Marsh Fritillary Project Update – July 2021

A lot has happened in the 2 months since the last update when the first butterfly had emerged in the rearing pens. Not long after that our newly-appointed species officer started and was lucky enough, on his first day, to see a pair of marsh fritillaries mating in one of the pens. This was a real milestone and a relief to see that the project was continuing as planned.

Marsh Fritillary mating in rearing pen

The week after that we found our first batch of eggs. They were bright yellow so had not long been laid; as they mature they turn brown and then a dark purple colour not long before hatching. We subsequently managed to get footage of a female laying her eggs on the underside of a devil’s-bit scabious leaf. Each female can potentially lay 300-400 eggs over a few days after mating.

Female Marsh Fritillary butterfly laying eggs

Over the next couple of weeks we kept finding more and more batches of eggs ranging in size from a scattered few on a leaf to one which had 4 large batches of eggs. The latter was clearly created by multiple females due to the size and varying colours of the batches, some of which were 2 or 3 layers deep. Marsh fritillaries are known to lay eggs on leaves which have already been used by other females.

Batches of Marsh Fritillary eggs on underside of Devil’s-bit Scabious leaf.

Anticipation subsequently rose as eggs were noticeably darkening and we were hoping that we would soon see the first caterpillars emerging. On the 26th June we finally saw the tiny (1-1.5mm) caterpillars start to emerge from one cluster of eggs which had turned grey just before hatching. These caterpillars immediately started spinning a web within which they will develop and feed on the underside of the leaf. In the few days since we have been seeing more eggs hatching and groups of caterpillars creating webs joining leaves together. The successful eggs soon become evident from a distance as the caterpillars feed on the underside of the leaves, leaving brown patches visible on the upper surface.

In addition to rearing the caterpillars, our species officer and Katie, the placement student who has been helping with the captive breeding, went to one of the donor sites with the aim of catching adult marsh fritillaries. This was a trial to see if it was feasible to capture adult butterflies from the wild and take them back to the rearing pens to breed them. The aim was to catch 2 males and 2 females which is easier said than done; there is considerable overlap in size and colouration of the 2 sexes so despite males generally being smaller and more well-marked we wanted to be as certain as possible. This was done by being lucky enough to catch a pair in the process of mating which had the added benefit of knowing that the female had not yet laid eggs as males ensure females can’t mate with other males by sealing their genital opening. We then caught a particularly large and particularly small individuals with the idea that that increased the probability that we had indeed caught one of each sex. These were successfully transported back to the rearing pens and have since been seen mating and laying eggs.

Species Officer, Vaughn Matthews searching for butterflies.

One thought on “Marsh Fritillary Project Update – July 2021

  1. Hello Rob,

    We met a few years ago now when I was with Valeways and you were looking to get more visitors to the Lavernock Nature Reserve. I am now a memmber of the Cowbridge U3A Natural History Group. We had a talk last year, or maybe the year before, about Marsh Fritillary butterflies in a Gloucester reserve. On hearing you talk about you project much closer to home, I wondered if it might be possible to bring a group to visit at an appropriate time of year.

    Hope this might be possible and look forward to hearing from you, Rebecca

    Mobile 07890 941502

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