Pale grass blues emerge in summer heat

The concrete and greenery of Shinjuku sit under full summer sun right now, and in the low vegetation along park edges and sidewalk plantings, something small is moving. Not the barn swallows cutting arcs overhead, not the brown-eared bulbuls calling from the shrubs. Closer to the ground, just above knee height, pale blue-gray wings open and close on a patch of oxalis. That is a pale grass blue, and this is its season.

The pale grass blue is one of the smallest butterflies you are likely to see in central Tokyo. A full-grown adult spans roughly two centimeters across the wings. The upper surface of the male is a soft violet-blue; the female is more subdued, brownish toward the margins. The undersides of both are pale gray with a fine pattern of dark spots and short streaks, which makes them nearly invisible when they fold their wings and settle on a stem. They fly low, rarely more than a meter off the ground, working across patches of their host plants in short, direct lines. In summer's longest days, adults are on the wing from morning through the afternoon heat, mating and laying eggs on the leaves of low-growing plants in the wood sorrel family.

That connection to wood sorrel is where this butterfly's urban persistence makes sense. Largeflower pink-sorrel, an invasive plant from South America now established across Shinjuku's planted beds and pavement cracks, is among the plants pale grass blue females will use as a larval host. The caterpillars feed on the leaves, and the plant is abundant here. This is not a comfortable relationship to describe simply. The invasive sorrel has displaced native ground flora in many places, but it has also extended the available habitat for a butterfly that might otherwise struggle in a city with little open meadow. The pale grass blue is not dependent on the invasive plant exclusively, and it uses native oxalis where those occur, but in an urban environment with limited ground cover, the abundance of largeflower pink-sorrel appears to support the butterfly's numbers through summer.

The pale grass blue produces multiple generations across the warm months, and the summer peak is when adults are most visible. Females lay eggs singly on the undersides of host plant leaves. The caterpillars are small and green, textured to match the leaf surface, and they feed at night. During the day they rest along the midrib of the leaf, difficult to find unless you look carefully. The pupal stage is brief in summer heat, and new adults emerge within a few weeks. This rapid cycling means the population through late summer is made up of overlapping generations, adults from eggs laid in early summer flying alongside adults that hatched only days before.

Other insects share this low stratum of the city. The common bluebottle, a larger native swallowtail, passes through occasionally at higher speed, but it does not linger in the same patches. Eurasian tree sparrows and white-cheeked starlings move through the same plantings and could take a pale grass blue if one were slow or exposed, but the butterfly's low flight and quick, irregular movement make it a difficult target. The carrion crows that patrol the park paths are after larger prey entirely.

If you are near a bed of low plants right now, a planted strip or a weedy edge along a wall, look at the level of your knees. The pale grass blue does not announce itself. It moves in short flights, lands briefly, and folds its wings so the spotted gray underside faces out. In the full midday heat of a Tokyo summer, when most of the park's activity has moved into shade, these butterflies are still working the sunny edges. Watch one land on a sorrel leaf and stay still. The wings close. The spots on the underside catch the light, and then the butterfly is just part of the plant.