Phacelia

Its flowers feed the adult hoverflies, which are flower-visitors; it's their larvae that devour the aphids. A strip of phacelia therefore kills no aphid directly, it attracts the adults in the hope that they lay nearby. The effect is real but inconsistent: in the reference study (wheat fields in southern England), phacelia strips drew more adult hoverflies in the first year, with no drop in aphids; in the second, they received more hoverfly eggs, and markedly fewer aphids were counted in the fourth week of monitoring (Hickman and Wratten, 1996). To be seen as support for the retinue of beneficials, not as protection on its own.

Phacelia is a non-host of this nematode: it penetrates the roots but cannot reproduce in them, which breaks its cycle when it is intercropped in a rotation. Caution on the scope: the result was obtained under controlled conditions, on a single cultivar, and for this precise species linked to beet and brassicas, it's not an effect against 'nematodes' in general.

A catch crop that traps nitrates: its fine, dense root network takes up the mineral nitrogen left by the previous crop and immobilises it in its biomass, which withdraws it from winter leaching. The consulted source gives no figure specific to phacelia, the lesson is qualitative: the effect exists but depends on the sowing date and the vigour of the cover.