Bryobia mite
Crop stage
Seedling.
Description
The adults mites are slightly smaller than a pin head with a dark grey body and pale red/orange legs. They are easily confused with red-legged earth mite and are difficult to separate without the use of a hand lens. Redlegged earth mites are not usually present in early autumn as they have a cold temperature requirement before hatching. The front pair of legs on bryobia mite are very long and held out in front of the body like a pair of feelers. The body of the mites is rounded and plump. However, if starved it's body shape changes to become flat on the top and rounded underneath with a flange around the sides.
Life cycle
Adult bryobia are active in late spring, summer and autumn. Eggs are present over winter, and hatch as conditions dry and warm up in spring and early summer. Winter eggs are usually laid in batches, while eggs over the dry period are laid singly on backs of leaves of host plants. Nymphs newly hatched have six legs and are bright red, but turn dark/grey in a few days. They moult to an eight legged nymph, then again to become a third stage nymph, before finally moulting to the adult stage. A month from eggs hatching to young adult is usual. There are several generations per year.
Damage
Bryobia mites have caused severe damage, when in high numbers, to emerging canola and lupin crops in autumn. Mites feed on the tops of leaves by stabbing into the surface cells with their sharp mouth parts, and sucking out sap. Whitish grey spots result, giving leaves a stippled wilted look. High risk situations include summer rains followed by warm mild autumns, giving bryobia mites the best conditions for survival and increase. They don't tolerate cold wet weather but can persist into June following warm autumn conditions. Crops planted into paddocks with a history of summer/early autumn weeds and warm dry conditions after crop emergence are most at risk. Reports of bryobia damage have increased since 1995, before which they were considered a minor and sporadic pest in some southern districts. Bryobia were reported as being a serious pest in central and some northern cropping zones during the autumn 1998 and 1999. The use of minimum tillage, earlier sowing times and tolerance to some insecticides have led to the increased importance of this pest.
Control
Early control of summer weeds in paddocks that are to be cropped will prevent the build up of mite populations. Weeds present in paddocks prior to cropping should be checked to determine the numbers of bryobia mites present. If they are found in large numbers then the incorporation of insecticide with herbicide immediately prior to sowing is a more effective control strategy than spraying when the crop is emerging and has very little cover of green material. Omethoate is registered for control of bryobia mite in pastures and some crops. Rates of insecticides commonly used to control redlegged earth mite and lucerne flea are not effective against bryobia mites.