Categories: Vegetables

Leaf miner control problems

There’s absolutely no reason for leaf miners to be a problem any longer – other than in isolated cases, that…

I used parasitoid wasps to control these creatures many years ago – and have not had a major outbreak since then. Occasionally, I’ll see the odd leaf miner tunnel, but never enough to do any damage. Simply put, leaf miners are no longer a pest for me.

Leaf miners not only attack crops, but also many weeds and ornamental plants. This is to our advantage as it enables parasitoid wasps to build up in these areas and so provides a constant source of the insects for our crops. Having said this, though, remember that wasps don’t always guarantee leaf miner control. The irony is, they can be so effective in eliminating leaf miners that they themselves die out, as they no longer have a host to facilitate reproduction.

This is why a leaf miner problem can suddenly flare up again after a long period – there is a shortage of wasps. Such infestations usually only occur when leaf miner-infected plants are introduced into the area or if a neighbour sprays their crops with a chemical that also kill the wasps. It’s very unusual for leaf miners to return for any other reason.

A client of mine has a fairly large farm, isolated from other properties. He plants Swiss chard, lettuce, beet and celery – which are all host crops for leaf miners. I inspect his crops regularly and have not seen any sign of leaf miners for at least two years. This farmer uses soft products (those which are less toxic to the environment and predatory organisms) on all his crops which protect any parasitoids or predators useful for controlling pests.

A COMMUNITY APPROACH
As noted, having a neighbour who doesn’t care about protecting parasitoids on their farm can lead to infestation in your lands. Furthermore, the sudden influx of leaf miner flies may be too big for the existing parasitoids to control. By the time their numbers have increased, the damage has been done.

So there may indeed be times when you have to resort to insecticides which reduce leaf miner populations while not harming the wasps. If everyone uses such products, leaf miners are unlikely to become a problem.

A community approach is thus necessary. Those farmers who aren’t aware of this natural way of controlling leaf miners must be enlightened. You may not necessarily want to help a competitor, but by doing so you’re helping yourself by eliminating a potential source of infestation.

If your neighbours are difficult to convince, tell them about the money they’ll be able to save. Even if insecticides are needed to control other pests on crops vulnerable to leaf miners, using wasps to deal with the latter means you won’t have to buy additional products for leaf miners.

Secondly, if there’s any risk of serious damage even when some wasps are present, you will need to apply an insecticide which spares wasps and reduces leaf miner populations. But usually only one application will be necessary – another saving.

HARD TO SPOT
Parasitic wasps are far more difficult to spot than leaf miner flies and you’ll need some experience before you can determine when the wasps are about to gain the upper hand. Also, using natural ways of controlling pests requires a good understanding of how pest and predator interact, as well as more frequent inspections of your crops – especially initially. But the rewards will be increased profit – and more enjoyable farming.

Contact Bill Kerr at farmersweekly@caxton.co.za. Please state “Vegetable production” in the subject line of your email.

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