160         Phragmataecia castaneae (Hübner, 1790)    REED LEOPARD     RDB2

A rare species that is well established at Wicken Fen and Chippenham Fen and local in the Norfolk Broads.  One or two eggs are inserted in the leaf sheath of common reed (Phragmites australis) and the resulting larvae then feed for nearly two years on the pith within reed stems.  The insect spends a further month as a pupa remaining within the stem before emerging in June.  Male adults are freely attracted to light, and females may occasionally be seen flying over the reed canopy at dusk, and thereafter rest on the host plant. 

The species is resident at a single extensive freshwater reedbed at Morden Bog, a SSSI near Wareham.  Searches for the species in the larger but more brackish reedbeds around the Poole Basin have so far proved negative, although sampling has been restricted to reedbed edges due to the wet terrain; detecting colonies and quantifying colony size remains a challenge.  The reedbed at Morden differs from these other sites in that the individual plants are scattered and not densely packed together.  Core samples have been taken from Morden Bog and it appears that the reedbed has been in existence for at least one thousand years (Haskins, 1978).

The threats to the host plant at Morden Bog include a lowering of the water table, drainage, fire, pollution and over-grazing.  The first two threats are unlikely given the current high level of protection afforded to the Bog.  A major fire in the severe drought of 1934 burnt much of the reed temporarily, but two years on, the moth was again seen in good numbers within the regenerated reedbed (A Russell).  Effluent seeping into the mire from a nearby caravan site has created nitrate-rich conditions where stinging nettle (Urtica dioica) flourishes in a small area on the western side of the reed habitat.  Stock has been introduced by Natural England on to Morden Bog and adjacent heathland to graze grass species, particularly purple moor-grass (Molinia caerulea).  The cattle do not tend to enter the heart of the reedbed as those used for grazing at Morden do not like getting their feet wet (Natural England, pers comm). However, sika deer do frequent the bog where new reed shoots form part of their diet.  Continued monitoring of this Dorset rarity is recommended at Morden Bog.

161         Zeuzera pyrina (Linnaeus, 1761)    LEOPARD

A species that is restricted to England, the larva feeding for two or three years in a stem or branch (often no more than ten cm. in diameter) of a wide range of deciduous trees and shrubs.  The caterpillar exudes an odour, a polyphenol called zeuzerine (C₁₄H₂₆O₂), and this may be used by the insect as a defence mechanism against potential prey (MOGBI Vol 2).  Damage to commercially grown crops is limited as larvae rarely ‘infest’ the host plant.  In Dorset, the moth is locally frequent in woodland and gardens, but not to such an extent that it constitutes a pest.  The male is readily attracted to light, but rarely more than a single moth is observed on any given night during its flight period. Larvae have been detected within sycamore (Acer pseudoplatanus), apple (Malus domestica) and oak (Quercus spp.) across the county.

162         Cossus cossus (Linnaeus, 1758)    GOAT     Notable/Nb

A local and decreasing species in the British Isles, the larva living up to five years inside a wide variety of deciduous trees.  A colony can occasionally destroy its host.  The caterpillar exudes a strong goat like scent, a polyphenol called cossine (C₁₆H₂₄O₂) that can be detected down-wind of an infected tree, and is reputed to be distasteful to birds (MOGBI Vol 2).  When full grown (up to 100 mm long), the caterpillar is occasionally encountered well away from its host tree as it seeks a place to pupate, although pupation usually takes place within its host.  The caterpillar spends its final winter in the larval state within a strong cocoon.  The moth is sometimes observed at ‘sugar’ bait apparently attracted by one or more of the sugar ingredients. Without a tongue it is difficult to understand how the moth might benefit from such behaviour, but as cossine is a complex alcohol, the alcohol within the ‘sugar’ may attract the moth.

In Dorset, the moth was regarded as “generally distributed and not uncommon in the larval state when it could be occasionally fatally destructive to poplar planted for roadside ornament” (per W Parkinson Curtis manuscript).  However, with fewer reports of the species in recent years, the declining trend nationally seems to be mirrored across the county.  The few larval records in the past ten years or so, suggest a preference for oak (Quercus spp.), birch (Betula spp.) and sallow (Salix spp.) in deciduous woodland on sandy soil.  An interesting observation by Brigadier H Warry at Upwey in 1953, related the occurrence of a freshly emerged female on the lawn ten yards from the nearest tree followed two days later by a freshly emerged male in the same place, and surmised infestation of the root of the tree. Conservation measures should include the retention of viable areas of mature oak-birch-sallow woodland on sites managed for conservation.