivory and yield in temperate crops, both perennial and annual.Effects of area of riparian habitat on other pest-predator taxa, and the possible synergistic or antagonistic effects of having more than one taxa
Effects of different plants used in restoration of habitat on pests and pest control on farms