What was it all about?

The white-fringed weevil is a major pest of potatoes in Australia. Grubs live in the soil where they can cause devastating damage to the roots and tubers of crops. Once established in a field, infestations are difficult to eradicate or suppress without long rotations of non-host crops or the use of expensive soil fumigants

Increasingly, potato growers are relying on the application of pre-plant insecticide sprays to insure against white-fringed weevil grub damage. However, the application of such sprays may be unwarranted if densities of white-fringed weevil grubs are too low to cause economic damage.

To make informed decisions on the need to use insecticides, this project, which ran from 2011 to 2013, set out to develop a method of determining the density of white-fringed weevil grubs present in a paddock. A reliable sampling plan exists for mainland potato crops but it has not been tested in Tasmania, which did not have the weevil until the mid 1980s.

A secondary purpose was to find out how white-fringed weevil grubs detect the presence of host-plant roots in the soil. Volatile compounds in plant roots could be used as deterrents or attractants for monitoring or controlling larvae.

Researchers began by making a review of the scientific literature on white-fringed weevil. They sampled paddocks sown to potatoes and pasture sown to legumes to test sampling methods and conducted laboratory experiments to assess the susceptibility of tubers.  

Main findings:

• The sampling plan developed for mainland potato growing regions was suitable for adoption in Tasmania and several workshops for potato industry stakeholders were held to inform growers.
• White-fringed weevil grubs could detect the presence of lucerne roots in the soil but were unable to discriminate lucerne from the roots and tubers of other plants, including unfavourable hosts such as sorghum. The attraction of potato roots and tubers to white-fringed weevil grubs was weak.
• Chemical analysis of compounds emanating from the roots of lucerne and potato plants found volatile compounds, but none appear likely to act as a significant attractant to the grubs.
• There was considerable variation in the composition and amount of volatile compounds emanating from potato roots, both within and between potato varieties, which might account for inconsistent susceptibility of tubers.

 Researchers set out recommendations for further research into a bait that could lure the weevil away from potato roots.