Adult survival and reproduction of many species of arthropod natural enemies depend exclusively on sugar-rich food such as honeydew. Here, we tested the impact of two types of honeydew as well as several sugars at different concentrations (10%, 25%, 50%) on the longevity and reproduction of two life-history omnivores. On the one hand, the aphid-feeding gall midge Aphidoletes aphidimyza (Rondani) and on the other hand the gall midge Feltiella acarisuga (Vallot) (Diptera: Cecidomiydae), a predator that can feed on many spider-mite species during the juvenile stage. Compared to a water control, females provided with the commercial sugar Biogluc® at a 10% concentration showed an increase in longevity by a factor of 3.6 for A. aphidimyza and 4.7 for F. acarisuga, respectively, whereas Biogluc® at a 50% concentration significantly reduced longevity in both species. Sucrose solutions had a positive, yet less pronounced effect. Feeding on Biogluc® or sucrose (all concentrations) raised the number of oocytes for A. aphidimyza and F. acarisuga by a factor 2.2–2.6 and 2.0–2.5, respectively. The highest realized fecundity during 72 h was recorded for A. aphidimiza females supplied with 50% Biogluc® (increase by 5.1) or in the case of F. acarisuga the strongest effect was observed with 10% Biogluc® (increase by 3.5). For the rest of the Biogluc® and sucrose solutions the effects were less pronounced. In contrast, aphid honeydew did not raise female longevity, nor the number of oocytes or eggs laid. Our results imply that sugar solutions at modest concentration (10%) are best suited for use to support these gall midges in the field as well as in mass rearing. Finally, given the positive association between the number of oocytes and the number of eggs laid for both species, we propose the use of the former as a proxy to assess the fertility for both species to simplify the procedures for quality assessments currently employed by the biocontrol industry.