Two field releases of rubidium (Rb)-labeled, laboratory-reared adultPlatynota idaeusalis(Walker) were conducted to measure female adult dispersal and egg mass deposition patterns within a 33-ha (about 7,450 trees) apple orchard in Adams County, Pa. In both studies>3,200 virgin male and female adults (♂:♀ ratio = 1.1:1.0) were reared from rubidium-incorporated diet (3 g Rb/liter) and released on four center trees. Trees were randomly sampled for hatched and unhatched egg masses up to 500 m from the release sites. Atomic absorption spectrophotometry was used to detect rubidium in egg masses, and masses were considered marked if concentrations were>10 ppb. Following the first release, 417 trees were sampled and 39% of the 222 egg masses collected were marked. Following the second release, 260 trees were sampled and 67% of the 346 egg masses collected were marked. The distribution of rubidium-labeled egg masses demonstrated that spatial patterns of egg mass deposition are primarily local and are affected by ambient air temperatures. For example, 90% of all recovered rubidium-labeled egg masses were found within 65 and 45 m of the release sites, and the most distant labeled egg mass was found at 250 and 170 m, respectively, following adult releases during a warm period in July (daily minimum temperatures,>16°C) and a cooler period in September (daily minimum temperatures,<12°C).