1. A stable ectoparasiticidal aqueous suspension formulation comprising an ectoparasiticidal amount of a spinosyn, or a physiologically acceptable derivative or salt thereof, milled to an average particle size of from about 1 to about 15 microns, and a surfactant in an amount effective to facilitate wetting the milled particles; a dispersant in an amount sufficient to form a spinosyn:dispersant weight ratio of from 3:1 to about 1:5; and water. 2. A formulation of Claim 1 wherein the average particle size of the spinosyn is about 2 to about 7 micrometers. 3. A formulation of Claim 1 or 2 wherein the amount of spinosyn is from about 0.02 to about 50 weight percent of the formulation. 4. A formulation of Claim 3 wherein the amount of spinosyn is from about 2 to about 5 weight percent of the formulation. 5. A formulation of Claim 1, 2, 3, or 4 wherein the dispersant is ionic. 6. A formulation of Claim 1, 2, 3, 4, or 5 which further comprises: a) about 0.1 to about 10 weight percent of a surfactant, b) about 0.3 to about 5 weight percent of a mineral thickener, c) about 0.05 to about 3 weight percent of a gum, and d) an antimicrobial agent acceptable for topical veterinary applications in an amount effective to prevent microbial growth in the suspension. 7. A formulation of Claim 6 wherein the surfactant is present in an amount of from about 0.1 to about 5 weight percent of the formulation. 8. A formulation of Claim 6 wherein the spinosyn is present in an amount of about 25 grams per liter of the formulation, the dispersant is a condensed formaldehyde/napthalene sulfonic acid or salt thereof, the gum is a xanthan gum, and the water is deionized, and which further comprises propylene glycol and a foam control agent. 9. An article of manufacture, comprising packaging material and a formulation for controlling an ectoparasite infestation on a small ruminant or companion animal contained within said packaging material, wherein said formulation comprises: a unit dose of a formulation of any one of Claims 1 to 8; and wherein said packaging material comprises a label or package insert with instructions for administering the dose to the animal. 10. A method of manufacturing a stable ectoparasiticidal aqueous suspension formulation, said method comprising: (a) wet-milling a composition containing a spinosyn, or a physiologically acceptable derivative or salt thereof, with a surfactant, a dispersant, an antifoam agent and water to form a "grind composition" in which the spinosyn has an average particle size is from about 1 to about 15 microns; (b) blending an aqueous suspension containing about 2 to about 10 percent by weight of a mineral thickener with a dispersion composition containing about 1 to about 4 percent by weight of a gum in a C2-C4 alkylene diol to form a "hydrated suspension composition" containing about 0.5 to about 8 percent by weight of the mineral thickener; and (c) diluting a first volume of the grind composition with a second volume of the hydrated suspension composition sufficient to provide the desired spinosyn concentration. 11. A method of controlling an ectoparasite infestation on a small ruminant or companion animal, comprising administering to the animal an effective amount of a formulation of any one of Claims 1 to 8. 12. The method of Claim 11 wherein the formulation is applied to the head, neck, shoulders or back of the animal by a spot-on or pour-on protocol. 13. The use of a formulation of any of Claims 1-8 in the preparation of a medicament for controlling an ectoparasite infestation on a small ruminant or companion animal.