Rhizophagus grandis Gyllenhal, 1827
Following recent introductions this widespread European and Asian native is now established in the UK, its use as a biocontrol agent of the scolytid Dendroctonus micans (Kugelann, 1794) (the great spruce beetle) is well-established and it has been established successfully following similar recent introductions in Belgium, France, Georgia and Turkey. The more or less specific predation of the bark beetle was first noted in Russia where it has been a long-standing pest of spruce and some other conifers; old and established stands of spruce are rarely affected by the scolytid but as new commercial plantations appeared and spread north and west through Europe the beetle followed with often devastating effects on young trees. D. micans is native to Siberia, it spread into Europe during the 19th century and its range is still increasing, it was first discovered in the UK in 1983 and is now established, though local, in southern and central England and Wales. Notwithstanding its local occurrence adults may become very abundant among plantations of young trees if left to breed over several generations. A population of Rhizophagus was obtained from Belgium and trialled in England during 1983 and 1984 and, following very promising results from several countries, a mass breeding and field introduction programme began in the UK and continues to the present time. Mass releases are now concentrated on the edges of known areas of infection and these are surveyed annually, by 1988 it was found that around 70% of the bark beetle broods were attacked and nowadays Rhizophagus grandis is established in the vast majority of woods hosting the pest and is thought to have reached a level of predation sufficient to control the keep the pest at low numbers and severely limit the damage it causes. Natural dispersal of Rhizophagus is of the order of hundreds of metres but they can detect and find prey over many kilometres, they are attracted to chemical signals (oxygenated monoterpenes) emitted from D. micans infestations and will fly over long distances to find them, furthermore adult Rhizophagus can survive for long periods under bark or in the soil and so it is very effective at exploiting its host. Adult Rhizophagus display some degree of parental care for their larvae; when they enter the bark they will consume any
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bark beetle eggs present but if larvae are also present they will mate and oviposit among them, when the larvae emerge the adults will then bite and wound bark beetle larvae to assist their young larvae in feeding. Young larvae aggregate among the host, boring into and completely consuming them, as they grow they are able to attack and kill the host directly; they rapidly pass through three instars and enter a prepupal stage which will leave the wood, fall to the ground and pupate in an underground chamber. New generation adults will emerge and disperse locally or, if fresh infestations are detected, over longer distances. The commercial value of fast growing conifers means that D. micans is very likely here to stay but good forestry methods e.g. thinning stands by removing the worst affected trees, and the fact that Rhizophagus can persist and breed at very low prey populations, would suggest that the effects will not become too commercially serious.
At 4.5-5.5mm, this is the largest of our Rhizophagus species, and coupled with this large size a defining feature is the second elytral interstice which is strongly widened towards the base and contains rather randomly arranged elongate punctures which continue beyond the middle towards the apex. Among our fauna only R. depressus (Fabricius, 1793) has a similarly (though less strongly) expanded and (though less extensively) punctured second striae, this species is smaller, 2.6-4.0mm, and has a quadrate to elongate pronotum which is isodiametrically microsculptured; in R. grandis the pronotum is slightly transverse with elongate cellular microsculpture. The large size and rather dull, entirely brown colouration should make grandis obvious even in the field.