The brown and black beetle (Bromius obscurus) is an occasional pest on grape vines causing feeding damage to the parts of the plant that are above ground and to the roots.
The five to six millimetre large leaf beetle (family: Chrysomelidae) is brown and black in colour and has no metallic shine. Its wing covers are wider than its scutum, outlining clear shoulders, which makes these beetles easy to recognise. Their white larvae live near the plant’s roots and have a brown head capsule, three pairs of legs and are up to 5 mm in size.
The beetles live on grape vines from the beginning of May to August. When they get disturbed, they let themselves fall to the ground. Following their maturation feeding, which lasts about two weeks, the female beetles deposit their eggs in small heaps of 15-20 eggs into the crevices of the old bark or on roots that are close to the surface. The young larvae hatch after 10 to 12 days, feeding on the vine’s roots. They begin to destroy the fine roots first and then move on to feeding on older roots, producing spiralling holes. The root-feeding continues until autumn or even through mild winters. The larva then pupates in the soil, close to the surface by the end of winter. The beetle hatches following a 3 week pupa stadium.
The beetles cause feeding damage on the parts of the plants that are above ground, such as leaves, shoots and grapes. These traces are about one millimetre wide and 10 mm long and are reminiscent of writing. Grapes carrying damage might be found from the high summer season onwards. The symptoms are similar to powdery mildew. Secondary fungi infections may cause additional damage to grapes that have already been gnawed at.
The larvae feed on the vine’s roots, causing problems in the plant’s water and nutrient transportation system, which can result damage from stunted growth to the death of the vine.
Prevalence and Host Plants
Bromius obscurus is widely found in Europe, Northern Africa, Northern and Central America. Its hosts also include members of the willowherb family (Onagraceae), such as the rosebay willowherb or fireweed (Epilobium angustifolium), in addition to Vitis sp.
The beetle must be subject to a morphological examination for exact classification.
Prevention and Control
Its natural predators includes insect eating vertebrae, as well as ants, which eat the beetle’s eggs. Plant protection measures are usually not required. In most cases, these beetles are also affected by plant protection measures taken against other pests.
(See list of authorised plant protection products in Austria)