A row of helicoverpa larvae, all different colours.
Major pests of sorghum
- Helicoverpa armigera
- Sorghum midge
Minor pests
- Soil insects
- Armyworms
- Corn aphids
- Rutherglen bugs
- Sorghum head caterpillars
Sorghum is susceptible to insect pests from emergence to late grain fill. Early sorghum pests include armyworms and soil insects. These pests are normally present in a grain sorghum crop in low numbers where their damage can be tolerated. However, seasonal conditions can sometimes stimulate the build up of a large population of one or more of these and they can cause significant damage.
Seed dressings to combat soil insects are now commonly available and may be the most effective control as well as the least disruptive to natural enemies.
Pests, such as aphids and rutherglen bugs, can be deterred by choosing open-headed type sorghum hybrids. These insects have a preference for compact or closed panicle types on which they are hard to control due to the difficulty of achieving spray penetration.
It is now possible to adopt an integrated pest management (IPM) strategy to control helicoverpa and midge on sorghum through the use of the Nucleopolyhedrovirus (NPV) that is selective for helicoverpa, and by planting of midge-tolerant hybrids. Such a strategy eliminates any impact on natural enemies that naturally attack both midge and helicoverpa.
Image showing key differences between Helicoverpa punctigera and armigera
Major pests of sorghum
Helicoverpa armigera
Helicoverpa do little economic damage as foliage feeders. Eggs laid on heads just prior to flowering produce larvae that cause economic damage. Sorghum is most at risk from helicoverpa from head emergence to early grain fill. Larvae of helicoverpa can be confused with sorghum head caterpillar and yellow peach moth.
Damage
- Small larvae (less than 10 mm long) feed on the pollen sacs in the flower head where they cause little damage.
- Larger helicoverpa larvae feed on developing seed.
- Each larva destroys about 2.4 gm of grain in its lifetime.
Monitoring
- Monitor weekly from head emergence through to early grain.
- Determine helicoverpa numbers by rotating five head stalks into a bucket.
- Count larvae in the bucket and work out an average per head.
- Determine larval sizes (important for control decisions).
- Repeat sampling at a minimum of six sites throughout the paddock.
Thresholds
- Action levels vary with factors such as grain prices and cost of control. The critical number of insects per head is the number of insects that can eat more dollars worth of grain than the cost of spraying. Control is recommended when insect numbers are at or above the critical threshold level.
- The critical helicoverpa larvae number can be calculated with the following formula:
C (cost of control) x R (row spacing) divided by V (value of crop in $) x N (no. of heads/m row) x 2.4
As a guide, one or two larvae per head may result in economic damage.
Helicoverpa punctigera and H. amigera adult moths
Control
The best product to use for control of helicoverpa is a naturally occurring virus that targets helicoverpa. There are a number of commercially formulated NPV products on the market that are cost-effective for the control of helicoverpa.
- NPV is most effective against small larvae (less than 7 mm). Avoid targeting larvae over 13 mm.
- Crop coverage is critical as this is an ingestion product.
- Spray NPV when about 50% of the sorghum panicles have completed flowering (see label for instructions).
- Spray when larvae are actively feeding between 25oC and 35oC.
- NPV is harmless to wildlife and natural enemies of helicoverpa.
- Use of additives with NPV will improve performance.
There are other chemical means of controlling helicoverpa; however, these chemicals severely disrupt natural enemies.
For current chemical control options see Pest Genie or APVMA (Australian Pesticides and Veterinary Medicines Authority).
Natural enemies
The combined action of natural enemies can have a significant impact on potentially damaging helicoverpa populations. It is therefore desirable to conserve as many of the natural enemies as possible. Natural enemies of helicoverpa include predators of eggs, larvae and pupae, parasites of eggs and larvae and caterpillar diseases.
Predatory bugs and beetles that attack helicoverpa eggs and larvae include:
-
Spined Predatory Shield Bug
spined predatory bugs
- glossy shield bugs
- damsel bugs
- bigeyed bugs
- apple dimpling bugs
- assassin bugs
- red and blue beetles
- predatory ladybird beetles
- ants
- spiders
- lacewings
Parasites include:
- Trichogramma spp. - tiny egg parasite wasps
- Microplitis and Netelia (wasps) - caterpillar parasites
- Species of tachinid flies - caterpillar parasites
With the exception of Trichogramma and Microplitis, most parasites do not kill helicoverpa until they reach the pupal stage. Predatory earwigs and wireworm larvae are significant predators of helicoverpa pupae.
Photographs:
Sourced from article at: http://www.dpi.qld.gov.au/26_16277.htm
