It’s not surprising that Bryobia mites are already damaging emerging canola – they are an early season pest and potentially problematic due to their natural tolerance to some insecticides
Where have they been reported?
Initial infestations of Bryobia mites have been recording damaging canola in the Lockhart (Riverina) and Beckom (Central West Slopes & Plains) regions of NSW. They have also been observed in several emerging canola crops near Manangatang (Victorian Mallee).
Bryobia mite habits and behaviour
Bryobia mites (Bryobia spp.) are early-season pests and tend to cause most damage in autumn when they attack newly establishing pastures and emerging crops, greatly reducing seedling survival and retarding development. They prefer broadleaf plants such as canola, lupins, vetch, lucerne and clover, but they will also attack cereals. In pastures, Bryobia mites tend to prefer clovers and medics to the grasses.
Bryobia mites are active during the warmer parts of the day and may be difficult to detect during the early morning or in wet conditions. Unlike other mite species that spend a lot of time on the soil surface, Bryobia mites are mostly found on the lower and upper leaf surfaces of plants, and are a lot less active than other earth mite species. They feed on the upper surfaces of leaves and cotyledons by piercing and sucking leaf material. This feeding causes distinctive trails of whitish-grey spots on leaves, compared to the general silvering caused by other mites. Extensive feeding damage can lead to cotyledons shrivelling. On grasses, Bryobia mite feeding can resemble that of redlegged earth mites.
It is important to distinguish Bryobia mites from other mite species before deciding on control options. Inspect paddocks during the warmer parts of the day. Look for mites and evidence of feeding damage on newly established crops and pastures. Established crops can tolerate moderate infestations of Bryobia mites and it is likely that Bryobia numbers will naturally decline in most areas with the onset of cooler late autumn and winter conditions.
Crops that follow pastures with a high clover content are most at risk. Early control of summer and autumn weeds within and around paddocks, especially broadleaf weeds such as capeweed and clovers, can help prevent mite outbreaks. Some insecticides are registered for Bryobia mites, however, be aware that recommended rates used against other mites might be ineffective against Bryobia mites. Bryobia mites have a natural tolerance to several chemicals. Generally speaking, organophosphate insecticides provide better control against Bryobia mites than synthetic pyrethroids.
It is a possible that some reports of control failures with bifenthrin are associated with particular species of Bryobia, which are numerous in cropping systems. In 2013 there were several reports of chemical control failures after alpha-cypermethrin was applied to paddocks. This insecticide should not be used to control Bryobia mites.
There are at least seven species of Bryobia mites (often called ‘clover mites’) found in broadacre crops in Australia, all very similar in appearance. They are often misidentified as the redlegged earth mite, however, Bryobia mites can be distinguished by their long forelegs, which are about 1.5 times their body length. Bryobia mites are smaller than other commonly occurring pest mites and reach no more than about 0.75 mm in length as adults. They have an oval shaped, dorsally flattened body that is dark grey, pale orange or olive in colour and have eight pale red/orange legs. If seen under a microscope, Bryobia mites have a sparsely distributed set of broad, spade-like hairs, appearing like white flecks.
* Sources of field reports of Bryobia mites
Andrew McMahen – Agronomist, Landmark (Victorian Mallee)
Elisa Strong – Agronomist, Delta Ag (NSW Riverina)
Craig Warren – Agronomist, Landmark (NSW Central West Slopes & Plains)