Leaf miner: still a threat to Swiss chard

The leaf miner was once a major headache for producers, having a wide range of hosts and a particular fondness for Swiss chard.

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Fortunately, it is now regarded merely as a ‘sporadic pest’, although still a serious one. The leaf miner is a small fly with a conspicuous yellow spot on its back and head. It burrows tunnels in the leaves that may go unnoticed until it’s too late. Infestation starts on lower leaves, hidden by new growth.

The fly punctures the leaf with its ovipositor and lays an egg in the wound. It also makes a number of tiny holes so that it can feed on the sap. These are seen as small white dots on the leaves. After hatching, the larva starts to eat its way between the leaf’s upper and lower epidermis in a serpentine pattern. As it grows, its tunnel becomes larger. The process stops when the larva pupates. The life cycle is rapid, normally taking just over three weeks depending on the ambient temperature. If unnoticed, infestation can increase markedly, rendering the entire crop unmarketable.

Liriomyza huidobrensis, which entered South Africa in 2000, has genes that give it resistance to a number of pesticides. These pesticides were routinely sprayed on Swiss chard for other pests and had no effect on the leaf miner – or at best only suppressed it. In fact, the pesticides actually helped leaf miners by killing off its main natural predators: parasitic wasps.

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Biological control
The leaf miner went on to become a major pest of potatoes, lettuce, beet and beans. Lettuce growers suffered immense damage, and at one stage were spending thousands of rands per hectare simply to ensure a marketable product. Fortunately, it became apparent that a number of parasitic wasps (parasitoids) found in South Africa were able to bring this pest under control.
Extremely small and not as easy to spot as the leaf miner flies, they deposit their eggs into the leaf miner larvae and multiply very quickly.

Once farmers learnt to work with these wasps in an integrated pest management system, the leaf miner became less of a problem. Farmers also found that the new method saved them money. Instead of routinely spraying for caterpillars and other pests without bothering to see whether they really needed to spray or not, farmers began to scout for pests and spray only when necessary. This obviously works out cheaper than spraying according to a set programme.

Pesticides and parastoids –a killer team
If you are farming in an area where leaf miner is prevalent, apply pesticides that are formulated not to harm parasitoids. A number of such products are available, and more are entering the market in response to the ongoing trend to use safer, more environmentally friendly pestidices. All the same, the fact that parasitoids are highly efficient at controlling leaf miner does not mean that you don’t have to worry about the pest anymore. Instead, you have to keep an even closer eye on your crops to determine the status of the pest and its predators.

Occasionally, the parasitoids locally eliminate leaf miners, but lacking sufficient prey, their numbers decrease. When leaf miners are re-introduced via transplants or come in on their own, there’s a sudden flare-up, with the parasitoids having to play catch-up. By the time they have the pest under control again, damage could have been done.

Where this happens, you need to spray a chemical that will reduce leaf miner numbers without harming the parasitoids. This will enable the wasps to regain control much more quickly.