Disclosed are solid forms of 1-[(2-chloro-5-thiazolyl)methyl]-3-(3,5-dichlorophenyl)-2-hydroxy-9-methyl-4-oxo-4H-pyrido[1,2-a]pyrimidinium inner salt (Compound 1). Methods for the preparation of solid forms of Compound 1 and for the conversion of one solid form of Compound 1 into another are disclosed. Disclosed are compositions for controlling an invertebrate pest comprising a biologically effective amount of a solid form of Compound 1 and at least one additional component selected from the group consisting of surfactants, solid diluents and liquid carriers. Compositions comprising a mixture of a solid form of Compound 1 and at least one other nematocide, insecticide and/or fungicide are also disclosed. Also disclosed are methods for controlling invertebrate pests comprising applying to a plant or seed, or to the environment of the plant or seed, a biologically effective amount of a solid form of Compound 1.
