Potential impact of fall armyworm on maize crops
Maize (corn, popcorn) is a preferred host of fall armyworm (FAW) with a high risk of significant crop losses. Both grain crops and those destined for forage or silage are at risk.
Eggs are laid on plant foliage and newly-hatched larvae feed on the leaves, creating a windowing effect. Growing larvae prefer to hide in the whorl, damaging the developing leaves, which display a shot-hole pattern of holes as they emerge. In severe cases, the leaf may break off above the damaged area.
If plants are infested very early in the vegetative stage and there is not much leaf present, larvae may hide in the soil at the base of the plant, where they can cause damage to the stem, or kill the young plant. Larval activity is higher at night.
Overseas, fall armyworm subjected to repeated and prolonged use of insecticides have rapidly developed pesticide resistance so consider management options carefully.
What to look for
Early detection is essential, particularly during crop emergence and establishment.
- Regularly check seedling crops for damage and caterpillars.
- Look on the leaves for egg masses and small larvae.
- Check the whorl for larger larvae and moths.
- Watch for windowing or damage to lower leaves, shot holes in new leaves as they unfurl, loss of tassels or damage to cobs.
The damage to leaves, whorl and cobs is similar to that caused by Helicoverpa armigera and common armyworm, especially at low FAW densities. These other caterpillar pests may also be present in the crop, so finding the larvae and correctly identifying them is important to ensure the most appropriate management decisions are made. Management decisions must not be made on the presence of damage alone.
Where heavy infestations occur, large amounts of frass (insect poo) will be visible on leaves and in the whorl (Helicoverpa frass is usually drier and in smaller quantities). If damage is evident but larvae are not visible, thoroughly check the whorl as larvae and moths will shelter there during the day. Destructively sample plants to find embedded larvae if damage indicates they may be present.
Fall armyworm larvae can attack maize at all growth stages. Vegetative crops can recover from defoliation, particularly if the crop is growing rapidly; however, larvae can defoliate and/or sever seedling maize plants at the base, producing damage similar to that caused by cutworm.
Impact on yield will be greatest if there is seedling death, a damaged growing point, or significant defoliation at critical growth stages (around 10 days prior to and 20 days after tasselling/flowering). Feeding damage to tassels and cobs during pollination and grain filling will also impact yield and grain quality.
Establishing crops are very vulnerable to FAW damage. It is essential to prevent medium and large larvae developing in seedling crops. These larger larvae have the capacity to feed at the base of the plants, on the growing point, resulting in seedling death. Severe defoliation at the seedling stage will compromise the rate of crop establishment.
Key to the control of any pest is an integrated pest management approach. Many of the natural enemies of Helicoverpa also attack fall armyworm, and over time may impact FAW populations. Maintaining clean fields (particularly preventing the establishment of maize volunteers in following crops) is also important.
Chemical control of FAW is most likely to be effective when targeting larvae on small plants. Use banding, if appropriate, to optimise application and reduce cost. Products with residual activity will make it more likely that larvae emerging at night to feed on plants (from the soil) will encounter a lethal dose.
While insecticides used for the control of other caterpillar pests may provide some level of control of FAW, the population that arrived in Australia is already demonstrating resistance to some chemical groups, including synthetic pyrethroids and carbamates. When choosing insecticides for FAW control, consider the efficacy against FAW, and the implications for chemical resistance development in FAW other pests (including Helicoverpa) that may be exposed. Broad spectrum insecticides will have major detrimental impacts on natural enemies.
The Australian Pesticides and Veterinary Medicines Authority (APVMA) has issued several permits for FAW in a range of crops. Check the APVMA permits portal for more details (search for 'fall armyworm' and tick the 'pest/purpose' button).
There are currently no locally-derived thresholds to guide management of fall armyworm due to lack of opportunity to assess the impact on maize in Australia. The Department of Agriculture and Fisheries is working with industry to identify appropriate thresholds and the most effective management strategies for Australian conditions.