Acronicta rumicis, the knot grass moth, is a species of moth which is part of the genus Acronicta and family Noctuidae. It was first described by Carl Linnaeus in his 1758 10th edition of Systema Naturae. It is found in the Palearctic region. A. rumicis lives and feeds on plants located in wide-open areas. At its larval stage, as a caterpillar, it causes such a large impact as a crop pest that it has received much attention and research. A. rumicis feeds on maize, strawberries and other herbaceous plants.
The moth's evolution has been affected by the industrial melanism that occurred in Europe during the late 19th and early 20th centuries, causing a dramatic increase in two aberrations (salicin and lugubris), which have darker grey wings. Today, A. rumicis is important to conservation efforts in the United Kingdom, because, like many other species, it is in decline. However, as it is part of a UK Priority Biodiversity Action Plan and receiving greater awareness and conservation, the species may become more abundant.
Description and identification A. rumicis has a wingspan of 34–44 mm. The forewings are blotched with a mixture of dark and light-grey shades, while the hindwings are dark brown.Alford, D. V., 2014. Pests of Fruit Crops: A Colour Handbook. p.356. A white spot is seen midway down the trailing edge of the forewing providing a distinct marker for A. rumicis not seen in other Acronicta moths. Chinese and Japanese moths tend to be larger than their European counterparts. The Asian moths also have lower forewings that have darker shades of grey.
Technical description and variation
C. rumicis L. (3i). Forewing dark grey, varied in places with whitish; lines and shades black; outer line marked by a white spot on the submedian fold; hindwings brownish fuscous Larva marbled dark and light grey: a dorsal row of red spots on black blotches, and a row of white spots on each side; a pale line below spiracles, containing orange-red tubercles; segments 5 and 12 of dorsum humped; tubercles with fascicles of fuscous and fulvous hairs— turanica Stgr., a form from Central Asia, is much paler, with the hindwings whitish. — Chinese and Japanese examples (3k) are larger than European and in all cases darker; in particular the lower half of forewing is blacker and the white spot of outer line then often obscured; possibly the scotch form figured by Curtis as salicis, a melanic form, may represent this aberration, though the larva figured as belonging thereto is unquestionably that of menyanthidis. — ab. alnoides Geest (3k) has the costal half of forewing grey, the inner half blackish, somewhat interrupted with a strong black dagger-shaped mark at base and above anal angle; superficially resembles a dark grey alni; recorded from Freiburg in Baden. — ab. euphorbiae Steph. nec. Hbn. is grey brown, with the stigmata and space between them pale; and ab. euphrasiae Steph. nec Dup. is pale yellowish grey with numerous black lines and the orbicular stigma faint or obsolete, occurring in Cornwall.Warren. W. in Seitz, A. Ed., 1914 Die Großschmetterlinge der Erde, Verlag Alfred Kernen, Stuttgart Band 3: Abt. 1, Die Großschmetterlinge des palaearktischen Faunengebietes, Die palaearktischen eulenartigen Nachtfalter, 1914
Taxonomy and phylogenetics A. rumicis is a moth of the genus Acronicta and family Noctuidae. Noctuidae has upwards of 21,000 species. Noctuids are commonly known as "owlet moths" because of their large eyes that shine in the dark and the colour of their wings which make them difficult to see.Singh, H., 2014. Taxonomic Studies on Family Noctuidae (Lepidoptera) from North-East India. In their usual resting position, noctuids fold their wings back over their bodies; the forewings overlap one another. The closed wings form a tent-like shape.Waring, Paul, et al. Field Guide to the Moths of Great Britain and Ireland. British Wildlife, 2009. Almost all Acronicta larvae are brightly coloured with spiky brown hairs all along the dorsal side of the body.Chapman, T.A., 1890. The Genus Acronycta and its Allies. The Entomologist's Record and Journal of Variation, 1(April), pp.1–4, 26–28. According to Thomas Algernon Chapman, the genus Acronicta can be split into three major groups: The first, characterized by a hoop-like structure of the pupa and clumped oviposition, includes: A. auricoma, A. myricae, A. menthanthidis, A. venosa, and A. rumicis. The second, characterized by terminal spines of pupa and dome-shaped ova laid apart from others, includes: A. psi, A. tridens, A. strigosa, A. alni, A. megacephala, A. leporina, A. aceris. The third group has only one species, A. ligustri.
Distribution and habitat A. rumicis is distributed throughout the Palearctic. It is common in northwestern China, the Korean Peninsula, and Japan, although it has also been found in other parts of Eurasia, including Russia.Anon, 1901. Transactions of the Entomological Society of London. A. rumicis is found in almost all parts of Europe, though it is absent in some areas of north-western Scandinavia. There has been much research about the species in England and Scotland.
These moths tend to spend their lives in plants located in wide-open areas like meadows, woodland clearings, gardens, and hedgerows, generally in non-humid areas.Winiarska, G., 1990. Communities of noctuids (Lepidoptera, Noctuidae) of linden-oak-hornbeam forests of the Mazovian Lowland. Fragmenta Faunistica, 15(11), pp.143–163.
Food resources
Caterpillars At the caterpillar stage, A. rumicis tends to feed on low-growing, herbaceous plants, including sorrel, dock, bramble, thistles, hop,Skinner, B., 2009. Colour Identification Guide to Moths of the British Isles: (Macrolepidoptera). p.149. and occasionally on the leaves of shrubs. In addition, A. rumicis larvae favour fruit trees like Prunus species and plants of the family Polygonaceae.Cho, Y., Kwon, O. & Nam, S.-H., 2006. Ecological and morphological characteristics of the endoparasitoids of larval Acronicta rumicis (Lepidoptera: Noctuidae). Entomological Research, 36, pp. 208–215.
In research, one of the most commonly used host plants is Rumex obtusifolius, a herbaceous plant commonly found within A. rumicis range.
Parental care Oviposition
Females tend to lay eggs on leaves of host plants in large clusters. The eggs have long longitudinal ridges, which originate and spread out from the micropylar area of the egg. The longest ridges tend to be in a zigzag formation.Dolinskaya, I. V., 2014. Egg Morphology of some Noctuidae (Lepidoptera). Vestnik Zoologii, 48(4), pp. 353–364. The eggs have a flattened shape, making clustering and clumping easier.
Life history A. rumicis species generally have two broods: the first generation flies in May and June, while the second flies in August and September. However, the number of generations can differ based on location; in southern Europe, it tends to have three broods, while in the north, it has only one, most likely due to diapause. The number of broods may be determined by the duration of light and temperature ranges. The larvae tend to feed from the summer onwards until winter, during which A. rumicis spend its time in the pupae stage in cocoons.
Egg Eggs are laid in clusters, in almost regular, overlapping rows on host plants. The egg stage lasts about 5 to 10 days. The eggs have a round flattened shape like a sand dollar and has a micropylar area that has 20 long ridges that radiate from the center, almost like a child's drawing of a sun and its rays. The flattened structure allows for stability while resting on the host plant.
Caterpillar When larvae hatch, they tend to nibble out the round opening of the egg. Usually, A. rumicis larvae are ready to pupate after 30 days. They tend to be about 40 mm long, having dark brown bodies, with white spots along both sides of the body and red spots along the centre of the back.Anon, 2010. The Knot Grass moth and caterpillar – Acronicta rumicis. Wildlife Insight. Body hairs grow in tufts, arranged in rows: two rows along each side of the body and one row along the centre of the back.
Pupa Once the winter months near, fully grown caterpillars pupate in cocoons on leaves or leaf litter. The cocoon has tough white silk with one simple layer of grass, twigs, or leaves for better concealment from predators.
Adult Adult moths tend to emerge in June but can also appear in earlier months if the weather is consistently warm.
Enemies Predators Although the general cause of trouble in the life of A. rumicis is parasitism via parasitoids, another group of predators includes small birds. These birds, like the great spotted woodpecker, tend to breed and nest in agricultural regions. In one study from Slovenia, the birds were nested in a hop field. Hop is a popular food resource for A. rumicis, while caterpillars are the primary food source for these birds.Vogrin, M., 2002. Breeding birds in hop fields. Ornis Svecica, 12, pp.92–94.
Parasites Parasitism in A. rumicis generally occurs in the months of May and October. Rates are lower in the months of June and July for reasons yet unknown. Most parasitoids affecting A. rumicis are parasitic wasps in the Braconidae and Ichneumonidae families, as well as parasitic flies.Cho, Y.-H., Kwon, O.-S. & Nam, S.-H., 2006. Species Composition and Seasonal Distribution of the Endoparasitoids on Acronicta rumicis (Lepidoptera, Noctuidae). Journal of Ecology and Environment, 29(1), pp.55–60.
One of the most common parasitoids of A. rumicis is Glyptapanteles liparidis, a larval parasitoid that is highly distributed throughout Eurasia.CHO, Y.-H., NAM, S.-H. & KWON, O., 2006. Interaction of Acronicta rumicis (Lepidoptera: Noctuidae) and its larval parasitoid, Glyptapanteles liparidis (Hymenoptera: Ichneumonidae). Entomological Research, 36, pp. 79–84. G. liparidis oviposits on the surface of the moth larvae; once hatched, the wasp larvae feed on the A. rumicis larvae and eventually cause death.
Based on one study in South Korea, G. liparidis affects A. rumicis in its first, second, third, and fourth instar. The same study also discovered that the second-instar larvae were apparently the best food source for this parasitoid, as the second-instar larvae had the highest mortality rate in comparison to the other three larval stages. As the G. liparidis continuously grows and utilizes the resources produced internally by A. rumicis larvae, the feeding habits of these larvae change as well. On average, parasitized larvae consume more plant material than those not parasitized; the second instar larvae tend to consume the most food in comparison to the first, third, and fourth instars, supporting the idea that most G. liparidis feed most efficiently on the second instar larvae of A. rumicis.
Genetics of colour patterns Pigmentation and structural colouration Industrial melanism exists within the species, meaning the moths evolved during the industrial period in England when dark soot was deposited into the environment. The aberration salicis Curtis, which is commonly found throughout the moth's distribution range, has darker grey-blotched sections on wings and retains the A. rumicis identifying marker, the white spot on its forewings.Skinner, B., 2009. Colour Identification Guide to Moths of the British Isles: (Macrolepidoptera), p. 149. Aberration salicis is commonly found distributed in England, Wales, and Ireland.