The idea of enhancing insecticide efficacy against phytophagous insects with feeding stimulators was proposed as early as the 1960s, and a number of insect feeding stimulators based on sugars, molasses, and cottonseed extracts, biologically active at relatively high (5% and higher) concentrations, have been advocated. Here, we show that an acidic amino acid, L-aspartate, stimulates feeding in codling moth neonates at much lower concentrations and acts as an effective tank-mixed additive for increasing efficacy of insecticides, reducing fruit damage, and increasing yield of the fruit. In laboratory experiments, 1 mg/ml L-aspartate increased foliage consumption by 40–60% and, when added to Assail 30 SG, Baythroid XL, Delegate WG, or Garbaryl 80S, maintained its feeding stimulatory properties and reduced LD₅₀s by ≈10 times. In a 3-yr field trial, addition of L-aspartate to the aforementioned insecticides at 395 g/ha reduced fruit damage from ≈6%, on average to <1% for first-generation codling moth, and from ≈20 to ≈5% for the second generation. Interestingly, addition of L-aspartate also increased the average weight of apples by 11–27%, as measured at the time of harvest.