According to Bug Guide: "Trigonalids are hyperparasitoids, parasitizing ichneumonid wasps and tachinid flies that are parasitoids of lepidopteran and dipteran hosts."

"Females lay their eggs along the upper edge of a leaf that are then eaten by a feeding caterpillar. After ingestion of the eggs, if the caterpillar is parasitized by a primary host (a wasp or fly), upon hatching, the trigonalid larvae seek out the host larvae and parasitize them. They then emerge from the primary host puparium. (Ross Hill)"