Test for exotic isoscalar resonance dominating $D^0 to pi^+ pi^- pi^0$ decays

The decay $D^0 to pi^+ pi^- pi^0$ appears to be dominated by $rho pi$ states in a configuration of zero total isotopic spin. The spin $J$, parity $P$, and charge-conjugation eigenvalue $C$ of this final state are therefore $J^{PC} = 0^{--}$, which cannot be formed of a quark $q$ and antiquark $bar q$. If a resonance near $M(D^0)$ dominates the final state, it must be a {it hybrid} composed of a quark-antiquark pair and a constituent gluon, or a {it tetraquark} $q q bar q bar q$. A test for this resonance in electroproduction is proposed.