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This year in my garden, there has been an unusual abundance of Orange Tips and an equally unusual shortage of Garlic Mustard - the wife has been a bit-over enthusiastic with her weeding! This seems to have driven the local female OTs to desperation: some of the remaining garlic mustard heads have had five or six OT eggs laid on them, something I have not seen before. They have also been laying eggs on the watercress in my pond.

Out of curiosity, I picked an example of garlic mustard with five eggs on it, just to see what happened when the eggs started to hatch. I have read that OT larvae are cannibalistic. However, all five eggs on one plant hatched and the larvae seemed to coexist quite happily until they reached about 7mm in length (second instar?) and the seed pods were depleted. They then simply abandoned the denuded stem and walked off to find fresh food, which I supplied.

They seemed completely disinclined to bite lumps out of each other! The five larvae are now on separate garlic mustard stems and are all flourishing.

Any thoughts?

Hi Keith,

Nice observations ! Maybe if the supply of food is plentiful there is no reason why the caterpillars would eat one another....
Last year I found caterpillars of the Mullein moth on Verbascum. There was nothing left of the plant and one of the caterpillars was eating the remains of another caterpillar. I was surprised by this behaviour but I think it was because the leaves of the plant had all been eaten.
Sylvie

KeithS wrote:
Out of curiosity, I picked an example of garlic mustard with five eggs on it, just to see what happened when the eggs started to hatch. I have read that OT larvae are cannibalistic. However, all five eggs on one plant hatched and the larvae seemed to coexist quite happily until they reached about 7mm in length (second instar?) and the seed pods were depleted. They then simply abandoned the denuded stem and walked off to find fresh food, which I supplied.

They seemed completely disinclined to bite lumps out of each other! The five larvae are now on separate garlic mustard stems and are all flourishing.

If they hatched roughly simultaneously, then they will all be approximately the same size. It's easy to defeat a small larva when you're a large one, but equality of size may make a difference?

The eggs hatched over five days - two on day one, one the next day and two on day five. I thought the first two larvae would eat the other eggs as they were on adjacent flower calyxes, but I suppose that if they didn’t physically bump into them and had plenty of other food available, there would be no need to eat their unhatched cousins.

I guess you are right about size differential - but I don’t plan to deliberately experiment...

Sylvie_h wrote:Hi Keith,

Nice observations ! Maybe if the supply of food is plentiful there is no reason why the caterpillars would eat one another....
Last year I found caterpillars of the Mullein moth on Verbascum. There was nothing left of the plant and one of the caterpillars was eating the remains of another caterpillar. I was surprised by this behaviour but I think it was because the leaves of the plant had all been eaten.
Sylvie

I think you are right. If there is plenty of food around, then similar-sized larvae will just go hunting elsewhere rather than eat each other.