Fall armyworm in Western Australia

Management in maize and sweet corn

Damage

Maize and sweet corn are among the more favoured host plants for fall armyworm. Larvae can reduce the yield of grain, forage and cobs by affecting plant establishment, damaging leaves and attacking cobs.

A schematic of the life cycle of fall armyworm in relation to damage caused to maize plants. Photo source: Integrated management of the Fall Armyworm on maize: A guide for Farmer Field Schools in Africa, Food and Agriculture Organization of the United Nations. 

Plant establishment is most likely to be affected when advanced stage larvae move into a newly planted crop from adjacent areas of crops or weeds. Fall armyworm damage to young plants is similar to that of cutworms, where the plant is either cut off at ground level or killed by burrowing into the growing tip.

The usual source of infestation is from moths laying egg masses on young plants. Newly emerged larvae feed on leaves and remove only the green tissue, leaving leaf tissue behind, resulting in a ‘windowing’ effect.

Damage to leaves caused by fall armyworm larvae feeding on green tissue can appear as a ‘windowing’ effect, as pictured here on corn.

As larvae increase in size, they remove chunks off the leaves. As plants grow, the larvae feed within the whorl (throat) of plants and are not seen during the day. They may emerge at night to feed. Their frass (droppings) may be visible near the leaf damage. Examination of leaves inside the whorl is needed to confirm the identity of the pest – other armyworms and ‘heliothis’ also infest maize, causing similar damage. Heavy infestations of fall armyworm result in a ragged appearance of plants. Fall armyworm larvae are cannibalistic and usually only one or two large larvae will occur within each whorl as the larvae develop.

Maize plants can tolerate some level of damage to leaves without affecting yield. The extent of damage depends on the timing and magnitude of fall armyworm infestation. This is incorporated in suggested action thresholds mentioned below.

Larvae also feed on the tassel, silks and cobs of maize/sweet corn plants. Damage to tassels is unlikely to affect yield, but larvae feeding on tassels may move to the silks and cobs. Feeding on silks may affect kernel fertilisation. Larvae are likely to feed on developing kernels in cobs by entering the cob from the side, unlike ‘heliothis’ larvae which usually enter cobs via the silks. Fall armyworm feeding on cobs results in some yield loss in maize crops but may cause significant loss through cosmetic damage in sweet corn crops for human consumption, where such cobs are rejected from market.

Mature fall armyworm larvae feeding on corn. 

Monitoring

The spread of fall armyworm in Western Australia (WA) is being monitored with the use of about 50 pheromone (lure) traps throughout northern WA, including Kununurra, Broome, Carnarvon, Geraldton. Monitoring and tracking of the pest will help determine its distribution and assist the Department of Primary Industries and Regional Development (DPIRD) to provide advice to industry about its presence. Additionally, knowledge of the insect’s whereabouts will help clarify the time of year specific crops in particular regions are most likely to be infested.

If there are indications of fall armyworm becoming a consistent pest in specific regions, a pheromone trap can be deployed in crops to check on the presence of moths as a warning of potential pest situations.

Growers and agronomists are suggested to start scouting as soon as maize/sweet corn seedlings emerge. Weekly checks thereafter are recommended. Check 20 consecutive plants (randomly selecting the first) from at least five locations across the crop, or 10 plants at 10 locations. 

Regularly check and record the:

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

A few days before tasselling, check for large larvae in the whorls, which will be pushed out when the tassels emerge. These larvae may attack young ears. 

Continue to check for fall armyworm larvae until silks begin to dry. On very rare occasions, an infestation of armyworm has been an issue in maize after silking, causing considerable leaf loss, which may have affected yield by reducing the plants’ ability to fill out kernels.

When to take action

The action thresholds recommended for applying control measures for fall armyworm vary with the growth stage:

• At the seedling stage, if more than 5% of plants are cut.
• At the early whorl stage (knee high), if more than 20% of plants are infested.
• At the late whorl stage (shoulder high), if more than 40% of plants are damaged and live larvae are present.
• At the tasselling/early silking stage, in sweet corn, if more than 5% of plants are infested and in maize, if more than 20% of plants are infested.

Management approach

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 maize or sweet corn, especially where natural control agents are used. 

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

An important management practice is to maintain farm biosecurity measures and implement good farm hygiene, and to 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, but this product is not available in Australia. The NPV that controls Helicoverpa larvae is not considered to be effective against fall armyworm.

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 larvae feed in concealed areas, foliar applications of Bt products are not recommended for use in maize/sweet corn 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 this pest.

Application of insecticide will be most effective if applied late in the day and into the night, when larvae become more active and emerge from 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  WA Health (Pesticides) Regulations 2011. 

The following tables list the details of pesticides available with current permits in Australia for maize (Table 1) and sweet corn (Table 2), 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. List of current permits in Australia for maize crops (as at 20 May 2020)

*Insecticide Resistance Action Committee  **Mode of Action

Table 2. List of current permits in Australia for sweet corn 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.