Insect Pest Management in Sorghum - Armyworms and Corn Aphid

Armyworms

Northern armyworm - Leucania separate, Common armyworm - Leucania convecta, and Dayfeeding armyworm - Spodoptera exempta.

Armyworm is the caterpillar stage of certain moths and can occur in large numbers especially after good rain following a dry period. During the day armyworms shelter in the throats of plants or in the soil and emerge after sunset to feed. The adults of the common and northern armyworms may be confused. Genitalia dissection by a specialist is required to separate the species. The larval stages likely to be encountered in cereals are all similar in appearance.

Damage

• Young plants may be defoliated or killed by larval feeding.
• Older plants can outgrow damage, but seed yield may be reduced.
• Signs of damage include chewed leaf margins and faecal pellets at the base of young plants or in the throats of older plants.
• The northern and common armyworm feed at night and hide in vegetation during the day.
• Dayfeeding armyworms feed during the day.

Monitoring

• Visually monitor during the early growth stage and again during head emergence and flowering.
• Egg lays are often associated with heavy rainfall so check for larvae several weeks after rainfall events.
• Since armyworms (except dayfeeding armyworm) hide during the day, look under clods of soil, under vegetation and at the base of plants. Also look for dark green-brown faecal pellets.

Thresholds

Treatment decisions should be made on whether the crop can compensate for damage, the prevalence of natural enemies, the value of the crop and the cost of treatment. As a guide, one-two caterpillars/m2 may warrant control.

Control

• Many chemicals will control armyworms. Their effectiveness is often dependent on good penetration into the crop to get contact with the caterpillars.
• Control may be more difficult in high-yielding thick canopy crops, particularly when larvae are resting under leaf litter at the base of plants.
• As larvae are most active at night, spraying in the afternoon or evening may produce the best results.
  

Natural enemies

Armyworm larvae are attacked by a number of parasitoids that may be important in reducing the intensity of outbreaks. When armyworms are in numbers likely to cause damage, parasitoids are unlikely to give timely control.

Predators include green scarab beetles, predatory shield bugs and perhaps common brown earwigs. Viral and fungal diseases are recorded as causing mortality of armyworm.

Corn aphid - Rhophalosiphum maidis

Although other species of aphids may be present on sorghum, the corn aphid is the most common and numerous.

Damage

• Adults and nymphs suck sap and produce honeydew.
• High numbers can cause plants to turn yellow and appear unthrifty.
• High populations on heads produce sticky grain and clog harvesters.
• Water stressed dry-land crops loose yield.

Monitoring

• Estimate the percentage of plants infested and the percentage of leaf area covered by aphids.

Thresholds

• The action level in the vegetative stage is 100% of plants with 80% of the leaf area covered by aphids. On the heads it is 75% of heads with 50% of the head covered by aphids.

Control

• Choosing hybrids with open heads can reduce aphid numbers as these are generally less infested than tight-headed hybrids.
• Chemical control options are cost effective but all insecticides that control aphids impact on natural enemies.
  

Natural enemies

A range of parasitoids and predators will help reduce aphid populations. Predators of aphids include:

• ladybird larvae
• damsel bugs
• bigeyed bugs
• larvae of green lacewings
• larvae of hoverflies.

Wasp parasitoids such as Lysiphlebus testaceipes mummify and kill aphids.

Photographs

Common armyworm larva sourced from :http://www.dpi.qld.gov.au/26_7690.htm
Corn Aphid sourced from :http://www.dpi.qld.gov.au/26_8166.htm