1. Intraguild predation (IGP) among generalist predators significantly affects the food web dynamics but the benefits of IGP for top predators are still unclear. 2. We offered diets consisting of aphids, of spiders, and their sequential mix (i.e. polytypic diet) to the spider Philodromus cespitum and measured its survival, weight, development, and fecundity. To avoid a species-specific effect, the aphid and spider prey consisted of several similarly sized species. 3. The spider diet prolonged survival in comparison to starvation while the aphid and polytypic diets enabled the highest survival of juveniles. The spider diet resulted in low weight gain and slow development in juveniles at the population level. However, the weight change was highly variable among individuals, and a substantial portion of the individuals engaging in IGP gained the most weight compared to other diets. The aphid and polytypic diets resulted in the largest weight increase and fastest development in juveniles at the population level, and there was little variability in the amount of weight gained among individuals. Diet did not affect survival of adult females, but females on the spider and polytypic diets had higher fecundity than those on the aphid diet. 4. While both the aphids and the spiders had energetic and nutritional value for Philodromus cespitum, a polytypic diet consisting of a mix of them improved overall fitness. IGP therefore seems to be an adaptive strategy of Philodromus cespitum, and IGP on small spiders may increase its performance in ecosystems where aphids are highly dominant pests.

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