This is from an article that just came out.

Ecology Letters - November 2007

Rachael Winfree, Neal M. Williams, Jonathan Dushoff, Claire Kremen (2007)
Native bees provide insurance against ongoing honey bee losses
Ecology Letters 10 (11), 1105-1113.

LETTER

Rachael Winfree-Department of Ecology and Evolutionary Biology, Princeton
University, Princeton, NJ 08544, USA4Department of Environmental Science,
Policy and Management, University of California, Berkeley, CA 94720,
USA*E-mail: rwinfree@princeton.edu,
Neal M. Williams-Department of Biology, Bryn Mawr College, Bryn Mawr, PA
19010, USA,
Jonathan Dushoff-Department of Biology, McMaster University, Hamilton, ON,
L8S-4K1, Canada and
Claire Kremen-Department of Environmental Science, Policy and Management,
University of California, Berkeley, CA 94720, USA

Abstract

One of the values of biodiversity is that it may provide 'biological
insurance' for services currently rendered by domesticated species or
technology. We used crop pollination as a model system, and investigated
whether the loss of a domesticated pollinator (the honey bee) could be
compensated for by native, wild bee species. We measured pollination
provided to watermelon crops at 23 farms in New Jersey and Pennsylvania,
USA, and used a simulation model to separate the pollen provided by honey
bees and native bees. Simulation results predict that native bees alone
provide sufficient pollination at > 90% of the farms studied. Furthermore,
empirical total pollen deposition at flowers was strongly, significantly
correlated with native bee visitation but not with honey bee visitation. The
honey bee is currently undergoing extensive die-offs because of Colony
Collapse Disorder. We predict that in our region native bees will buffer
potential declines in agricultural production because of honey bee losses.