I'm starting a daily series on urban bush-crickets. Checking through the blog, I realised I haven't paid much attention to these newly arrived species in the city. In 2022, I undertook a survey of urban bush-crickets, after I was made aware of the presence of several species in the city of Hull. It was an exhilarating summer: armed with a heterodyne bat detector, I explored grassy areas, marshland and brownfield sites across the city, listening for them or trying to spot them. The last two summers, I have continued with the surveys, as I have really got the bush-cricket bug! With this series, I want to write what I wish I had known when I started regarding each of the species.
Female Roesel's Bush-cricket basking, Rockford Fields, 9 October 2023. Note the curved, relatively short ovipositor and short wings.
The Roesel's Bush-cricket, Roeseliana roeselii, is a stocky species (top shot, a calling male at Sculcoates Cemetery, 28 September 2024). On first impression, if you see one scuttling through grass you think it's a small toad, as they physically disturb the vegetation with their bulk as they move. The size and colour is variable. The most common form is brown and yellow, with some green touches, but some individuals can be very green. The rim of the black side of the saddle-shaped thorax is white, yellow or green. Three yellow spots just above the base of the legs, on the thorax, are distinctive, as is a black comb-shaped stripe across the jumping legs. The species is wing-polymorphic. Nymphs growing in dense populations tend to develop long wings and are able to fly long distances. The long-winged form is responsible for long-distance dispersal and colonisation of new areas. The most common form has short wings, extending half way through the abdomen, and is unable to fly. Females are larger than males and have a short ovipositor, curved upwards. Males have modified forewings, which they use to sing (stridulate). They lay their eggs in plant stems. During mating, males provide females with a nuptial gift called 'spermatophylax' which makes a substantial proportion of males' weight.
A male long-winged Roesel's form calling.
Behaviour. Individuals like to bask in sunshine, often quite exposed, especially in cool mornings in the autumn. Males start to sing while basking, choosing grass tussocks as a perch. Tussocks offer individuals protection from predators: at the slightest disturbance, they drop onto the base of the tussock and scramble into it.
Roesel's are very well camouflaged in their environment and can be tricky to spot. Try to find a calling male Roesel's male from a perch on tall grass. (tip: the Roesel's is at the bottom third of the centre of the image. There is also a Long-winged Conehead on the centre top third.
Habitat. Thought to be associated to salt-marshes and dunes, the species has been shown to be little limited by habitat and can now be found along road and path verges, grassy fields, brownfield sites, and any habitat that includes rank grasses.
Field craft. How do you find Roesel's bush-crickets? The first nymphs appear in mid to late May. Nymphs can be found by sweeping grass with a net. Adults appear at the end of July, and are best searched for. Male's stridulation is within the human audible range, but it is a high-pitcher whirr, that is easier to hear for children. Many middle age people are unable to hear them at all, so using a bat detector set to 20 KHz makes it easier to detect them. The call is a continuous whirr that has been compared to an electric buzz. Once you are close enough so that the sound is quite loud, you can try and pinpoint where the calling male is perched by swaying the bat detector left and right and up and down. I find binoculars very useful, to scan suitable perches from a couple of meters with minimal disturbance to spot individuals. With a bit of luck, once located, using slow movements, they can be approached to close range. Males tend to call during the day in sunny weather, unless they are already warm, then they will carry on calling. Searching at suitable sunny spots for basking individuals can also be rewarding, especially on bramble leaves next to grass, where they can be more obvious.
A long-winged female Roesel's. I saw it flying and landing on the short grass of a playing field (31st July 2020), this is the only photo I took, as it immediately flew north.
Range expansion. Roesel's have undergone a spectacular range expansion in the last decades, from the south east corner of the UK. The first record in Hull was in summer 2015. They are now widespread in East Yorkshire and the northernmost populations are currently in North Yorkshire (at Robin Hood's Bay), but the colonisation is ongoing, aided by climate change.
No comments:
Post a Comment