This photo shows ants on a silky lupine flower. See the whitish grub-looking thing? That’s a caterpillar! It might grow up to be a beautiful butterfly one day. In particular, it’s a caterpillar in the family Lycaenidae.
Watch how jazzed these ants are about this lycaenid caterpillar:
Why are these ants hanging around with caterpillars? Well, the caterpillar honeydew is full of sugar and amino acids. The prevailing idea has been that ants benefit from eating the honeydew, and in turn offer protection to the butterfly larvae. But there might be a little more to it than that . . .
One labratory study of Japanese species suggests that the caterpillars actually secrete drugs along with honeydew that 1) encourage the ants to tend to the caterpillar, and 2) make the ants aggressive towards predators and parasites of the caterpillar. These non-honeydew secretions appear to affect dopamine regulation in ants. Caterpillars tend to produce these secretions when threatened. Basically, the caterpillars secrete chemicals that manipulate the behaviour of ants.
An interesting pattern is that butterfly species whose caterpillars are tended by ants often eat nitrogen-fixing plants (like lupines!), and also usually feed on the most nitrogen-rich parts of the plant.
Nitrogen is a key component of amino acids. The lycaenids may eat nitrogen-rich food in order to meet the high demands of their diet – they are feeding their growing caterpillar-selves ON TOP OF providing that amino acid-rich honeydew to their ant protectors.
But it’s also possible that lycaenid caterpillars eat nitrogen-rich foods because those foods contain the precursors necessary to make the secretions that manipulate ant behaviour.
There are other ideas as well. I have linked a couple articles by or involving Naomi E. Pierce, who has been doing interesting research into this topic for a while now. Regardless, if you look hard enough and at the right time, it’s not that hard to find caterpillars being tended to ants on lupines and other nitrogren-fixing plants in the legume family.
Below are some photos of lycaenid butterflies as adults, to give you a sense of what these caterpillars grow up to look like:
Silvery blues actually overwinter as pupae in ant nests! When Silvery blues feed heavily on lupines, those lupines will develop very high concentrations of distasteful alkaloids to discourage futher feeding. Other studies have shown that
Lycaenid Caterpillar Secretions Manipulate Attendant Ant Behaviour by Masaru K. Hojo, Naomi E. Pierce, Kazuki Tsuji. https://www.cell.com/current-biology/pdf/S0960-9822(15)00819-2.pdf
Lycaenid Butterflies and Ants: Selection for Nitrogen-Fixing and Other Protein-Rich Food Plants by Naomi E. Pierce. https://www.journals.uchicago.edu/doi/10.1086/284387