Outbreaks of Thecodiplosis piniresinosae Kearby apparently have occurred in North America since 1932 on Pinus resinosa (Ait.). Serious losses of new needles and some mortality of shoots resulted since 1957 in central Wisconsin. Fall browning caused by the midge is distinguished from needle blight and needle droop.

Three larval stages and the pupa are described. Adults emerged in late May and oviposited on elongating shoots. Eggs hatched within a week and larvae mined into the bases of the fascicles, where they fed until early October. In October, mature larvae left the fascicles, dropped to the litter, and entered the soil to overwinter. Pupation and transformation to the adult occurred in the spring.

One larva usually was present per fascicle during periods of low populations, but during outbreaks up to 11 larvae infested a single fascicle. A monothalmous or polythalmous gall-like enlargement was noticeable during the outbreak. Infested needles formed an abscission layer prematurely and dropped during the winter, 3 to 5 years before normal abscission. Stand conditions, age and spacing conducive to the buildup of midge outbreaks are discussed.