Abstract: A method and test kit for determining insect resistance based upon an esterase in the body fluid of the insect in the field. The method and kit includes an absorbent material pad (12) containing a dry substrate for the esterase which is preferably mounted on a fluid impermeable sheet (11). A roller (20) is preferably used to express the fluids from the insect onto the pad 12. The fluids are sufficient to enable the esterase to react with the substrate to produce an intermediate compound which is reacted with a chromogen to produce a visually detectable image with an intensity which varies directly as a function of the amount of the esterase and which is easily determinable by the farmer. The method and kit is particularly useful with homopteran, particularly aphids which are a problem in potatoes.