Diamondback moths

District agronomist, Ian Menz (NSW DPI), has reported observing diamondback moth caterpillars (Plutella xylostella) in young canola crops around Condobolin, in the Central West Slopes and Plains district of New South Wales. Although the current rate of development of diamondback moths (DBM) will be relatively slow due to the cool weather conditions, it will be important to monitor for signs of increasing damage to crops as the average temperatures begin to rise.

DBM are distributed across Australia, but are most common in southern states. Although they can be found throughout the year, they are most abundant during spring and summer. DBM can have significant effects on canola yields and current control measures are variable. DBM can evolve insecticide resistance readily and some populations are difficult to control with insecticides. Furthermore, no single insecticide application will completely eliminate the population. In years when populations are large, a two-spray strategy is recommended. 

DBM caterpillars 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.

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 DBM that incorporates factors such as crop type, age, market destination and parasitism levels. To learn more about diamondback moths, including identification and control, visit the pestIQ website.

Winter canola considerations for DBM

As new winter canola varieties gain interest among growers (particularly in the HRZ of southern Australia), consideration should be given to potential insect issues that could arise. Winter canola varieties can be sown in October/November and will not flower until the following spring. DBM could potentially reduce both establishment and feed production of these crops, as their numbers can reach very high levels under the right conditions. The rate of development from eggs to moths is largely dependent on temperature. At 12°C the lifecycle of DBM takes more than 100 days, whereas at 28°C the lifecycle takes only 14 days.

Growers who are considering using winter canola varieties should consider developing a DBM control strategy to respond to any pressure they may pose. Philip Jobling (cesar) says recent trial work in south-west Victoria has demonstrated winter canola varieties can tolerate quite strong grazing pressures. Thus, grazing by itself could prove to be an effective strategy against DBM. If the crop is heavily grazed, the reduced feed availability will naturally reduce DBM caterpillar numbers.