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Introduction

During 2011 a transect along 500 metres of boardwalk at Hickling NNR was checked from 21st June to 15th August 2011 for Swallowtail Papilio machaon britannicus eggs and larvae in the reedbed fringes, one metre either side of the boardwalk.
This was not intended to be a scientific study, but purely an observational exercise brought about by a comment from the warden: 'Had I seen any swallowtail eggs?'
What ensued was an almost daily check on the number of larvae and eggs found on the Milk Parsley Peucedanum palustre from July to August. However, this article is not based on the numbers of eggs and larvae present, but on observations of events that occurred during the time of monitoring the eggs and larvae along the section of boardwalk.

Overview for Papilio machaon britannicus at Hickling in 2011

Due to a very warm spell towards the end of April, the first swallowtail had been recorded at Hickling on the 25th April. Although this relates to a very early individual, swallowtails began to appear earlier than the normal expected emergence time and in 2011 there was a second brood, which began towards the end of July and continued into August, with adults being seen until 15th August, when a single final instar larvae was also found.
Two eggs were found 22nd June, followed by 5 eggs and 9 1st instar larvae 29th June. In most instances, there was usually only one larvae per milk parsley plant. However, on the 6th July 5 larvae (1st and 2nd instars) were found on just one plant, followed by 3 larvae on another single plant on the 8th July. The numbers of eggs and larvae along the study area remained fairly constant throughout, allowing for developmental changes.

Predation of larvae by Picromerus bidens

During a check on the number of eggs and larvae (9th July), a 1st instar larvae was found dead on a milk parsley plant. On closer inspection a heteropteran species was seen next to the dead larvae. Although it was not clear at the time, whether or not the larvae had been killed by the bug, further evidence was found two days later.
On the 11th July, just behind Cadbury's hide (outside the study area) another swallowtail larvae was found dead, this time a final instar. Once again, the same bug species was found in attendance (see Figures 1 & 2). The bug was later identified as an immature Picromerus bidens. However, this was not the only intriguing factor, as the larvae was feeding on Fennel Foeniculum vulgare. Although there are only a few fennel plants on the reserve (apparently originating from a former wardens herb garden), the plant had been selected by an adult swallowtail, in the absence of any 'preferred' plant species as there are no milk parsley growing anywhere near the area where the fennel was growing. It is also quite clear that the plant sufficiently sustained the larvae through to final instar.

Larvae assuming position
bidens (3)
bidens (1)

Figure 2

Figure 3

Figure 1

Chrysalis colour changes

At 13.00 on the 30th July, a final instar larvae was found on a reed stem, at the edge of the boardwalk. However, its position and 'hunched-up' shape suggested that it was 'assuming the position' to pupate (see Figure 3). No change had occurred in the late afternoon, and checking the larvae in the morning the following day, found no further discernible changes, but it was still maintaining the same shape.
During the morning of the 1st August a chrysalis had formed (see Figure 4) but was still moving occasionally. At this point the colour was predominately a greenish colour, one of two colour forms cited in many texts, e,g, Tomlinson & Still ( 2002). However, as time progressed there was a colour change from the originally greenish chrysalis to a greyish form with areas of brown (see Figures 5 & 6).

1

Figure 4. 1-8-2011

2

Figure 5. 11-8-2011

3

Figure 6. 30-12-2011

Discussion

Considering the restricted distribution of the britannicus race of Swallowtail in the UK, there would appear to be very little recent research carried out and the observations made during 2011 present more questions than answers.
Whilst the swallowtail has many predators, despite its defence mechanisms (e.g. osmeterium), the extent and effect of predation by Picromerus bidens on britannicus larvae is unclear, at least in the UK. Puchkov (1961) & Lariviere & Larochelle (1989) describe P. bidens as a wide polyphagous species that preys on glabrous moving larvae and sometimes adults of many orders of insects, while studies in Czechia by Vrabec & Jindra (1998) suggest that P. bidens can decimate entire nests of the rare Euphydryas maturna butterfly. However, P. bidens thermal requirements for development appear to be a limiting factor in its ability to reach maturity (Mahdian et al 2008).
The well cited reference to britannicus larvae having an 'exclusive' food plant is also called into question here. Although, the evidence is a 'one-off' in this particular study and it cannot be ruled out entirely that the final instar larvae found (albeit dead) on fennel at Hickling may have been from a continental swallowtail Papilio machaon gorganus and as Burton (2008) states, he is not aware of any obvious differences between the larvae of the two races. There have been other occasions where britannicus larvae have been found on fennel at Hickling (pers. com. David Jenkin).
In addition, whilst researching this article, a website (http://www.wwb.co.uk/spring-and-summer-eggs-and-larvae-order-now-for-supply-in-season), which supplies butterfly eggs and larvae states that P. m. britannicus larvae are easy to rear on Fennel, Carrot tops, the flowers of Parsnip (wild and cultivated) or on its fenland food plant Milk Parsley Peucedanum palustre.
If britannicus larvae can survive and finally reach imago on umbellifer species other than milk parsley, it poses the question to why are they restricted to their present range? Or is there something else in the fenland ecosystem that limits their distribution? Chinery (1986) states that the British race can only survive in moist habitats, for the male genitalia are very sensitive to humidity and become hard and unable to function in drier climates.
Considering how difficult the swallowtail chrysalis is to spot, it begs the question of how many have been located and observed in their natural environment and the basis on which two different colour forms are cited in many books and articles. The photographs (Figures 4, 5 & 6) clearly show that this particular chrysalis changed colour (or faded with age) over time. This would make far more strategical sense, with respect to camouflage, corresponding with the change in colour of the reeds. Is this again, a 'one-off' or is it that the two colour forms of the chrysalis, referred to in literature are individual observations relating to a brief stage in the chrysalis colour change over a longer period of time?

Acknowledgements

Thanks to David Jenkin for his valuable information, relating to Swallowtails on the reserve at Hickling, during his 23 year presence working there.

References

Burton, J. F., 2008. Notes on the Continental Swallowtail Papilio machaon gorganus Fruhstorfer. Atropos 33: 4-11.
Chinery, M. (1986). Insects of Britain & Western Europe pp. 112. Harper Collins, London.
Lariviere, M.-C. & Larochelle, A., 1989. Picromerus bidens (Heteroptera: Pentatomidae) in North America, with a World Review of Distribution and Bionomics, Entomol. News, vol. 100, no. 4, pp. 133–146.
Mahdian, K., Tirry, L & De Clercq, P., 2008. Development of the predatory pentatomid Picromerus bidens (L.) at various constant temperatures. Belg. J. Zool., 138 (2): 135-139.
Puchkov, V.G., 1961. Stink-Bugs, Fauna Ukrainy (Fauna of the Ukraine), Kiev: Vidavnitstvo Akad. Nauk UkrSSR,, vol. 21, part 1, pp. 1–338.
Tomlinson, D. & Still, R., 2002 Britain's Butterflies. Wild Guides, London.
Vrabec, V. & Jindra, Z., 1998. The caterpillars of the rare butterfly Euphydryas maturna (Lepidoptera: Nymphalidae) as food for predatory bug Picromerus bidens (Heteroptera: Pentatomidae). Ento mological Problems 29: 87–90.
http://www.wwb.co.uk/spring-and-summer-eggs-and-larvae-order-now-for-supply-in-season. 30-5-2012.

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