Oobius agrili wasps are so tiny that they are difficult to see (0.95 mm long). Adult females search ash bark crevices to find EAB eggs. A female will insert one of her eggs into an EAB egg. The wasps develop in the EAB eggs then emerge as an adult. There are at least two generations per year resulting in parasitism rates up to 60%. Oobius agrili overwinters as mature larvae in EAB eggs then adults emerge the following spring and summer. Each O. agrili female can parasitize up to 62 EAB eggs and can reproduce without males (parthenogenesis).