Insect Pest Management in Sorghum - Rutherglen bug and Sorghum head caterpillar

Rutherglen bug (RGB) - Nysius vinitor

Damage

• Seed heads may be damaged by feeding, resulting in reduced yield and quality.
• Affected seed is red and spotty and hollowed out internally.
• Grain will be small and shrivelled and does not continue to fill beyond the point it was damaged.
• Damaged seed is subject to fungal and bacterial attack.

Monitoring

• Monitoring for RGB can be done at the same time as for helicoverpa.
• Monitoring methods consist of beating sorghum heads into a bucket and counting the insect pests.
• Because RGB can start to build up at any stage of crop maturity and impact on seed set during flowering and early seed development, monitoring should start at early flowering.
• Distribution is typically patchy across the field which means that the more samples taken, the greater the level of confidence in the overall infestation.

Thresholds

• Preliminary research on RGB has shown that adult RGB will reduce seed set by around 20% at densities of 50-100 bugs/head.
• Research has indicated that there is no impact on yield or quality once the crop is in the ‘hard dough’ stage.
• New thresholds for earlier stages are proposed with a threshold during flowering and milky dough stage of greater than 30 bugs/head and a threshold of greater than 80 bugs/head during the soft dough stage.
• More research into the density-damage relationship is required to come up with robust thresholds for RGB in sorghum.

Control

• Control of RGB should occur in the early seed development stages.
• There are no soft chemical options for the control of RGB. Repeated influxes of migrating adults can make repeat applications necessary.

Natural enemies

• Egg parasitoids are the most commonly recorded natural enemy of RGB. Their potential contribution to population control will be limited in seasons when there are large influxes of adults. Predation has rarely been recorded, but spiders may play a role.

Sorghum head caterpillar - Cryptoblabes adoceta

This pest in more prevalent in tropical and sub-coastal areas of north-eastern Australia. It may be confused with helicoverpa and yellow peach moth.

Damage

• Larvae feed on developing seed, each larva destroys anout 0.5 gm of grain.
• They web clusters of seed together.

Monitoring

• Presence of caterpillars is indicated by webbing of seed clusters, webbing of whole heads and presence of small white/pink excreta.
• Count larvae on heads by dislodging them from sorghum heads into a bucket.

Thresholds

• Action levels vary with commodity prices and the cost of insecticides.
• The threshold level can be calculated using the factor of 0.5 g of grain destroyed by the larva.
• The threshold is usually in the range of 5-10 larvae per head.

Control

• Open-headed sorghum varieties are generally less infested that tight-headed varieties and also allow better penetration of insecticides.
• There are currently no registered products against sorghum head caterpillar but chemicals targeting helicoverpa are likely to be effective.

Natural enemies

• Parasitic wasps provide some biological control, but are unlikely to control populations above 10 per square meter.
• The avoidance of broad-spectrum pesticides prior to flowering may help conserve natural enemies.

Yellow peach moth - Conogethes punctiferalis

This pest may be confused with helicoverpa and sorghum head caterpillar.

Damage

• Larvae feed on developing seed. Each larva destroys about 1 gm of grain.

Monitoring

• Count larvae during milky dough stage and by dislodging them from heads into a bucket.

Thresholds

• The level varies with factors such as commodity price and cost of insecticide. The level can be calculated using the factor of 1 gm of grain destroyed per larva. The threshold usually works out to be 2-5 larvae per head.

Control

• Open-headed sorghum varieties are generally less infested than tight-headed varieties as they allow better penetration of insecticides.
• Chemical control is cost effective.
  

Natural enemies

• The avoidance of broad-spectrum pesticides prior to flowering may help conserve natural enemies. Parasitic wasps provide some biological control.

Photographs

Rutherglen bug sourced from: http://www.dpi.qld.gov.au/26_5084.htm
Sorghum head caterpillar sourced from: http://www.dpi.qld.gov.au/26_13758.htm
Yellow peach moth sourced from: http://www.dpi.qld.gov.au/26_13760.htm