Lasius brunneus

Workers of this species look superficially like the common black garden ant Lasius niger but can be distinguished by, amongst other characters, their lighter brownish-red head and alitrunk contrasting with a dark/black gaster

Can be confused with Lasius niger and L. alienus.

• Frontal triangle well defined
• Scale of petiole notched
• Gaster darker than head, thorax and petiole

Unless identified by a recognised expert, a photo is required and the specimen should be examined with a microscope. In the comments box, state the key or ID method used and describe the size and identifying characters.

An arboreal species. Found almost exclusively nesting in living old oak trees or occasionally fallen timber. Often the only outward signs of a tree being inhabited are small piles of frass resulting from excavations within the tree. Tunnels created by the ants can be found by removing the bark but the nest itself is difficult to locate and can be deep within the roots, trunk or boughs. 

Mating flights occur in June or early July and are reported to take place early in the day. Workers may be seen any time between early spring and late autumn.

Workers are fugitive and rarely seen away from their host tree or even on its surface. Most activity occurs in bark crevices or tunnels under the bark where the ants tend large tree aphids such as those of the genus Stomaphis. The excreta of these Homoptera forms the majority of their diet, although they may also take other small insects found on or under the bark such as psocids and beetle larvae.

Nationally Notable 'A' species – Nationally scarce and generally restricted to Central and southern English counties.

Rare or rarely recorded in Leicestershire and Rutland. 

https://www.bwars.com/ant/formicidae/formicinae/lasius-brunneus