Alliums such as onions, leeks, shallots and chives are a staple part of a veg patch and are genreally easy to grow and look after. But there are a few insects that like to lay their eggs on these plants – and the developing larvae can do some damage.

Simply building up populations of predators such as hoverflies, lacewings, ladybirds and ground beetles can keep ‘pest’ insects at manageble levels – but sometimes you will need to take more action.

Allium leaf miner (Phytomyza gymnostoma)

In recent years, allium leaf miner has become more widespread and become a regular nuisance. This is mostly a pest of leeks where it can cause considerable damage. You will first notice the leaves twisting in mid autumn, then the stems start to collapse. When you come to harvest them, there will be lots of pale creamy maggots and dark brown pupae, the size of rice grains between the layers rendering them inedible. This could be mistaken for leek moth, but the lack of head and legs, and the presence of pupae in the leek are the tell tale signs that it is allium leaf miner.

This is a more recent pest in the UK, having first been spotted in an allotment in Wolverhampton in 2002. Since then, its presence has radiated out from the Midlands and spread around most of England and Wales. Nobody in our most recent survey reported any cases in Scotland, although that doesn’t necessarily mean that it isn’t present. They have two generations, one in March, and a larger one spanning September November.

If this is a ‘pest’ in your area and you’re growing leeks, you will most likely need to cover your crops with fine eco mesh to discourage it. Covering the crop during the egg laying seasons is the most effective way of controlling this pest.

Leek moth (Acrolepiopsis assectella)

The leek moth used to be restricted to the sunny climes of the southernmost counties of the UK, but it has slowly moved northwards and has now been spotted in counties of north Wales.

Adult leek moths are brown with cream patches and lay their eggs in April and again in a second generation in August/ September. These hatch into small grey-green caterpillars. These bore through the folded outer leaves leaving shot hole effect. They will migrate to the centre of the leek, where they damage developing leaves.

If this pest regularly strikes in your area, the most fail-safe solution is to cover your crops with fine mesh in April and August, when the flies are actively laying eggs.

Onion fly (Delia antiqua)

The onion fly is a dull grey fly that you’ll find widely distributed around the UK. It lays its eggs in May and again in July and August. The small maggot-like larve often feed on the seedlings which can cause them to die off. The later generation also feed on the bulbs, eating away at the roots and the bulb tissue causing rots and losses.

Sets are more reslient than seedlings to the early generation, although producing sets takes more energy than producing seeds, and the choice of varieties is more restricted. Your other option is, of course, covering with mesh.

Thrips (Thrips tabaci)

Onion thrips do most damage to leafy allium crops such as spring onions and leeks where their feeding causes scarring and discolouration to the leaves.

During the summer, when the adults are actively laying eggs, you will find both adult thrips and their larvae. The adults are around 1mm in length, darker brown and have two pairs of wings while the larvae are smaller, wingless and pale yellow/cream. They overwinter as adults, often on leeks, but they will find other plants such as cereals if these aren’t available. A quick blast with an irrigation hose can help reduce infestation.