Mantalodus weevils

Consultant, Geoff Pitson, has received a number of reports from growers concerning weevil damage to canola seedlings around Temora in the South West Slopes of NSW. The weevils have been identified by Senior Technical Officer, Ken Henry (SARDI), as the mandalotus weevil (Mandalotus spp.). Ken has also recently identified mandalotus weevil for Kate McCormick (John Struchbery and Associates) which were causing damage to a barely crop near Murrayville in the Mallee region of Victoria.

Little is known about the biology of this weevil. It is commonly found on rubbly sandy loam soil; hence it is often referred to as the ‘rubble bug’. Geoff says he has also noticed higher numbers in lighter soil types, where large patches of canola crops are completely bare due to weevil feeding. The mandalotus weevil appears similar to Polyphrades laetus, which is a native weevil that caused severe damage to cereal crops in the mid-to-late 1980s on the Eyre Peninsula in SA.

Mandalotus weevil was first recognised as a significant pest in 2003, where it caused severe damage to approximately 500ha of canola crops in the Mallee region. Ken says the distribution of the mandalotus weevil has likely widened in southern Australia in the past few years. It appears that the population increase of this weevil is a direct result of minimum tillage. The amount of trash in paddocks is also thought to create a more favourable habitat.

The adults are mostly active at night; during the day they can be found hiding under dirt and trash. In some cases numbers can exceed 500/m^². Ken says mandalotus weevil may superficially resemble sitona weevil, but it is more rounded in appearance, 3-4mm long, and resembles a clod of dirt.

Usually, feeding damaged is first noticed on the leaves of emerging crop plants. In severe cases the seedlings are often ‘ring barked’ just above ground level. This causes the plants to fall to the ground where the weevils continue feeding on the leaves. Adults are difficult to control with chemicals. Over the past few years a number of pesticides have been tried; some at considerably high rates. In more recent times, growers in SA have had success with bifenthrin.