Saturday, 5 November 2016

Can you dig it?

The grasshoppers (Orthoptera) is a large order of insects and a familiar one. The Orthoptera contains two sub-orders, the Ensifera (crickets and katydids) and the Caelifera (grasshoppers and locusts). These suborders can be distinguished by the length of the antennae, mechanisms of sound production and the ovipositor shape in females. Within the Ensifera are the Grllotalpidae, also known as mole crickets. These fascinating insects are the subject of this blog.

Mole crickets are large insects, being 4-5cm long and 2-5g in weight, and are dark brown with velvety hairs. Their subterranean lifestyle is betrayed by their spade-like forelegs, modified for digging. Adults can fly, with the exception of the short-winged mole cricket Scapteriscus abbreviatus, though the wings of males and females are different.

Mole crickets have a hemimetabolous lifestyle - incomplete metamorphosis with no pupal stage. As an example, female Gryllotalpa gryllotalpa lay 100-300 eggs in a chamber curing the summer. They exhibit maternal care as the nymphs move through 6-8 stages, with it taking more than a year for them to complete development. The adults are nocturnal and hibernate over the winter.

The burrow system of mole crickets in complex. The network has a horn entrance (A), where the male can also amplify his 'voice' as he sings. There is the nest chamber (5), grazing areas linked by horizontal tunnels 6), hideaways at the end of vertical tunnels (7) and at least another horn entrance, which can act as an escape route (B).

The Gryllotalpidae are distributed across the USA, South America, Europe, North Africa, Asia and New Zealand. The European mole cricket Gryllotalpa gryllotalpa comprises 13 sibling species/chromosaomal races through Europe and North Africa.

Whilst native species are not generally seen large-scale pests (though they formally were in the UK), some introduced species are serious pests, mainly in the south eastern USA.

  • Gryllotalpa gryllotalpa - European mole cricket (not a major pest)
  • Scapteriscus abbreviatus - short-winged mole cricket
  • Scapteriscus borellii - southern mole cricket
  • Scapteriscus vicinus - tawny mole cricket (major pest)  
Mole crickets are seen as pests because of the damage they cause through their tunnelling behaviour, whilst tawny and short-winged mole crickets also feed on crops. Control of pest species uses soil applied insecticides and a range of biological controls - nematodes, fungal pathogens, Tachinid flies, Sphecid wasps and Bombardier beetles (which feed on the eggs).

In the UK, the European mole cricket Gryllotalpa gryllotalpa is a conservation priority. They are on the Red List, and protected under Schedule 5 of the Wildlife & Countryside Act 1981. Their population declined due to habitat degradation, with the last native sighting in Wareham, Dorset in 1981. Species recovery programmes were run by English Nature, though these were not successful and have now finished.

In 2014 a small population was found in the New Forest , Hampshire. This is suspected to be from imported insects, though it is not clear which species is involved and taxonomic study is needed to establish the species and the origin. 

Potential future conservation action would involve a captive breeding programme, from suitable foreign populations, with a relaease programme and raising public awareness.

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