Disclosed are polynucleotides encoding a pesticidal fusion polypeptide comprising (i) a toxin domain; and (ii) a heterologous binding domain capable of binding non-specifically to a cell membrane without disrupting that membrane. Preferably the toxin domain is derived from a Bacillus thuringiensis cry toxin (e.g. CryIA (b) or (c)) and the binding domain is derived from a lectin (e.g. ricin toxin B chain). The use of such fusions may help to inhibit the acquisition of resistance in a pest population treated with the polypeptide. A further aspect of the invention is a method of assessing the toxicity of a polypeptide to a pest species by expressing a nucleic acid encoding said polypeptide in a host cell from that species, observing the viability of the cell and correlating the results of the observation with the toxicity of the polypeptide, wherein the viability is determined by assessing esterase activity or membrane.