Aryl alkyl malononitrile compound of formula (I), or a salt thereof: ** (See formula) ** for use in the control or prevention of a parasitic infection of an animal with a selected ectoparasite of Haematobia irritans and Stomoxys calcitrans, wherein the compound of formula (I), or salt thereof, is administered to the animal topically, and wherein Y is phenyl unsubstituted or substituted with 1, 2, 3, 4 or 5 R5 substituents； Q is phenyl unsubstituted or substituted with one or more R6 substituents； R1 is hydrogen, halogen, cyano, hydroxy, C1-C6 alkyl, C2-C6 alkenyl, C2-C6 alkynyl, C3-C8 cycloalkyl, C3-C8 cycloalkenyl, C1-C6 alkoxy, C2-C6 alkenyloxy, C1-C6 alkylthio, (C1-C6 alkoxy) carbonyl, in which the carbon atoms of the aliphatic or cycloaliphatic radicals mentioned above are unsubstituted or are substituted with 1, 2 or 3 R7 substituents； R2 is hydrogen or halogen； or R1 and R2 form together with the carbon atom to which they are attached, a methylene group or a cyclopropyl group； R3 is hydrogen, halogen, cyano, hydroxy, C1-C6 alkyl, C2-C6 alkenyl, C2-C6 alkynyl, C3-C8 cycloalkyl, C3-C8 cycloalkenyl, C1-C6 alkoxy, C2-C6 alkenyloxy, C1-C6 alkylthio, (C1-C6 alkoxy) carbonyl, in which the carbon atoms of the aliphatic or cycloaliphatic radicals mentioned above are unsubstituted or are substituted with 1, 2 or 3 R7 substituents； R4 is hydrogen or halogen； or R3 and R4, together with the carbon atom to which they are attached, form a methylene group or a cyclopropyl group； each R5, R6 is independently halogen, cyano, azido, nitro, -SCN, SF5, C1-C6 alkyl, C2-C6 alkenyl, C2-C6 alkynyl, in which the carbon atoms of the aliphatic radicals mentioned above are unsubstituted or substituted with one or more Ra； C3-C8 cycloalkyl or C3-C8 cycloalkenyl, in which the carbon atoms of the above-mentioned cycloaliphatic radicals are unsubstituted or substituted with one or more Rb； phenyl unsubstituted or substituted with up to 5 Rc； a 3-, 4-, 5-, 6-, or 7-membered saturated, partial
