Success is counted sweetest
By those who ne'er succeed
To comprehend a nectar
Requires sorest need
Not one of all the purple Host
Who took the Flag to-day
Can tell the definition
Although “nectar” goes hand in hand with sweetness, it doesn’t necessarily mean that it is sweet. However, it represents something desirable, and that desire is stimulated by the greatest need. 
 Although Dickinson doesn’t name the bee, the implication is that the bee seeks nectar because it “requires sorest need”.  So something abstract is presented through a concrete idea. 

Success is counted Sweetest