Emergence boxes are simple tools that can be used to increase the population of parasitoids in the crop.
While inspecting the crop for pests and diseases, farmers will often find egg masses of pest insects. It is for example very easy to find egg masses of stem borers in a rice field. Destroying these egg masses (by hand pressing or by burying them) will prevent the development of the pest population. However, this will also destroy any egg parasitoids that may be present. Often a part of the eggs will be parasitized by miniscule wasps, such as Trichogramma. The farmer can keep these egg masses and release any emerging wasps in the field while destroying any emerging caterpillars.
This could be done by placing the leaf with the egg mass in a plastic bag and inspecting it regularly. But another way is to place the egg masses in an “emergence box”. These devices are constructed in such a way that the emerging parasites (adult wasps) can escape by flying away, while the caterpillars remain trapped inside as they can not pass a barrier of glue or grease.