How to stop caterpillars eating your vegetables

You put weeks into a bed of broccoli and cabbage, you walk out one morning, and every leaf looks like somebody has been at it with a hole punch. That's caterpillars, and around here it's nearly always the cabbage white, because the white butterflies you see drifting over the patch in the warm months are laying eggs on the underside of your brassica leaves while you watch them and think how nice they look.

The good news is that this is one of the few pests you can genuinely get on top of without spraying everything, as long as you know when to be looking.

When they turn up here

Cabbage whites are about from spring right through to autumn, and they get worse as the weather warms up. A brassica bed planted in late summer for an autumn crop cops the heaviest pressure, because the plants are small and soft exactly when butterfly numbers are at their peak. Winter plantings get off much lighter. So the timing to remember is simple enough: if the white butterflies are flying, your brassicas need looking at twice a week.

Know what you're actually dealing with

Caterpillars are the grub stage of butterflies and moths, and not all of them are a problem. The cabbage white grub is a velvety pale green thing that lies flat along the leaf rib and is very hard to see until you know the shape. The eggs are tiny yellow skittles stuck under the leaf, laid one at a time, and squashing those takes 30 seconds and saves you the whole argument later.

What actually keeps them off

Check the undersides twice a week

This is the unglamorous one and it's also the one that works. Turn a few leaves over, rub off any eggs you find, and pick off the grubs. Ten minutes on a Saturday morning through spring is worth more than anything you can buy, and I say that as somebody who sells the sprays.

Feed the insects that eat them

Parasitic wasps are the big one, they're tiny and you'll barely notice them, and they lay into caterpillars for a living. They need small flowers to feed on as adults, so a patch of alyssum or a row of marigolds along the end of the bed does real work. Birds take a lot of grubs too, and a shallow dish of water somewhere shaded will bring them in.

Grow a decoy

Nasturtiums are the classic. Cabbage whites will lay on them readily, so a clump at the end of the row pulls a fair share of eggs away from the vegetables you actually want, and you can pull the caterpillars off the nasturtium without caring what the leaves look like afterwards.

Cover the bed

A fine mesh thrown over hoops stops the butterfly landing at all, which means no eggs, which means no grubs. It's the most reliable method there is. It has to go on before the butterflies arrive rather than after, and it has to come off or be lifted while anything is flowering so bees can get in.

When you do reach for something

There's no shame in it, and a bed that's already covered in grubs is past the hand picking stage. A vegetable dust or an oil spray will sort it out, and two rules make the difference between a spray that helps and one that costs you the bees.

• Spray late in the day. Bees are done by then, and the leaf stays wet longer so the product has time to work.
• Spray the undersides, not the tops. That's where the grubs are sitting and it's where most people miss.
• Read the withholding period on the label. It's the number of days between spraying and picking, and it's on every pack for a reason.
• Keep it off anything in flower. If the broccoli has bolted and is flowering, the bees are working it, so leave that plant alone.

Shop related products

• D.T. Brown Alyssum, 750 seed pack. Small flowers that feed the parasitic wasps doing the work for you.
• D.T. Brown Nasturtium Jewel Mixed, 35 seed pack. The decoy crop, planted at the end of the row.
• Multicrop EcoPest Oil Insect Spray. An oil spray for eggs and small grubs, used late in the day.
• David Grays Tomato Dust 3 in 1. A dust for when the bed is already under attack.

Did you know we also stock vegetable dust, a long life pyrethrum spray and Brunnings Bug Kill in the shop, and they're not all on the website, so give us a ring on (08) 8536 4333 to check what's on the shelf. The seed racks and the flower seed are in the garden centre if you'd rather come and have a look.

More from the Garden Haven blog

• How to control garden pests without hurting bees
• How to get good bugs into the garden and keep pests down
• When a garden pest is worth spraying, and when it is not

I still find myself standing in the broccoli turning leaves over like I'm looking for lost car keys, and honestly that's most of the job. Come and tell me what's eating yours and we'll work out what it is.

Candeece