Received for publication 13 July 1983. Contribution of the Massachusetts Agricultural Experiment Station as Paper 2596. This work is part of a thesis submitted by the senior author in partial fulfillment of the requirements for the PhD degree. Mention of a trademark or name does not constitute a guarantee or warranty of the product by the University of Massachusetts and does not imply its approval to the exclusion of other products that may also be suitable. The cost of publishing this paper was defrayed in part by the payment of page charges. Under postal regulations, this paper therefore must be hereby marked advertisement solely to indicate this fact.