Maize (corn, popcorn) is a preferred host of fall armyworm (FAW) with a high risk of significant crop losses. FAW will potentially impact both grain and forage yields and quality.
Eggs are laid on the underside of lower leaves (less specific when FAW populations are large) and newly-hatched larvae feed on the leaves, removing green material and leaving skeletonised areas, small to medium hole and windows. If plants are infested very early in the vegetative stage, larvae may leave the plant and shelter in the soil at the base of the plant, where they can cause damage to the stem, or kill the young plant. These larvae may move up onto the plants at night and feed on foliage.
Established larvae prefer to hide in the whorl, or at least in sheltered sites on the plant (leaf axils, under wrapper leaves on the cob). Leaves damaged in the whorl reveal the extent of the feeding damage as they emerge and expand. Where damage is severe, the leaf may break off or be so damaged that no new leaves emerge from the whorl.
If medium-large larvae are in the whorl when the tassel is emerging, they can feed on the tassel. Severe and prolonged feeding on the tassel can result in significantly reduced tassel size.
Tassel damage caused prior to emergence by FAW feeding in the whorl (Photos: Melina Miles).
What to look for
Early detection is essential, particularly during crop emergence and establishment. Regularly check seedling crops for damage, eggs and caterpillars. Look on the underside of leaves for egg masses and small larvae and check the whorl for larger larvae. Destructive sampling is essential to adequately assess an infestation.
Record the number of eggs and larvae per plant. Recording the size of larvae is also useful in decision making. Where infestations are patchy (a cluster of damaged plants here and there), be careful to randomly select plants rather than focusing on the damaged areas.
Ensure you have correctly identified any larvae found as Helicoverpa armigera and native armyworm species may also be present in crops.
Management decisions cannot be made on the presence of damage alone. Leaf damage will be visible long after larvae have finished their development. The condition of newly emerged leaves, and the condition of the whorl (damaged/undamaged) are better indicators of an active infestation than damage to older, fully expanded leaves. Dissection of plants is essential to get an accurate estimate of the FAW infestation.
Plant death, caused by larvae burrowing down inside the stem from the whorl to the growing point, has not been observed in the field. Plant death has been observed in establishing crops (younger than 4-5 leaves) when large larvae feed at the base of seedlings whilst sheltering in the soil. This damage occurs when infestations have not been detected early enough post-emergence and large larvae eventuate.
Vegetative crops have the potential to compensate for defoliation, but just how much defoliation can be tolerated at different growth stages is the focus of current research into FAW management in maize. Persistent defoliation, and particularly feeding that damages the growing point of seedlings, can result in plant death. This type of damage is unlikely in crops older than 4-5 leaf stage, and more likely when FAW densities are high and repeated egg lays are occurring (late summer, or in north Qld and coastal regions).
Feeding damage to tassels prior to emergence is not uncommon, and at high FAW densities, this feeding can reduce the size of the tassel. However, in almost all instances where this damage has been observed, sufficient pollen is still produced to effect pollination. FAW are more damaging to cobs than Helicoverpa. In addition to burrowing in at the terminal end of the cob (silk end), FAW larvae will burrow through the side of the cob, increasing the amount of grain loss and increasing the risk of subsequent disease establishment and weather damage to the cobs. Assessing the risk of cob damage requires inspection under wrapper leaves to find larvae that may burrow into the cob when sufficiently large.
Whilst not observed in Qld, corn crops in northern WA (2020), where FAW populations were very high, suffered yield losses just prior to harvest because of larval damage to the stalk of the cob (causing cobs to fall off plants). Damage from larvae burrowing into the stem also caused stems to break and plants to lodge.
See the FAW damage page for pictures of crop damage.
Management
Establishing crops are very vulnerable to FAW damage. Severe and/or repeated defoliation at the seedling stage will compromise the rate of crop establishment.
It is less clear how much damage can be tolerated before crop loss occurs at later crop growth stages. This is the focus of current research.
Insecticides and insecticide resistance management
Chemical control of FAW is most effective when targeting small larvae. Good efficacy of insecticides on larger plants has also been very achievable. Research on the relative efficacy of the different MOAs available for FAW, and optimisation of application, is in progress. On smaller plants, banding may provide an opportunity to reduce the cost of insecticides without compromising efficacy.
Using 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, FAW has developed resistance to many of modes of action (MOA) overseas. Baseline screening of the Australian FAW populations in 2020-21 identified moderate resistance to carbamates and organophosphates and high resistance to synthetic pyrethroids (Bird et al. 2022). Ongoing monitoring and implementation of resistance management strategies will be essential to minimising the risk of resistance development in Australian FAW population.
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 also have major detrimental impacts on natural enemies.
Pesticide permits for FAW are available for the use of a number of actives in a range of crops. Details are available at the Australian Pesticides and Veterinary Medicines Authority (APVMA)’s permit portal (search for ‘fall armyworm’ and tick the ‘pest/purpose’ button).
The Department of Agriculture and Fisheries is currently working with industry to identify appropriate locally-derived thresholds and the most effective management strategies for Australian conditions.
Reference
Bird L, Miles M, Quade A, Spafford H. 2022. Insecticide resistance in Australian Spodoptera frugiperda (JE Smith) and development of testing procedures for resistance surveillance. https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0263677