Liriomyza spp., leafminer flies (Mik; Diptera: Agromyzidae), are economically important quarantine pests that puncture and mine leaves and fruits of various horticultural crops worldwide, affecting yield and trade. Halticoptera arduine (Walker; Hymenoptera: Pteromalidae), a key parasitoid from the pests' areas of origin in South America, was introduced as a potential alternative management strategy. Prior to H. arduine release, its potential interactions with the dominant local ectoparasitoid, Diglyphus isaea (Walker; Hymenoptera: Eulophidae), were assessed. Halticoptera arduine and D. isaea were released in single, sequential and simultaneous combinations on Liriomyza huidobrensis (Blanchard; Diptera: Agromyzidae) to evaluate possible effect on the parasitism rate, reproduction and host mortality. The combination of both parasitoids did not significantly affect the specific parasitism rates of either of them, an indication that H. arduine and D. isaea can coexist. Parasitism rates of the exotic H. arduine were significantly superior to the indigenous D. isaea in all release combinations except when both species were released simultaneously. While 50 individuals of D. isaea resulted only in 21.23 ± 2.1% parasitism, 50 parasitoids composed of 25 H. arduine and 25 D. isaea caused 53.27 ± 4.99%. Both parasitoids further induced significant nonreproductive host mortalities. Both parasitoids' F1 progenies sex ratios were female-biased in all parasitoid release combinations except in single release of D. isaea with a balanced sex ratio. The improvement in D. isaea's sex ratio induced by the presence of H. arduine suggests a synergetic effect on D. isaea's reproductive performance. The introduction of H. arduine in horticulture production systems may therefore improve natural control of Liriomyza leafminers in East Africa.