One Spot Grass Yellow (Eurema andersonii)

The One Spot Grass Yellow (Eurema andersonii) is a small, bright butterfly belonging to the family Pieridae — the group that encompasses the familiar whites and yellows. Widespread across South and Southeast Asia, it is a regular presence in Sri Lanka's open lowlands, forest margins, and roadsides, where patient observers can watch it dart between wildflowers throughout the day.

How do you identify Eurema andersonii?

True to its name, the One Spot Grass Yellow is predominantly yellow on both wing surfaces, with one diagnostic black spot on the underside of the hindwing. The forewing uppersides carry a thin dark border that is more pronounced in some individuals, while the underside ground colour is a softer, paler yellow. Males and females are broadly similar in appearance, though females occasionally show slightly heavier markings along the wing margins.

• Size: Small to medium — typical of the Eurema genus.
• Upperside: Bright yellow with a slender dark forewing border.
• Underside: Pale yellow; a single, distinct black spot on the hindwing is the key field mark.
• Sexual dimorphism: Subtle — females may have marginally more marginal markings.

Where does the One Spot Grass Yellow live in Sri Lanka?

Eurema andersonii favours open, sunny environments rather than dense canopy forest. In Sri Lanka it is most reliably encountered in grassy clearings, scrublands, paddy-field margins, and along roadsides where flowering herbs and low shrubs are abundant. Elevations from sea level up through the mid-country foothills are all suitable; the species is less commonly recorded in cloud-forest zones above roughly 1,500 m.

Good places to look include the open grasslands around Udawalawe National Park, forest-edge tracks near the Sinharaja Forest Reserve, and the scrubby margins of wetland areas. Populations are generally resident rather than strongly migratory, so sightings are possible year-round.

What does Eurema andersonii eat?

Adults feed on nectar from a variety of small wildflowers, typically foraging close to ground level. They are generalist nectar feeders, visiting plants from several families, and in doing so contribute to pollination in grassland and scrub habitats. Larval host plants belong primarily to the legume family Fabaceae. Plants such as Pongamia pinnata and related leguminous shrubs are among those used by the caterpillars, which feed on the foliage before pupating on stems or leaf litter.

How does the One Spot Grass Yellow behave?

This species has a characteristically rapid, darting flight that makes it appear restless and unpredictable. It rarely flies in a straight line for long, instead weaving low over vegetation before settling briefly on a leaf or flower head. Activity peaks during the warmest parts of the day, and individuals frequently bask with wings open on sunlit foliage to regulate body temperature. At dusk they roost on low vegetation, wings closed, where the pale underside provides reasonable camouflage.

Like most pierid butterflies, Eurema andersonii occasionally congregates in loose groups at damp soil or mud puddles — a behaviour known as mud-puddling, through which males absorb salts and minerals that are later transferred to females during mating.

What is the broader range of Eurema andersonii?

Beyond Sri Lanka, the One Spot Grass Yellow is distributed across peninsular India, the Andaman Islands, and much of mainland Southeast Asia, extending through Myanmar, Thailand, and into the Malay Peninsula. Across this range it occupies broadly similar open habitats and is generally considered a common species, though local abundance varies with land-use change and the availability of host plants.

What role does this butterfly play in the ecosystem?

As a nectar feeder, Eurema andersonii is a minor but consistent pollinator of low-growing flowering plants in grassland and scrub communities. Its caterpillars, feeding on Fabaceae, form part of the food web supporting insectivorous birds, lizards, and spiders. The species' tolerance of disturbed habitats — roadsides, cultivated margins, secondary scrub — means it persists in landscapes that have lost many more specialised butterflies, making it a useful indicator of baseline invertebrate health in modified environments.

When is the best time to see this butterfly in Sri Lanka?

The One Spot Grass Yellow is active throughout the year in the lowlands, with no single peak season. Populations tend to be most conspicuous during and just after the two monsoon periods, when fresh vegetation and flowering plants are abundant. The dry-zone lowlands — such as those around Horton Plains National Park's lower approach roads — offer good sightings in the early morning before temperatures climb too high. In general, a midday walk along any sunny forest edge or grassy track in Sri Lanka stands a reasonable chance of producing an encounter.

Tips for photographing Eurema andersonii

• Approach slowly and from downwind — the darting flight is triggered easily by movement or shadow.
• Focus on mud-puddling sites near stream edges or damp soil in the morning, when groups may settle long enough for frame-filling shots.
• Use a macro lens or a telephoto with close-focus capability; the butterfly is small and rarely allows approach within 50 cm.
• Overcast light softens harsh shadows on the wings and makes the yellow tones easier to expose correctly.
• Photograph the underside as well as the upperside — the single hindwing spot is the definitive identification feature and is visible only from below.