  We consider correlator of two concentric Wilson loops, a small and large ones
related to the problem of flux-tube formation. There are three mechanisms which
can contribute to the connected correlator and yield different dependences on
the radius of the small loop. The first one is quite standard and concerns
exchange by supergravity modes. We also consider a novel mechanism when the
flux-tube formation is described by a barrier transition in the string
language, dual to the field-theoretic formulation of Yang-Mills theories. The
most interesting possibility within this approach is resonant tunneling which
would enhance the correlator of the Wilson loops for particular geometries. The
third possibility involves exchange by a dyonic string supplied with the string
junction. We introduce also t'Hooft and composite dyonic loops as probes of the
flux tube. Implications for lattice measurements are briefly discussed.
