On the Darling Downs there have been a number of reports of common armyworm causing significant defoliation in sorghum, corn and millet. Jimbour grower John Alexander treated a severely defoliated sorghum two weeks ago. Millet in the same area and corn have also been affected. Although patchy, the outbreaks have the potential to cause significant yield loss if the armyworm defoliate before the crop has reached physiological maturity
At this time of year, high numbers of common armyworm can defoliate seedling crops, summer crops that are still green as well as pasture. This is the same species of armyworm that causes damage to winter cereals in spring. The high numbers now may be a carryover from the outbreak in northern NSW last spring.
Common armyworm larvae look similar to Helicoverpa larvae. Correct identification is important because armyworm cannot be controlled with NPV, and helicoverpa have resistance to synthetic pyrethroids (the two most commonly used control options for these species). For identification of common armyworm refer to a previous beat sheet posting http://thebeatsheet.com.au/armyworm/avenge-of-the-caterpillars%e2%80%a6/
Size of the larvae. Eighty percent of feeding is done by medium– large larvae. If the armyworms are longer than 30-35mm, they have completed most of their feeding and the damage is already done. Treating at this stage may be of little benefit. To be effective, populations need to be detected and controlled before large larvae have caused significant defoliation.
Threshold? Specific thresholds have not been established for defoliating armyworm in sorghum or corn in Australia. US data suggests controlling armyworm when crops have more than 50% defoliation.
Timing of control. Small larvae may feed during the day, but large army¬worm feed at night and shelter on the ground under stubble and soil during the day. Applying a treatment later in the day maximizes the likelihood of contacting large, damaging larvae as they climb the plants to feed. Control options can be found on http://services.apvma.gov.au/PubcrisWebClient/welcome.do
Good coverage is required to get contact with the caterpillars, particularly in crops with thick canopies.
If treating pastures beware of the long withholding periods for cattle grazed on this pasture with reference to export slaughter intervals and export grazing intervals. These intervals are not always marked on insecticide labels.
Around 20 species of predator and parasitoids have been recorded attacking armyworm, but they are unlikely to prevent damage in an outbreak situation like those being reported.
The most frequently observed predators are predatory shield bugs, ladybeetles, carabid beetles, lacewings and common brown earwigs.
Parasitoids include tachnid flies and a number of wasp species (e.g. Netelia, Lissopimpla, Campoletis).
Viral and fungal diseases can also cause mortality of armyworm. Such outbreaks are more common at high armyworm densities.
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Article by Kate Charleston and Melina Miles