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.

The ice crawlers

I have recently been learning about the Grylloblattodea, one of the smallest and the most threatened insect orders on earth. These fascinating insects are given the fantastic colloquial names of ice or rock  crawlers, and are found in cold mountainous areas of the Western USA and Asia.

In looking into the species in more detail, I found an excellent blog by Piotr Naskrecki, which is highly recommended reading here.

Grylloblatta campodeiformis - Piotr Nascrecki
Grylloblattodea get their name from the combination of features similar to Orthopteran (Grylloid) insects - external sword shaped ovipositor in the female, tentorium 'skull' structure - and Blattodean (cockroach) insects - five-segmented tarsi, multiarticulate cerci, assymetric male genitalia. There are number of features that can be used to distinguish the Grylloblattodea:

  • long and slender antennae
  • reduced (or absent) compound eyes, with no ocelli (simple eyes) present
  • apterousness (being wingless)
  • metathoracic spine (unique in the hexapods)
  • eversible sac on first abdominal sternum
  • long and slender legs, adapted for walking
  • long, multiarticulate cerci (at the rear of the animal)
  • long, sword-shaped female ovipositor (almost as long as cerci)
  • assymetric male genitalia
  • simple, chewing, foreward-facing mouthparts
Typical habitat in Western USA - Piotr Naskrecki

These are hemimetablous insects, meaing that they have an incomplete metamorphosis and their life cycle is a quite simple one of egg to nymph to adult. They are long-lived, with a lifespan of around five years, though this is thought to range to up to ten years. As typical of long-lived insects, the nymphal stage takes two years to mature.

The Grylloblattodea are cryophilic, meaning that they are adpated to cold environments. Their optimal temperature range is just 1-4 degrees C, though they can tolerate between -8 and 10 degrees C (and species in Asia can apparently tolerate even higher temperatures of 9-15 degrees C).

They are found at high elevations on glaciers, where they are mainly nocturnal, or at lower elevations in caves with permanent ice. Given the extreme environments in which they are found, their diet is quite broad, they probably cannot be too choosy. Species can be omnivorous scavengers of dead insects as well as plants, fungi and detritus.

The order Grylloblattodea contains one family - the Grylloblattidae. This contains  around 26 species (probably more) in five genera.

  • Grylloblatta - 11 species found in the USA
  • Grylloblattella - 2 species, found in China
  • Grylloblattina - 1 species, found in Siberia
  • Namkungia - 1 species, found in Korea
  • Galloisiana - 12 species, found in Japan and China
Galloisiana nipponensis, a blind species from Japan - Piotr Naskrecki

The populations of species in Grylloblatta have been reaersched in the Western USA. They are found in the Cascade mountain range, streching from British Columbia to Oregon, and the Sierra Nevada in California. In these areas they have been found in high altitude ice caes (300m-1000m) and north-facing talus and glacial margins (1500-3000m). Migration is limited, and they certainly cannot move at lower elevations, mneaning that populations are isolated.

Because of their limited ability to move, narrow environmental niche and small distribution, the insects are very vulnerable. The impacts of habitat loss and global warming are seen as strong threats to many species. SPecies in the order are placed in the IUCN red list in the near threatened, vulnerable, endangered and critically threatened categories.

Whilst the Grylloblattodea have successfully occupied particular niches through becoming specialists, habitat changes ate global warming pose a significant threat. This is an example of how specilaist species can be more extinction-prone that generalists.