tansy beetle

The tansy beetle (''Chrysolina graminis'') is a species of leaf beetle. It measures 7.7–10.5 mm in length and has a characteristic bright metallic green colouration. The common name derives from the tansy plant on which they often feed as both larvae and adults. In addition to the nominotypical subspecies, which repeats the specific name, ''C. graminis graminis'', there are five further distinct subspecies of tansy beetle, which, collectively, have a Palearctic distribution, although in the majority of countries where it is found the species is declining. In the United Kingdom it is designated as 'Nationally Rare' and this localised population, centred on York, North Yorkshire, has been the subject of much recent research.

Taxonomy

The tansy beetle was first described by Carl Linnaeus in his landmark 1758 10th edition of ''Systema Naturae'' as ''Chrysomela graminis'' and was later transferred to the genus '' Chrysolina''. The genus ''Chrysolina'' currently contains 39 subgenera. ''C. graminis'' (with its several subspecies) is located in the subgenus ''Euchrysolina'' which contains only one other species – '' C. virgata''; this subgenus was first established in 1950.

Its specific name ''graminis'' is a form of the Latin noun ''gramen'', meaning "of grass" or "grass-like". ''Chrysolina graminis'' sensu lato has at least three later binomial names which are considered to be synonyms: ''Chrysomela fulgida'' Fabricius, 1801, ''Chrysolina nigrocuprea'' Mallet, 1924, and ''Chrysolina taupini'' Mallet, 1924.

There are six subspecies of tansy beetle. The nominotypical subspecies ''C. graminis graminis'' was established via the original description of the species in 1758. ''C. graminis santonici'' (named after the Italian name, Santonico, for its host plant '' Artemisia caerulescens'') was described by N. B. Contarini in 1847. In 1860 Victor Motschulsky described two subspecies – ''C. graminis artemisiae'' and ''C. graminis auraria''. These were added to in the 20th century with ''C. graminis christianae'' (Mallet, 1933) and ''C. graminis mediterranea'' Bechyné, 1950.

The subspecies of ''C. graminis'' have localised distributions: ''C. graminis artemisiae'' is located in south-east Europe, central Asia, and southern Siberia; ''C. graminis auraria'' is located in Dauria, eastern Mongolia, and China; ''C. graminis christianae'' is located in France; ''C. graminis mediterranea'' is located in Corsica and Spain; ''C. graminis santonici'' is located in the central Alps.

Description

All tansy beetles are small, rounded beetles approximately 7–12 mm in length. There are both internal and external morphological differences between the six subspecies.

Larvae

The larvae of ''C. graminis'' have four instar stages, though the first instar may not possess the characteristics of the later instars. Larvae are brown, and dorsally convex with spiracles evident on eight segments. They usually have small, indistinct tubercles with very short setae. The head of each larva is a darker brown than the body and has six ocelli on each side. The mandibles have five apical teeth.

Distinguishing ''Chrysolina graminis'' and ''Chrysolina herbacea''

''Chrysolina graminis'' and '' C. herbacea'' are similar in size and colour and may be confused in UK populations. Distinguishing the two species in the United Kingdom is particularly important as ''C. graminis'' is a vulnerable species whereas ''C. herbacea'' is much more common. This has previously led to the misidentification of ''C. herbacea'' as ''C. graminis''. ''C. herbacea'' measures 7–11 mm in length. It is coloured iridescent green on dorsal surface throughout. The punctuation on the pronotum and elytra is distinctly similar, unlike in ''C. graminis''. There is a smooth lip running only half the length of the ventral edge of the elytra, whereas this lip runs the full length on ''C. graminis''.

Distribution and habitat

The tansy beetle has a Palearctic distribution. Evidence from archaeological excavation has shown that its presence in western Europe is confirmed at least as early as the Neolithic period. The beetles may be found on tansy (''Tanacetum vulgare'') and water mint (''Mentha aquatica'') in fen land and the banks of rivers with broad floodplains in Britain. Larvae are also recorded from other host plants: ''Achillea ptarmica'' (sneezewort) in France and various plants of the genus '' Artemisia'' in Russia. Adults and larvae feed on the leaves of their host plants.

Distribution

In continental Europe, ''C. graminis'' is widespread from Scandinavia to the Mediterranean Sea. It may also be found in central Asia and China. In Russia it may be found in the tundra zone from the Polar Urals to the Kolyma River, and in the nearby countries of Kazakhstan, and Mongolia. It is listed as vulnerable in Austria, Denmark, Sweden and Norway, and in Germany it is rare in one district, endangered in another.

United Kingdom

In the United Kingdom, its range is currently restricted to about 45 km of the banks of the River Ouse centred on York, North Yorkshire. Although there are scattered records from across England some of these may represent mis-identifications of the mint beetle, a more widespread species. Tansy beetles had previously been recorded from Wicken Fen, Cambridgeshire, where species of mint (''Mentha'' spp.), not tansy, acted as the host plant. The last accurate record for the beetle at this site was in 1981. In August 2014 a new sighting was made nearby at Woodwalton Fen, following which a relocation programme involving hundreds of adult beetles from Yorkshire to this site was undertaken. As of 2006 there were 19 British hectads (10 km squares) with records of the tansy beetle, but it has only been seen in 11 of these since 1970, six of which are centred around York. In 2015, the total number of individuals estimated from a survey of this area on the banks of the River Ouse was 24,000. In 2016 this number increased significantly to 40,000.

Threats to habitat

The decline in ''C. graminis'' is likely to be due to habitat loss resulting from land improvement and arable conversion, over-grazing, development, drainage and lowering of water-tables due to over-abstraction. Neglect may also lead to loss or degradation of habitat such as through over-shading or competition of food plants with invasive species such as Himalayan balsam (''Impatiens glandulifera''). Flood-bank works may deplete or destroy local sub-populations.

Within the York distribution the beetles are dependent on tansy as their sole food source; if a clump disappears the beetles are forced to walk to a new location as they rarely fly, despite having fully working wings and being capable of doing so. The loss of habitat impacts upon the ability of the tansy beetle to find an alternative source of the host plant. Additionally, tansy is a ruderal species and thus has a naturally high rate of turnover of plants, forcing beetles to regularly seek out new tansy patches to colonise.

A 2009 study of tansy beetle occupancy amongst 1305 patches (stems separated by no more than 50 cm) of the tansy plant on the banks of the River Ouse (York, UK) intended to establish data on the distribution of tansy plants and relate this to existing tansy beetle populations in order to contribute to the conservation effort of the species. The results were analysed using generalised additive models to conclude that the tansy patches should be managed towards volumes of 3 m and that these patches should be targeted within 200 m of existing beetle subpopulations on the same river bank in order to help the beetle population disperse and survive.

Life cycle

Both adults and larvae use the same host plant during their life cycle. As tansy often grows in discrete clumps, the total population of ''C. graminis'' in an area may be divided and individuals may spend their entire life cycle within an area of a few square metres.

Adults mate between March and June. In the monitored populations at York, mating of the same pair could last over 24 hours, during which time some pairs moved between tansy patches. Mating in a Russian population of ''C. graminis'' is preceded by an elaborate ritual not evident in other populations of the species, involving the male tapping the female's eyes, pronotum and antennae with its antennae. Mated females lay batches of 3–15 elongated, yellow eggs (each 2 mm long) on the underside of the tansy leaves. Eggs hatch into grey larvae. In captivity, one newly mated female produced 561 eggs over 136 days and another produced 158 eggs over 49 days. Females will lay in several locations; the average clutch size is 5–6 eggs. Female ''C. graminis'' will cannibalise the eggs of other females. In laboratory conditions, newly hatched larvae have been shown to survive for at least four days without food and thus have a long window of opportunity in which to reach a tansy plant. In July, the final instar larvae burrow underground beneath the plant to pupate, although very little is known about the biology of this process in ''C. graminis''. Between August and September, this new adult population emerges to feed before returning underground to overwinter in October; emergence of adults is from March to April the next year. Long-term monitoring has indicated that survival during winter hibernation is surprisingly high, as autumn and spring population sizes are very similar. This is despite annual winter flooding of the River Ouse, implying that overwintering individuals must be extremely tolerant of long periods of inundation and oxygen deprivation. Approximately 5% of overwintering adults do not emerge from the soil after the winter, but remain underground for the next year in a state of extended diapause and emerge in the following spring.

A few adult beetles may overlap between the spring and summer populations, but most adults from the earlier emergence die before the end of summer. Both adult and larval tansy beetles are unable to detect their host plant, or each other, at a distance, either by smell, sight or a combination of the two. Starvation is thus likely to be a major contributor to mortality when beetles wander away from their host and become lost.

File:Tansy beetle eggs.jpg, Tansy beetle eggs laid on the underside of a tansy leaf
File:Tansy beetle larva 3.jpg, Tansy beetle larva
File:Tansy Beetle mating pair.jpg, Mating pair of tansy beetles

Behaviour and ecology

Diet

Tansy beetles are herbivorous and primarily use tansy (''Tanacetum vulgare'') as their host plant. The species has, however, also been recorded consuming a wider range of food plants including '' Lycopus europaeus'' (gipsywort), '' Stachys palustris'' (marsh woundwort), ''Achillea ptarmica'' (sneezewort), ''Mentha aquatica'' (water mint), '' Mentha rotundifolia'' (false apple mint), as well as other species of the genera ''Chrysanthemum'', ''Scutellaria

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