Grubs in winter cereals are not unusual at this time of year, and already there have been reports of high numbers (up to 20/m2) in Central Queensland wheat (Figure 1). More grubs can be expected in southern districts as the season warms up.
The two most likely larvae (grubs) found in winter cereals are the corn earworm, Helicoverpa armigera, and the common armyworm, Leucania convecta. See previous blog postings for more information on these pests.
http://thebeatsheet-ipmnews.blogspot.com/2007/10/can-you-confidently-identify-armyworm.html
Figure 1. Large corn earworm larva on a wheat head. (Photo: R. Lloyd)
All Helicoverpa larvae found feeding in wheat, barley or triticale crops will be corn earworm. The native budworm, H. punctigera, is not normally found on monocots (grasses). This is important to know, because the corn earworm has developed resistance to pyrethroids, and unless the larvae are small, a pyrethroid spray is unlikely to control them.
If large larvae are present, identification becomes a somewhat academic issue. However, large H. armigera larvae can be identified by the white hairs behind the head (Figure 2). In contrast, the hairs on large H. punctigera larvae are black. These compare with armyworm larvae which have three pale stripes just behind the head, and smooth skin, without any hairs or bumps.
If corn earworm infestations are detected early and larvae are small, preferably less than 7 mm in length, Helicoverpa nucleopolyhedrovirus (NPV) sold as Vivus Max could be considered as it will not harm beneficials (predators and parasites) in the crop. Some caution is needed as NPV will not kill corn earworm larvae greater than 13 mm in length, and will have no effect on armyworms.
Invariably when larvae are found on cereal crops, they are medium or large (>13 mm in length) and a more robust option is needed to control them. Both corn earworm and common armyworm are usually present in winter cereals, and control measures will be influenced by the relative abundance of each.
Follow the link below for more information related to thresholds and control options.
http://thebeatsheet-ipmnews.blogspot.com/2007/10/are-corn-earworm-problem-in-winter.html