We had intended to add Viburnum tinus (Laurustinus) to our collection of Boring Plants, until we discovered that it had an intriguing secret. More about that later.
Until recently, the genus Viburnum was included in the honeysuckle family (Caprifoliaceae); now molecular biologists tell us that it is more appropriately placed in the Adoxaceae, a small family comprising about 200 species, mostly (~150)Viburnum species.
Viburnums are mostly shrubs and trees, many originating in China. Viburnum tinus grows in oak forests and maquis (shrubland) surrounding the Mediterranean: North Africa, Israel, Lebanon, Turkey, Spain, Portugal, France, Italy, Greece and Albania.
There are two subspecies: Viburnum tinus subspecies rigidum from the Canary Islands; V. tinus subsp. subcordatum from the Azores. Worldwide it is a popular horticultural plant, widely planted in suburban parks and gardens and much favoured for hedging.
And now for the intriguing secret. On the undersurface of the leaves, barely visible, are tufts of tiny hairs growing from minute pockets in some of the angles between the main vein (mid rib) and the lateral veins. These tiny depressions are known as domatia (singular – domatium, from the Latin domus, a house) which serve as refuges for tiny arthropods which have a mutualistic relationship with the plant. Domatia occur in more than 90 plant families.
Two Spotted Mites, sometimes known as Red Spider Mites (Tetranychus urticae) are minute arthropods that destroy plant leaves by sucking cell contents, leaving damaged tissue and a much reduced photosynthetic capacity.
However, the predatory mites (Metaseiulus occidentalis) that reside in domatia of Viburnum tinus, eat the eggs of this serious pest. Experimental work in Melbourne suburban gardens found that leaves of V. tinus with domatia had up to 36 times more predatory mites than those without domatia, and high numbers of predatory mites were accompanied by a substantial decrease in leaf damage.
So, far from being a boring plant, perhaps Viburnum tinus may be worthy of a place in the garden for its role in the biological control of Two Spotted Mite.
ZipcodeZoo: http://zipcodezoo.com/index.php/Viburnum_tinus Paul Grostal, Dennis J. O'Dowd 1994 Plants, mites and mutualism: leaf domatia and the abundance and reproduction of mites on Viburnum tinus (Caprifoliaceae) Oecologia (97:308-315)
Alison Downing, Brian Atwell, Kevin Downing Department of Biological Sciences, July 2015