Received for publication 19 May 1988. Scientific article no. A-4797, contribution no. 7818 of the Maryland Agr. Expt. Sta. Mention of a tradename, proprietary product, or vendor does not constitute a guarantee or warranty of the product by the author or the Univ. of Maryland and does not imply its approval to the exclusion of other vendors 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.