Selamectin Impurity B (EP):(2aE,2'S,4E,5'S,6S,6'R,7S,8E,11R,15S,17aR,20Z,20aR,20bS)-6'-cyclohexyl-7-[(2,6-dideoxy-3-O-methyl-a-L-arabino-hexopyranosyl)oxy]-20b-hydroxy-20-(hydroxyimino)-5',6,8,19-tetramethyl-5',6,6',7,10,11,14,15,17a,20,20a,20b-dodec.