Identify and Control Leek Moths and Onion Leaf Miners

The leek moth (Acrolepiopsis assectella) also known as the onion leafminer, is a pest of several species of Alliums, The distribution of the pest includes Asia, Africa, Europe, and North America, it was first discovered in North America relatively recently, in 1993, in Canada.

In Europe, generations are limited to 1-2 per year. The leek moth attacks leeks (Who would have guessed ? ) as well as garlic, chives, green onions, shallot, and some related plant species.

Being an invasive aspecies, not native to the North American continent, it doesn’t have many natural enemies, which gives rise to the fear of its unchallenged spread across the continent, much the same as Japanese Beetle and gypsy Moth did (other “invasive” species).

Macro photo of leek moth or onion leaf miner.

In its native habitat of Europe, it is considered a major pest, with up to 40% infestation in some areas.

Praying Mantis will attack the adults, they are nocturnal , and so too is the Mantis. The effects of other predatory insects is not known.

The adult leek moth is a small, reddish-brown moth with a white triangular mark on the middle of the folded wings. It has a wingspan slightly over 1/2 inch and is less than an 1/8 inch long with its wings folded . The hind wings of the moth are pale grey to grayish black in color.

Adults overwinter in plant debris and reemerge in early spring. Mating generally takes place within a day of emergence. Females live less than a month during which time , they mate, lay eggs on the lower leaf surfaces near the plant base . The Moths are nocturnal and are restricted to the evening and night. Eggs hatch in generally less than a week

The larvae will tunnel mines in the leaf tissue, sometimes causing distortion, and are reported to occasionally attack the bulb and stems. Their activity stunts plant growth and compromises the storage life of the crop.

They bore through leaves and migrate towards the center of the plant feasting en route. They leave behind a trail of pinholes on inner leaves. Larval mines in the center leaves become vertical grooves as the plant matures.

Macro photo of onion leaves damaged by leek moth larvae.
Tunnels made by leek moth larvae.

On leeks, they generally feed on the youngest leaves . They also enter hollow leaves, such as those of onions and scallions and feed internally, creating translucent sections on the plants surface. They rarely attack flowers of the host plant. Damaged plants may appear stunted and distorted and are much more susceptible to plant diseases.

Chemical control is expensive, rarely cost effective for home gardeners and not eco-friendly. Hand picking of infested plants is not feasible as the larva are inside the plant itself.

The best control for Leek Moths is Bacillius thuringiensis. It has proven effective against almost all species of Moth and worm , it is a bacterium / organic pesticide that is readily available to home gardeners. It’s harmless to people and pets.

Apply it at either bloom or petal fall, or both. Bt It is a stomach poison and must be ingested . It is more effective when applied during warm, dry weather while the larvae are actively feeding. Bt breaks down quickly in nature so multiple treatments per season are necessary.

Spinosad is another biopesticide , correctly termed a microbial pesticide, harmless to humans. Spinosad will interfere with pollinators as well as pests – evening / night time use is advisable. Working Diatomaceous Earth into the soil around host plants will help deter this pest. Applying beneficial nematodes to the soil before planting will also greatly reduce surviving pupae.

Diverse gardens that include a symbiotic mingling of flowers and vegetables are a poor environment for insect pests as they also attract many natural predators,LacewingsandLadybugs in particular.

Parasitic wasps sometimes attack the fruitworms, but are not exceptionally effective as the fruitworm is inside the fruit itself . Praying Mantis will prey on the moths. Adult moths can be trapped with light traps by night.

Sprays/Pesticides must be applied in the pre-bloom stage to prevent injury. Do not apply insecticides, even bio-pesticides during bloom time to protect vital pollinating insects.

Pesticides such as pyrethroids (Pyrethrin) that are more effective in cool than warm weather will work more efficiently against Fruit worms earlier in the season, and is the recommended early season chemical for most types of worms.

Use Broad-spectrum insecticides, such as pyrethin only as a last resort.

Bio Pesticides such as Neem Oil derived products and Spinosad are harmless to pets and humans are advisable. Broad-spectrum insecticides, such as pyrethrin or malathion only as a last resort.

Proper sanitary practices are vital to the health of your garden. Proper sanitation can help to ensure disease-free pest-free and productive gardens. Try to keep the garden free of any diseased dead or damaged plant materials.

Remove cuttings from pruning and trimming and either destroy them – if diseased, or send them to the compost pile. Leaving rotting fruits and vegetables in the garden is like a written invitation to unwanted pests and diseases.

If a diseased or dead plant part has to be cut, the microorganism that caused the problem is probably on the tool you just used. Like a surgeon, sterilize all tools by washing in soap and water – rubbing alcohol wouldn’t hurt either.

If you pinch off diseased plant parts, wash hands before handling any other plants. Keep Weeds under control. Till the soil in the spring before planting to expose and kill larvae that wintered over in the soil.