What's eating my 'walla walla' onion?

Walla Walla Onion, a large sweet variety, is most at risk from onion thrips and onion fly maggots. Thrips silver and streak the strap-like leaves, reducing photosynthesis during the critical bulb-swelling phase. Allium leaf miner is an increasing threat in the UK and Northeast US. Birds may pull seedlings. Row covers offer good all-round protection.

Fine silver-white streaks and flecking on leaves; foliage withers in hot dry weather; large populations significantly reduce bulb size at harvest. Cover with insect mesh in spring. Use sticky yellow/blue traps to monitor and reduce adult numbers. Avoid drought stress — well-watered plants tolerate thrips better. Remove crop debris promptly.

White maggots tunnel into the base of young plants and bulbs; leaves yellow and wilt; seedlings collapse; bulbs rot in storage. Grow from sets rather than seed where possible (sets are less attractive to egg-laying females). Cover with mesh after planting. Avoid thinning in May–June when flies are active. Rotate onion crops.

Lines of small white puncture marks along leaves in spring and autumn; larvae mine into the stem and bulb, leaving a pathway for rot fungi. Cover from early spring (February in UK) with fine insect mesh until mid-June. Repeat cover in autumn for the second generation. There is currently no approved insecticide for garden use.