Silvery Streaks on My Leek Leaves

Whitish-silver streaking or patches on leek leaves — particularly along the length of the leaf — are caused by thrips (Thrips tabaci, the onion thrips). Thrips are tiny, barely visible insects (around 1 mm) that rasp the surface of leaf cells, piercing them and ingesting the contents. The damaged cells collapse and dry, leaving the characteristic silver-white streaking as light reflects off the empty, collapsed cell walls. Thrips are most active during warm, dry summer weather and can build up rapidly when conditions favour them.

Confirming thrips

To confirm thrips are present, hold a leek leaf over a sheet of white paper and tap the leaf firmly. Tiny, pale golden-brown or cream-coloured insects — elongated and narrow — will fall onto the paper and can just be seen moving. They are faster-moving than aphids and very small. Adult thrips have narrow wings; larvae are paler and wingless. Dark faecal specks may also be visible on the leaf surface alongside the silvery areas.

How serious are thrips on leeks?

At moderate levels, thrips damage on leeks is cosmetic. The silvering appears on the outer leaves and the plant continues to function. Leeks are robust plants and a moderate thrips population rarely affects the final quality of the shank. Severe thrips infestations — extensive silvering across most of the leaf surface of the majority of plants — can reduce photosynthesis and potentially affect shank development, but this level of attack is uncommon except in unusually hot, dry years when thrips populations explode.

Control and management

Thrips populations are naturally regulated by predatory insects — pirate bugs (Orius), lacewing larvae, and ground beetles all feed on thrips. Avoiding unnecessary insecticide use (which kills beneficial insects) is therefore the most important long-term strategy. Water during dry spells — thrips thrive in dry conditions, and misting the foliage during hot weather temporarily reduces the surface temperature and humidity conditions they prefer. In severe infestations, insecticidal soap sprays directed at the leaf surfaces (where thrips shelter inside the leaf sheath) can reduce population density, though complete eradication is rarely possible.