Question

Are resonance structures needed to describe the structure hco2-

Answer 1

Resonance or mesomerism is applicable to structures that cannot be formed with a single bond alone. Thus, for the structure of HCO2, there can be 2 resonance structures. In each structure, the formal charges are

C=0;
C=0
Oxygen=0
C-O=-1
H=0

HCO2-'s structure is a mixture of the two resonance structures. Hope this helps.