In regions with a humid summer climate, the use of water-soluble bait to control apple maggot is often limited by rainfall. We studied increasing the rainfastness of GF-120 fruit ßy bait by adding parafÞn wax emulsion. First, we veriÞed that adding 10% wax to a mixture containing 16.7% GF-120 did not reduce the mortality of female apple maggot compared with GF-120 without wax. In addition, we determined that ßymortality caused byGF-120 plus wax subjected to simulated rain was similar to that caused by GF-120 without wax not subjected to rain. Other assays showed that higher ßy mortality resulted from increasing the proportion of wax from 10 to 15%, and lower mortality resulted fromdecreasingGF-120 from 16.7 to 10 or 5%. The availability of spinosad on or near droplets of a mixture consisting of 5, 10, or 15% GF-120 and 15% wax was determined before and after the droplets were subjected to three 15-min periods of simulated rain. We found an initial steep decline in dislodgeable spinosad and smaller decreases after subsequent periods of rain. In a small-plot Þeld trial, fruit infestation by apple maggot in plots treated with a mixture consisting of 16.7% GF-120 and 19.2% wax was less than in plots treated with 16.7% GF-120 without wax but not less than in control plots. Overall, we found that adding parafÞn wax emulsion to GF-120 increased rainfastness in laboratorybioassays, and speciÞcally that it retained the active ingredient spinosad.However, ourÞeld data suggest that optimal rainfastness requires the development of mixtures with 19.2% wax, which may preclude application using standard spray equipment.