Acronicta rumicis

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Knot grass moth (Acronicta rumicis)

The knot grass moth is a small nocturnal moth found across the Palearctic region, including much of Europe and Asia. It has a wingspan of 34–44 mm. The forewings are grey with dark and light patches and a distinctive white spot, while the hindwings are brown. Adults usually emerge around June, and sometimes earlier in warm years.

Eggs are laid in clusters on host plants. They are flattened with ridges and hatch into caterpillars (larvae) about 40 mm long. Larvae are dark brown with white and red spots and grow tufts of hair. They feed on many herbaceous plants and fruit trees, including sorrel (Rumex obtusifolius), dock, brambles, thistles, hop, and Prunus species.

Acronicta rumicis typically has two generations per year in many places, but southern Europe can have three and northern areas often have just one, because the larvae enter diapause (a winter pause) in response to day length and temperature. Pupation occurs in a tough white silk cocoon among leaves or leaf litter.

The moth is a minor crop pest during its caterpillar stage, capable of defoliating strawberry plants and sometimes feeding on maize and other crops. It also faces natural enemies, especially parasitoid wasps and flies; a common one is Glyptapanteles liparidis.

Some darker color forms appeared during the industrial era when pollution darkened habitats, but these melanistic forms have become less common in recent years.

Distribution-wise, Acronicta rumicis is widespread in Europe and Asia but is absent from some areas in northwestern Scandinavia. It prefers open habitats like meadows, woodland clearings, gardens, and hedgerows.

UK populations are in decline, having fallen sharply since 1969. It is now a Priority Biodiversity Action Plan species, with conservation efforts focusing on protecting hedgerows and diverse herbaceous plants to support moth populations. Hedgerows are especially important, providing food, shelter, and routes for moths moving between habitats.