PS 59-15
Avian pest control in walnut orchards: Do habitat patches in orchard margins facilitate the provision of pest control services?
Conservation biological control focuses on manipulating habitats to favor natural enemy invertebrate communities to maximize pest control services for growers. A growing body of evidence suggests that avian insectivores can also provide important pest control services. Largely untested is the extent to which local habitat enhancement influences top-down effect strengths of bird predation on crop pest abundance. In a walnut orchard system in California’s Sacramento Valley, we measured avian densities and sentinel larvae predation rates in orchards with woody plant insectary hedgerows and/or riparian streamside vegetation in their margins (i.e., habitat orchards) and made comparisons with control orchards devoid of habitat enhancement features in their margins. We performed the study during three winter months overlapping with the cocooned diapausing larval stage of the major walnut pest Cydia pomonella L. In each of 20 orchards, we glued 100 C. pomonella cocoons to trees at orchard edges and interiors, and enclosed half in wire cages to exclude vertebrate but permit invertebrate access. During this sentinel larvae exposure period, we conducted three monthly line transect bird counts in orchards, and deployed video cameras to identify vertebrate predators.
Results/Conclusions
Densities of bark-gleaning insectivorous birds were significantly higher in habitat orchards than in control orchards by an estimated 2.13 times as many birds; two of these species were captured by video cameras consuming sentinel C. pomonella larvae. The predation rates for uncaged and caged larvae were 41% and 5% respectively, and the cocoons of a majority of the depredated uncaged larvae had video-corroborated predation sign that implicated birds. Larvae were significantly more likely to be depredated in orchard interiors than in orchard edges. The indirect effect of hedgerows and/or streamside vegetation in orchard margins on overall larval predation rates was positive but weak and 95% CI’s for effect estimates widely overlapped zero. The standard deviation in predation rates among orchards was high, with model estimated orchard predation rates ranging between 1 and 73%. These results suggest that birds were major predators of overwintering C. pomonella, that bark-gleaning avian predators were more prevalent in orchards with habitat components on orchard margins, but that more complex orchard margins did not translate to significant pest predation effects.