Description
Adults are small (1 mm) and have a blue-black body with a distinct red spot and eight orange legs. Larvae are very small (0.3 mm), oval and have three pairs of legs. They hatch pink-orange in colour, quickly turning to brown then green. Often mistaken as redlegged earth mites (RLEM), BOM are easily identified by the presence of a red spot on their back and they also feed either alone or in very small groups. In contrast, RLEM have a black body and generally feed in large groups (up to 30).
During cool and wet conditions (April – October) BOM are active and through this time, 2 - 3 generations of BOM can occur, each lasting eight to ten weeks. Throughout the hot summer months, diapause eggs, resistant to heat and desiccation, are produced. When cooler conditions and rainfall occur in early autumn, diapause eggs hatch releasing large numbers of mites which attack young emerging crops and pastures. BOM reproduce asexually which means offspring are all female clones. Eggs are laid either singly or in clusters on leaves, stems and roots of plants or the soil surface.
BOM rip the leaf surface tissue and feed off the sap. Damage results in cell destruction and affects photosynthesis, causing leaf silvering and other discoloration. Leaves often become shrivelled under severe infestations. BOM are most damaging to emerging crops and if populations are high, BOM can cause seedling death. All symptoms are exacerbated when plants are under stress.
Control
There are number of known predators which attack BOM including; other mites, small beetles, ants, French anystis mite, snout mites and the fungal pathogen Neozygites acaracida (common in wet winters). Where possible aim to preserve these populations by limiting the use of broad-spectrum insecticides.
In paddocks known to have high infestations of BOM, rotate with non-host crops to reduce insect levels. Cultivation and hot stubble burns will significantly decrease the number of over-summering eggs and therefore insect pressure. Control host plants, such as broadleaf weeds which serve as a food source for young stages of BOM. Grazing is another option which reduces BOM food sources therefore decreasing populations the following year.
Currently, chemical control is the most common method used to control BOM. There are however limitations as insecticides are only effective on the active growth stages of BOM, not the eggs. Insecticide tolerance levels differs between the three species; P. falcatus has higher tolerance to insecticides and aren’t easily controlled, whilst the other two are. This can complicate the control of BOM depending which species is dominant, which requires proper identification.
BOM are most active in the cooler mornings, therefore crops/pastures should be monitored at the start of the day. Early in the season bare earth insecticide applications can provide residual control of BOM, protecting the vulnerable seedling stage from attack. Systemic seed dressings such as Gaucho® provide protection during the seedling stage and can be applied to the seed. Gaucho is registered for the control of BOM in canola and lupins.
