Tansy beetle

Taxonomy

The tansy beetle was first described by Linnaeus in 1758 in the 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 species name is the Latin noun graminis meaning "grass-like". Chrysolina graminis sensu lato has at least three later binomial names which are considered to be synonyms: Chrysomela fulgida Fabricius, 1801, Chyrsolina nigrocuprea Mallet, 1924, and Chrysolina taupini Mallet, 1924.

There are six subspecies of tansy beetle. The nominate subspecies C. graminis graminis' was established via the original description of the species in 1758. C. graminis santonici was described by Contarini in 1847. In 1860 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.

Description

Tansy beetles are small, rounded beetles 7.7-10.5 mm in length. The body is dorsally bright green, the elytra mostly with golden-red or golden lateral margins and stripe near the suture, and a bluish suture. The aedeagus is narrowed at the apex, without teeth on the underside.

The subspecies C. graminis auraria is larger, 9.5-11.4 mm in length, with the deep lateral depressions of the pronotum developed only in its basal half.

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 mis-identification of C. herbacea as C. graminis.

Differences include the similarity of punctuation on the pronotum and elytra of C. herbacea, whereas the punctuation between these two parts on C. graminis is very different. C. herbacea tends to be unicolour - an iridescent green, whereas C. graminis has a copper or bronze sheen on its elytra. When both species are viewed from the side, there is a smooth lip running along the bottom eldge of the elytra; this lip runs the entire length of the wing cases on C. graminis, but only half the length on C. herbacea.

Distribution and habitat

The tansy beetle has a Paleartic 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.

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.

Continental Europe

In continental Europe, C. graminis spp. 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. In the majority of countries where it is found the species is declining. It is listed as vulnerable in Austria, Denmark, Sweden and Norway, and in Germany it is rare in one district, endangered in another.

Subspecies

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

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.

Life cycle

Both adults and larvae utilise 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 spent 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. gramnis will cannibalise the eggs of other females. In laboratory conditions, newly hatched larvae have been shown 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 unble 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.

Diet

Tansy beetles are phytophagous and primarily use tansy (Tanacetum vulgare) as their host plant. In continental Europe, the species has also been recorded consuming a wider range of food plants including gipsywort, marsh woundwort, and sneezewort.

The tansy plant naturally contains a number of volatile components including 1,8-cineole, trans-thujone, camphor and myrtenol, with the quantities and proportions of each varying seasonally and from plant to plant. 1,8-cineole is a toxin believed to defend the plant leaves against attacks by herbivores. The tansy beetle is resistant to these chemicals. However, tansy is a repellent to other Chrysomelid beetles. For example, the steam distillate of fresh leaves and flowers of tansy contains high levels of camphor and umbellulone and is strongly repellent to the Colorado potato beetle (Leptinotarsa decemlineata).

Predation

Beetle remains bearing what appeared to be the marks of bird attacks have been found by the River Ouse and hardened adults often bear symmetrically indented elytra, interpreted as 'pinch marks' inflicted by a beak whilst newly emerged adults are still hardening. Dead beetles have also been found in the webs of spiders in a captive population. The accidental predation of C. graminis by livestock consuming the tansy plant is also possible.

Predation of a larva by a scorpionfly Panorpa germanica has been observed in the field. The tachinid genus Macquartia exclusively parasitises beetles of the family Chrysomelidae; M. dispar may parasitise C. graminis directly. The adult fly deposits fully incubated eggs or newly hatched larvae into the vicinity of the larvae of their host. Larvae of other Chrysomelidae are predated by birds, coccinellids, predatory bugs, lacewing larvae, syrphid larvae, carabids, ants, wasps, spiders and harvestmen, all of which are common on the tansy around the York population. The pupal stage may be directly predated by the European mole.

Parasites

A mite, Chrysomelobia mahunkai (family Podapolipidae) has been recorded from a single adult specimen of C. graminis, and Eulophus chrysomela (a species of hymenopteran of the family Eulophidae) is recorded as an endoparasite of the pupal stage.

Conservation in the United Kingdom

The species is designated as 'Nationally Rare' in the United Kingdom and categorised as a 'Species of Principal Importance' in accordance with the Natural Environment and Rural Communities Act 2006. The Tansy Beetle Action Group was set up in 2008 to initiate and oversee conservation efforts and comprises representatives from the University of York, North Yorkshire County Council, City of York Council, Environment Agency, National Trust and Buglife. A recovery programme started, involving annual surveys of both tansy and beetles, control of tree shading and invasive plants that compete with tansy, such as Himalayan balsam, and limited re-introductions within the current species range. New clumps of tansy have been planted, particularly between isolated existing patches which may be beyond the 200 m walking range of the beetle. In order to publicise the conservation project Rachael Maskell, the Member of Parliament for the constituency of York Central became a 'Tansy Beetle Species Champion' in 2016.

In 2016, a team of 30 volunteers surveyed a 90 km stretch of the banks of the River Ouse. The surveying has identified an upward trend in population numbers, rising over 60% between 2015 and 2016 to 40,000 individuals.