Lime swallowtail lays eggs in summer heat

The air over Narayan Peth sits heavy and bright in the middle of summer. In the gardens and along the roadsides, wherever a citrus tree grows, look closely at the leaves. Near the tips of young shoots, on the smooth upper surface of a single leaf, you may find a pale yellow egg, about the size of a pinhead, sitting alone. It was placed there by a lime swallowtail, and it is the beginning of a new generation.

The lime swallowtail is one of the most widespread butterflies in this part of the world. It is a large butterfly, with yellow and black patterning across its wings and a distinctive dark band running through them. In flight it moves with a loose, unhurried wingbeat, and it is often seen in gardens, around flowering shrubs, or drifting along tree lines. But in early summer, the females are doing something more deliberate. They are searching for citrus. Lime, lemon, orange, curry leaf, and the native bengal quince all belong to the same plant family, and all carry the chemical compounds the lime swallowtail needs. A female will land on a young leaf, press her forelegs against the surface, and taste it. She is checking for the right volatile compounds before she commits to laying. If the leaf passes, she curves her abdomen and deposits a single egg, then moves on to the next plant. She will lay dozens of eggs this way, spreading them across different plants rather than concentrating them in one place.

The timing matters. Young, actively growing leaves are softer and have lower concentrations of the defensive compounds that older leaves accumulate over time. A caterpillar hatching on a mature leaf faces tougher tissue and stronger chemistry. On a new flush of growth, it has a better start. Summer in Narayan Peth brings heat and, in many gardens, irrigation, which keeps citrus pushing out new leaves even when the rains have not yet arrived in full. The butterfly is tracking that growth. The caterpillars that hatch will go through several distinct stages. Early instars are dark and mottled, resembling bird droppings on a leaf surface. Later instars turn green, matching the leaf they rest on. Both are forms of concealment, but they work differently at different sizes. The caterpillar also carries a structure behind its head called an osmeterium, a forked orange gland that it can extend when threatened, releasing a sharp, unpleasant smell. It is a last resort, but it works against some predators.

The lime swallowtail does not live in isolation here. The common mormon swallowtail uses the same citrus host plants, and both species can be found laying on the same trees during the same weeks. There is overlap but not direct competition in the strict sense: each female is selecting individual leaves, and a tree in active growth produces enough new foliage to support both. The shikra, a small hawk present in this area, takes large insects and could take a butterfly on the wing, though the swallowtail's erratic flight makes it a difficult target. More consistent pressure comes from birds like the greater coucal, which moves through dense vegetation and is capable of taking caterpillars from foliage. This is part of why the egg-scattering behavior matters. A female that lays all her eggs on one plant loses everything if a forager works that plant thoroughly. Distributed eggs mean distributed risk.

The plain tiger butterfly and the lemon migrant are also moving through gardens right now, each tied to different host plants, each running its own cycle through the heat. The lime swallowtail's cycle is simply the one written on citrus leaves. If there is a citrus tree near you, or a curry leaf plant in a pot, run your eye over the youngest leaves at the growing tips. The egg, if it is there, will catch the light slightly differently from the leaf surface around it, a small pale dome sitting on green.