Mayetiola thujae

Males are 2.44 millimetres (0.096 in) long on average. Females are somewhat larger, averaging 2.77 millimetres (0.109 in) in length, and have retractible ovipositors.[1] Both sexes are dark gray, and are very difficult to visually distinguish from other midge species.[2]

Eggs are laid on the inner surface of female western redcedar cones in the March or April,[3] shortly after pollination occurs.[4] The eggs are oblong and translucent, but appear to be orange due to the color of the larvae,[4] and are laid in clusters of up to 45, with up to 150 eggs per cone.[3] The eggs hatch in April or May after a month of incubation.[3] The larvae, which grow up to 4 millimetres (0.16 in) long,[2] initially feed on the tissue found on the inner surface of the cone's scales, moving between scales as they do so. As they mature, they move toward the distal end of the cone, where seed development and the majority of cone growth occurs, and eat the seeds. They begin enclose themselves light gray silk cocoons in late summer.[4] The adults emerge from their cocoons and mate after pollination occurs in March or April, leaving the cones for the first time as they do so.[3]

As a control measure, insecticides can be applied to western redcedar cones 1–2 weeks after larvae begin to hatch.[2] The larvae are often infected by parasitoid wasps of the family Chalcididae.[4]