Fall armyworm in Western Australia

Management in sorghum

Damage

Sorghum is among the more favoured host plants for fall armyworm. Larvae can reduce the yield of forage and grain by affecting plant establishment, damaging leaves and attacking sorghum heads.

Plant establishment is most likely to be affected if more advanced stage larvae move into a newly planted crop from adjacent areas of crops or weeds. In this situation, damage to young plants is similar to that of cutworms, where the plant is either cut off at ground level or killed when larvae burrow into the growing tip. Sorghum plants will compensate by producing other tillers but this slows the crop growth and may reduce yield.

The usual source of infestation is from moths laying egg masses on young plants. Newly emerged larvae feed on leaves and remove the green tissue only, resulting in a ‘windowing’ effect with the leaf epidermis left behind (see the photo below illustrating leaf damage on sorghum where the white areas on leaves are the result of feeding by early stage larvae). Fall armyworm larvae typically infest the whorl (throat) of sorghum plants, resulting in emerging leaves having large, irregular-shaped holes.

Leaf damage caused by fall armyworm larvae in young sorghum, grown in Kununurra, Western Australia. Notice both the windowing and shot hole damage. 

Other armyworm species and Helicoverpa armigera will cause the same type of damage in sorghum, so it is important to open the whorl to identify the larvae. Leaf damage to younger sorghum plants does not usually impact on plant growth and yield, unless pest pressure is high and defoliation severe. Heavy infestations result in a ragged appearance to plants. Fall armyworm larvae are cannibalistic and usually only 1 or 2 large larvae will occur within each whorl as plants develop.

As panicles emerge, fall armyworm can infest the heads and damage the developing grain. Like H. armigera, small fall armyworm larvae feed on the pollen and larger larvae feed on the developing grain. Managing infestations of fall armyworm before heads emerge will reduce the risk of this type of damage. Check for the infestation level in crops and the age of larvae prior to the emergence of sorghum heads. 

Monitoring

The spread of fall armyworm in Western Australia is being monitored by the use of pheromone traps. This surveillance will clarify the distribution of fall armyworm across the agricultural regions. In addition, experience with the insect should clarify the time of year crops are most likely to be infested.

If fall armyworm becomes an important/consistent pest in a particular region, a pheromone trap can be deployed to check on the presence of moths as a warning of potential pest situations.

It is suggested that growers and agronomists start scouting as soon as sorghum seedlings emerge. Weekly checks thereafter are recommended. Check 20 consecutive plants (selecting the first randomly) from at least five locations or 10 plants at 10 locations across the crop.

Regularly check and record: 

• Percentage of plants infested with fall armyworm larvae, and the 
• Characteristic damage:
  • windowing indicates the start of an infestation
  • gross leaf damage the presence of older larvae, and
• Accumulation of frass/excreta around the whorl.

Confirm the presence of fall armyworm on the damaged plant. The damage could be caused by another species. 

A few days before panicle emergence, look for larvae in the whorls, which will be pushed out when the panicles emerge. These larvae will attack the developing grain. The infestation of fall armyworm in sorghum heads may be assessed by tapping heads over a container. On very rare occasions, with heavy infestations, when grain is hardening, fall armyworm may continue attacking foliage. Extensive leaf loss may affect yield by reducing the plants’ ability to ripen the grain.

When to take action

Economic thresholds have not yet been developed for sorghum grown in Australia.  The action thresholds recommended overseas for applying control measures for fall armyworm vary with the growth stage:

• At the seedling stage, if more than 10% of plants are cut.
• At the early whorl stage (knee high), if more than 30% of plants are infested.
• At the late whorl stage (shoulder high), if more than 40% of plants are damaged and live larvae are present, and
• At the panicle emergence stage, if more than 5-10% of plants are infested.

Management

An integrated pest management (IPM) approach should be considered for protecting crops from infestations of fall armyworm. 

Fall armyworm has entered an existing suite of pests and associated natural enemies, systems of production and pest management programs.  Careful consideration needs to be given to any actions taken for fall armyworm control that may have adverse effects on management options already in place for other pests of sorghum, especially where natural control agents are used. 

Natural enemies of other insects in the same group as fall armyworms already present in sorghum may also attack fall armyworm. There is already evidence of wasps parasitising fall armyworms in sorghum and maize crops in Kununurra. However, the effect of natural enemies with fall armyworm will become more clear as our experience with the pest accumulates. 

An important management practice is to maintain farm biosecurity measures and implement good farm hygiene, and remove alternative hosts such as weeds and volunteer crop plants, especially during periods, and in places, where fall armyworm would not easily be able to survive year-round. 

Other cultural practices, such as trap cropping, may reduce fall armyworm numbers. Understanding the value of the various cultural practices for fall armyworm management that have been tested overseas will requires further study under Australian conditions.

In the Americas, Spodoptera frugiperda nuclear polyhedrosis virus (SfNPV) has been successful in significantly reducing the damage caused to maize crops in the Americas, but this product is not available in Australia. The NPV that controls Helicoverpa larvae is not effective against fall armyworm.  This approach to management could also be used in sorghum if the product became available in Australia.

Insecticides that contain the bacterium Bacillus thuringiensis (Bt) would most likely kill larvae of fall armyworm, especially if applied when larvae are small. However, considering they need to be ingested and the concealed areas where larvae feed, foliar applications of Bt products are not recommended for use in sorghum crops.

In the short term, insecticides are available to help protect crops from fall armyworm. However, this insect has a reputation for developing resistance to insecticides. Resistance management strategies will therefore be required to maintain effectiveness of insecticides for controlling fall armyworm.

Application of insecticide will be most effective if applied late in the day and into the night, when the larvae become more active and emerge from the protected areas of the plant.

Insecticides available for use against fall armyworms include those available under recently approved APVMA minor use permits. Also available for use in WA, are any insecticides registered for use on crops for control of other insects if those products are considered effective on fall armyworm and provided they are applied according to label details. See section 87 ‘Use in accordance with label’, page 53 of Western Australia’s Health (Pesticides) Regulations 2011.

Table 1 lists the details of pesticides available with current permits in Australia for sorghum and their specified application rates. The permits should be read in conjunction with the relevant product label for information on withholding periods and other critical comments.  

More detail on the permits is available from the information sheets found on the APVMA Portal. A direct link to each minor use permit PDF is provided in the tables below. 

Note: New permits are regularly issued for fall armyworm control. Check the APVMA Portal for the most current information. 

Table 1 Current permits in Australia for sorghum crops (as at 20 May 2020)

 *Insecticide Resistance Action Committee  ** Mode of Action

Important disclaimer

The Chief Executive Officer of the Department of Primary Industries and Regional Development and the State of Western Australia accept no liability whatsoever by reason of negligence or otherwise arising from the use or release of this information or any part of it.