Syrphid flies suppress lettuce aphids

Syrphid flies are abundant in lettuce fields, where their larvae are key predators of aphids. However, the presence of predators in the field does not always result in economically significant levels of prey suppression. Even when predators are numerous, their effects on prey population dynamics may be variable. Over a two year period we surveyed lettuce fields in coastal California, USA to test whether syrphid flies are capable of colonizing fields with aphids and suppressing aphid population growth. The survey showed that female syrphids oviposited more eggs at locations with more aphids, and that greater numbers of syrphid larvae resulted in lower rates of increase in the aphid populations. We also directly manipulated syrphid densities by adding syrphid eggs to uncaged lettuce plants, and these syrphid additions resulted in lower aphid population growth. This research shows that syrphid flies have the ability to suppress aphid populations in lettuce fields.