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I've got a few broccoli plants which are under attack from caterpillars. They aren't currently netted.

I manually "weeded" all the caterpillars off the most distressed plant (and some of the other plants nearby) and figured that as long as I repeated the exercise fairly regularly I'd be able to save the crop.

So, a few days (and many squished caterpillars later) I'm not sure this is a winnable battle. There are probably slightly fewer caterpillars on each sweep but they're nothing like eradicated. And I realise I know almost nothing about my foe.

Also I'm wondering .. are the butterflies laying new eggs daily? Or are eggs laid long ago simply reaching hatching point? If the former, I should try netting (even now at this late stage). If the latter, there's no point.

Also, one plant is far and away under the most attack. The other plants much less so. How come? If hordes of butterflies are visiting my brassicas on an ongoing basis then surely they'd move onto the healthier looking plants wouldn't they?

Actually, this question is less about saving this particular crop on this occasion and more about educating myself about my enemy.

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I had a similar experience with my cabbage & kale last year. It seemed like a losing battle and as a last resort I sprayed them all with BT. In a couple days I had no more caterpillars to worry about for the rest of the season and the plants all came back. That’s my weapon of choice now for these. –  Brian Surowiec Aug 23 '12 at 20:28
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The theory I've heard is that the most infested plant is probably weaker than the others; stronger plants can manage to defend themselves from insects to some extent. And yes, just spray with Bt every other week and your problem will be greatly reduced. (For me, it greatly reduces the damage, but there are still a few worms at any given time in my broccoli heads, which is really unappealing...) –  bstpierre Aug 24 '12 at 20:53
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2 Answers

You can use BT as regularly as necessary, every time you see more caterpillars attacking. The BT does persist in the plant for a few weeks, providing residual protection. The white butterfly lays eggs on brassicas all the time, as long as it's warm enough. This is from late spring to late autumn. The yellowish oval eggs are laid on the underside of leaves, attached at one end of the egg. I'm trying silica in water spray to try to harden up my plants and vegies, hopefully reducing damage by pests.

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Brassica crops in the UK are subject to attack by three different caterpillars; those of the Large White or the Small White Butterflies (cabbage whites) and Cabbage Moth, and all three may be present at once. Netting in about April with insect proof mesh, or with fine net mesh (5-7mm) will prevent most attacks.

Note that the Small and Large Cabbage Whites have 2 generations a year - Cabbage Moth 1. The caterpillars of the Large Whites are yellow/black, and can usually be seen on the plants in June/July and again in late August and September. Small cabbage white caterpillars are usually May/June, and again July to September.

There is a nematode solution (Steinernema carpocapsae) available, which can be bought on line from suppliers like Nemesys, or sometimes available at garden centres. I'm not sure when the best time is to use the nematodes, so check the optimum time to use these when/before you buy. Otherwise, control is by hand picking, but if the infestation is heavy, pesticides available suitable for this use are:

  1. Bayer Sprayday Greenfly Killer
  2. Westland Plant Rescue Fruit and Veg Bugkiller
  3. Scotts Bug Clear Gun for Fruit and Veg
  4. Doff All in One Bug Spray

The first two can only be used twice a year, with a minimum 7 day gap between spraying and harvesting; the latter two can be used as often as necessary, with a gap of one day between spraying and harvesting.

As for the butterflies preferring one plant above all the others, that's probably just because there's something about access that's better for them - or maybe they feel safer in numbers!

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