Diamond back moth warning

Diamondback moth (Plutella xylostella) has recently caused significant problems in many parts of southern Australia. Recent reports from Western Australia indicate high numbers of diamondback moth larvae and extensive feeding damage to canola crops. Entomologist, Ken Henry (SARDI), says many regions in South Australia have also experienced problems, with a substantial amount of spraying and control difficulties. Crops in Victoria and New South Wales should be closely monitored for infestations.

Diamondback moth can have significant effects on canola yield and current control measures are variable. Diamond back moth grubs are pale yellowish green and tapered at each end of their body, which grows to about 12 mm long. They often wriggle rapidly when disturbed. The moths are about 10 mm long and are grey-brown in colour. They have a characteristic whitish strip of uneven width down the back, which resembles diamond patterns. For images of diamondback moth, click here.

Diamondback moth numbers can increase or decrease at any stage during the growing season. Trials have shown that population sizes depend on both the area and localised conditions. Crop damage depends on the level of infestation and the amount of time the pests spend in the crop. Grubs eat irregular holes in leaves and at high densities can eat all the leaf material, parts of the stem and some flowers.  Later in the season, they will graze on the canola pod walls. This causes external scarring damage but minimal or no yield loss, unless at very high populations.

Diamondback moth populations are notorious for their rapid resistance to synthetic pyrethroid insecticides. Further, insecticides are not effective in killing adults, eggs and 1st instar larvae (which occur (mine) inside the leaf and are not visible). Sampling crops at several locations is important to determine whether numbers are increasing or decreasing before any spray decision is made. A monitoring guide has been developed for diamondback moth that incorporates factors such as crop type, age, market destination and parasitism levels. For further information on diamondback moth, click here.