Many natural enemies of pests feed on the pests in their larval stages, while the adult stage is dependent on nectar. Weeds can be a good in-field source of nectar for these natural enemies.

In the photo at top, an adult syrphid fly is feeding on nectar in the flower of a Canada thistle growing in a field of fababeans. In the bottom photo, a syrphid larva feeds on black aphids on the fababean plants. The presence of flowering weeds in the fababean field or close by contributes to natural control of the black aphid, a serious pest on fababeans.