The mirid Macrolophus caliginosus Wagner (Hemiptera: Miridae), and the syrphids, Sphaerophoria rueppellii (Wiedemann), Sphaerophoria scripta (L.), and Episyrphus balteatus (DeGeer) (Diptera: Syrphidae), belong to the aphidophagous guild, feeding on aphids (Hemiptera: Aphididae) in Mediterranean vegetable crops. The direction, symmetry, and intensity of intraguild predation between different instar combinations of syrphids and mirid were studied in an arena without plants. Moreover, predatory interactions between syrphids and mirids on plants were also evaluated. The results obtained in the arena without plants demonstrated mutual intraguild predation, i.e., syrphid eggs were highly susceptible to predation, whereas mirid nymphs and adults were attacked by syrphid larvae. All syrphid species showed the same propensity to predation, which was unaffected by parasitism of syrphid larvae by Diplazon laetatorius (Fabricius) (Hymenoptera: Ichneumonidae). Sphaerophoria scripta eggs were less susceptible to predation than those of the two other syrphid species. The experiment performed on plants showed different results. Predation by syrphid larvae on mirids never exceeded 10%, while 100% of syrphid eggs were preyed upon after 48 h. Intraguild predation rates were not influenced by the presence or absence of aphids on the plants. This study demonstrated that results obtained from studying intraguild predation in arenas that are highly artificial should be interpreted cautiously. Depending on the colonization history of a particular habitat in the field, syrphids should be more vulnerable to predation by mirids than vice versa.
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