The catnip plant, Nepata cataria, produces a fragrance that elicits a flurry of rolling, scratching, and purring in about two thirds of domestic cats (and bigger felines, apparently). As it turns out, nepetelactone – the compound produced by the plant – serves as an insect repellent. The effect on cats is an accident of its similarity to cat pheromones.
An excelent idea to your cat collar! I bet he'll love it! :D