Risk analysis: Spodoptera frugiperda (J.E. Smith) (fall armyworm)

Risk analysis: Spodoptera frugiperda (J.E. Smith) (fall armyworm)

                                                         SHAHID HUSSAIN    
1.1 Introduction
Scientific name: Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae)
Common name(s): Fall armyworm
Spodoptera frugiperda (Lepidoptera: Noctuidae) is a highly mobile moth, native to tropical and
subtropical regions of the Americas. It has become established in regions where temperatures rarely
fall below 10°C, and it has a long history of sporadic outbreaks in the USA (Jeger et al. 2018). The first
recorded outbreak occurred in 1899 when most of the area east of the Rocky Mountains was invaded,
but the most serious outbreak occurred in 1912 in the same area. To date, there have been other
irregular outbreaks, including a serious one in 1977, where the costs of control were estimated at
US$297 million (Johnson 1987).
Fall armyworm was first recorded outside its native range in West Africa in 2016. By 2018, it had
spread across much of Sub-Saharan Africa (Intelligence Report 016/20 2020). It was first detected in
India in 2018 and had been detected in China by early 2019. The pest, particularly the larval stages,
can damage crops heavily, especially maize, posing major economic challenges to subsistence
farmers. Fall armyworm’s spread and invasiveness is attributed to its ability to survive in a broad range
of climatic conditions, its wide host range and its long-distance dispersal capabilities (Li et al. 2020;
Zacarias 2020).
Results from species distribution models indicate that a key factor limiting FAW establishment is the
lowest temperature in winter months, which is consistent with observations in nature. In North
America, FAW populations only persist year-round in southern Florida and Texas but expand as far
north as Canada every summer. The temperate zone populations are seasonal; individuals die off in
winter, and new migrants re-establish every summer. A similar pattern of seasonal variation is
emerging in FAW populations in China (Biosecurity Intelligence Team 2020).
After FAW’s arrival in Australia, preliminary evidence suggests that the pest is unlikely to be limited by
the distribution and availability of potential host plants species. In Australia, FAW has been detected
on several crops including maize, sorghum, chickpea, soybean, melon, green beans and pastures
(Rhodes grass). Plant damage has been characterised by grain, ear, kernel or fruit damage that alters
plant architecture (Kearns et al. 2020).
The success of FAW as an invasive pest is due to a combination of the following characteristics:
i) wide plant host range for development;
ii) wide plant range for food;
iii) multiple generations per year, which can result in ‘army’ behaviour (see section 1.4
below);
iv) ability to develop tolerance/resistance to insecticides, insecticidal proteins and transgenic
crops;
v) migratory capability; and
vi) the ability to persist in temperate through to tropical climates.
Species distribution models predicting FAW abundance suggest the pest is likely to persist through
many of the grain-growing regions of Australia, with year-round population in northern parts of
Queensland, Northern Territory and Western Australia, and make periodic appearances in more
southern regions (Biosecurity Intelligence Team 2020).
Biosecurity New Zealand Pest Risk Assessment: Spodoptera frugiperda • 12
1.2 Purpose
This document assesses the biosecurity risks associated with S. frugiperda entering New Zealand via
all likely entry pathways. This work will be used to determine surveillance, monitoring and risk
management options for the wider Biosecurity New Zealand team.
1.3 Scope
This assessment examines the risk of S. frugiperda entering, becoming established and causing
unwanted impacts in New Zealand. The assessment covers all likely entry pathways including wind
dispersal, nursery stock, fresh produce, fresh cut flowers and foliage, passenger and inanimate
pathways.
1.4 Taxonomy
The genus Spodoptera was described by Guenée in 1852, who also described the genera Laphygma
and Prodenia, which were later treated as synonyms of Spodoptera. Spodoptera species feed on
numerous vegetables, grain crops and pasturelands. The genus is mainly found in tropical and
subtropical regions, with some species migrating into more temperate areas throughout the growing
season (Pogue 2002).
The common name for the Spodoptera genus is ‘armyworm’ due to the tendency of many species to
move en masse from one location to another in search of preferred hosts when population numbers
are high and uncontrollable. This potential to ‘army’ gives larvae access to a broader range of host
plants, and some of the most serious economic armyworm pests are species in the genus
Spodoptera. Highly mobile large larvae tend to eat a wider range of plants than smaller sedentary
larvae (Pogue 2002).
The genus Spodoptera is comprised of 30 species, inhabiting six continents. Morphological
identification of late larval instars and adults to species level is possible, especially if the country of
origin and host plant is known. Fall armyworm has two subpopulations or strains (commonly referred
to as the corn strain (C-strain) and rice strain (R-strain)), which are morphologically indistinguishable
but differ in their host plant preference and certain physiological features.
Synonyms:
• Laphygma inepta Walker, 1856
• Laphygma frugiperda var. fulvosa Riley, 1876
• Laphygama frugiperda var. obscura Riley, 1876
• Laphygma macra Guenée, 1852
• Phalaena frugiperda Smith, 1797
• Prodenia autumnalis Riley, 1871
• Prodenia plagiata Walker, 1856
• Spodoptera autumnalis (Riley, 1856)
• Spodoptera inepta (Walker, 1852)
• Spodoptera plagiata (Walker, 1856)
• Spodoptera macra (Guenée, 1853)
• Spodoptera signifera (Walker, 1856)
Biosecurity New Zealand Pest Risk Assessment: Spodoptera frugiperda • 13
2 Hazard identification
2.1 Description
Fall armyworm (FAW) is native to tropical and subtropical regions of the Americas. FAW migrates to
temperate regions in North and South America during the relevant hemisphere’s summer. In southern
Florida, FAW can breed year-round, and in Central and South America, four to six generations can
occur in a year (Jeger et al. 2018). The pest eats many different plants (353 plant hosts in 76 families)
but favoured hosts for larvae, which is the most destructive life stage, include maize, rice, wheat and
sorghum (Montezano et al. 2018). The ability to disperse over long distances enables FAW to exploit
new habitats and increase its ever-expanding range (Spafford 2020).
2.1.1 New Zealand status
Fall armyworm is not known to be present in New Zealand:
• Fall armyworm is listed on the Country Freedom register as not being present in New Zealand
(MPI 2020).
• Fall armyworm is not recorded in NZInverts (2020) or NZOR (2020).
• Fall armyworm is recorded in PPIN (2020) as not present.
• Fall armyworm is listed as an unwanted organism in New Zealand (UOR 2020).
• Fall armyworm has a quarantine and regulated pest status on the Official New Zealand Pest
Register (ONZPR 2020).
2.2 Commodity association
Montezano and colleagues (2018) compiled a comprehensive list of records to show that FAW has
plant hosts in 353 species from 76 families (Appendix Table 1). The families with the greatest number
of species reported as hosts of FAW include Poaceae (106 species), Asteraceae and Fabaceae (31
species each). However, FAW larvae prefer to feed on grasses such as maize and sorghum (Figure
1), and they cause the greatest damage to these crops.
1. Maize
2. Sweet corn
3. Sorghum
4. C4 pasture grasses
5. Sugarcane
6. Rice
Most preferred
Least preferred
Figure 1. Preferred host plants for fall armyworm
While the host range of FAW includes 353 plant species within the Poaceae
family and several non-grass hosts, studies show that FAW tends to favour
summer crops in above general order (Kearns et al. 2020).
Biosecurity New Zealand Pest Risk Assessment: Spodoptera frugiperda • 14
2.2.1 Plant associations
Fall armyworm eats a wide range of plants. It is reported from 353 host plant species across 76
families (Appendix Table 1). However, FAW prefers to consume wild and cultivated grasses, rice,
sorghum, millet and sugarcane. Among vegetable and fruit crops, only maize is regularly damaged,
but tomato, apple, grape, orange, papaya, peach, strawberry and a number of flowers are occasionally
attacked (Montezano et al. 2018). Records from the USA show evidence of defoliation, fruit damage
and fruit drop/rot as a result of FAW feeding (Miles 2020). Weeds known to serve as FAW hosts
include bentgrass (Agrostis sp.), crabgrass (Digitaria spp.), Johnson grass (Sorghum halepense),
morning glory (Ipomoea spp.), nutsedge (Cyperus spp.), pigweed (Amaranthus spp.) and sandspur
(Cenchrus tribuloides). Other crop host species include Allium (Liliaceae), Brassica spp.
(Brassicaceae), capsicum, aubergines, potatoes (Solanaceae), Cucumis (Cucurbitaceae), Gossypium
(Malvaceae), Phaseolus (Fabaceae) and Ipomoea (Convolvulaceae) as well as various ornamental
plants (chrysanthemums, carnations and Pelargonium) (Jeger et al. 2017). In laboratory host
preference studies examining larval feeding choices, maize and wheat were preferred above soybean
and cotton (da Silva et al. 2017).
The broad range of plant hosts FAW can exploit is influenced by several biological factors, including
female moth egg-laying behaviour and larval movement. Larvae need to quickly disperse in order to
find food and to avoid intraspecific competition and high levels of cannibalism. Adult female fall
armyworms lay eggs on several species of plants, including species that are not known hosts but are
abundant at times, if preferred hosts are scarce or absent. Hence, the behaviour of ovipositing in large
masses is associated with polyphagy and is common within the Spodoptera genus (Montezano et al.
2018). The number of host plants reported for FAW is higher when compared to congeneric species of
agricultural importance such as Spodoptera albula (Walker) (65 species), S. cosmioides (Stoll) (126
species), S. dolichos (Fabricius) (94 species) and S. eridania (Stoll) (202 species) (Montezano et al.
2018).
When larval populations are abundant, the preferred host plants are stripped. This causes the
population to behave like an army and disperse in large numbers, consuming nearly all vegetation in
their path (Montezano et al. 2018). Many host records reflect such periods of abundance and are not
truly indicative of egg-laying and feeding behaviour under normal conditions.
Since FAW arrived in Australia, it has predominantly been observed in maize crops, but also in
sorghum, chickpea, soybeans, melons, green beans and pasture seed crops, with reports of larvae on
Rhodes grass in Western Australia. Formal inspections, or surveys for FAW, are yet to be conducted
on native vegetation or introduced grasses and broadleaf weeds throughout the pastoral zone of
Australia (Kearns et al. 2020).
2.3 Potential for establishment and impact
2.3.1 Establishment
Currently, the environmental conditions and host plant availability in northern regions of New
Zealand are suitable for FAW to establish.
Climate matching (Roigé and Phillips 2020) indicates that some regions within the known native and
invaded distribution of FAW have a 70–90% similarity with New Zealand’s climate. Climate is therefore
unlikely to be a barrier to FAW establishment in northern and some southern parts of New Zealand.
Current climate data was used to construct two species distribution models (CLIMEX and MaxEnt) for
FAW, with moderate uncertainty in the prediction (Narouei-Khandan 2020). The models predicted that
most of North Island climate is conducive to FAW establishment. In the South Island, Nelson and
Canterbury were predicted to be suitable. Te Kao, Northland, was identified as the most suitable area
in New Zealand. The growth index summarises how suitable a location is for the population growth of
a species. In the case of FAW, the growth index is highest from October to mid-March in the Te Kao
region of Northland. For the rest of the North Island, the growth index is considerably lower than in
Northland. In northern parts of the North Island, FAW could produce up to three generations per year
and, consequently, limiting the potential to ‘army’. In the rest of the North Island, FAW would likely
have only one or two generations.
Biosecurity New Zealand Pest Risk Assessment: Spodoptera frugiperda • 15
Optimistic and pessimistic climate change scenarios for 2030 and 2050 showed that highly suitable
areas would extend from Northland to the Auckland, Gisborne and Hastings regions (NaroueiKhandan 2020). In both optimistic and pessimistic scenarios, it is predicted that northern parts of the
North Island can sustain up to five FAW generations per year, which increases its potential to ‘army’.
2.3.2 Impact
Fall armyworm is a significant economic pest in other regions around the world where it has
become established.
Fall armyworm is recognised as one of the most significant moth pests in the Americas. In Brazil,
several tonnes of insecticides are used for FAW control each year. It is estimated that in maize the
annual losses can be between 19–100% (Montezano et al. 2018), although variable seasonal and
environmental conditions can make these types of estimates difficult.
It has been suggested, in the absence of proper control methods, FAW has the potential to cause
maize yield losses of 8.3–20.6 tonnes per hectare (21–53% of annual production), per year, in 12 of
Africa’s maize-producing countries (Day et al. 2017). The monetary value of these losses is estimated
to be between US$2.48–6.19 billion (Harrison 1984; Montezano et al. 2018). The economic impact of
FAW in its native range is estimated to be between US$300–500 million per annum (Cruz and Turpin
1983).
During seedling emergence in maize, young mid-stage FAW larvae (instars 1–3) can infest seedlings
and feed on leaf whorls resulting in substantial defoliation and damage, plant death and occasional
yield loss (Malo and Hore 2020). Mature larvae can behave like cutworms by completely severing the
stem of maize seedlings (Goergen et al. 2016). The extent of the damage depends on geographical
region, planting season, cultivar planted and in-field cultural practices (de Almeida et al. 2002).
Fall armyworm can be difficult to control with chemicals in maize due to the plant’s whorl and
characteristic ears and protective husks, which assist the pest’s ability to seek shelter and avoid
insecticide exposure.
In sorghum, damage affects plant development by delaying plant maturity, reducing plant height and
increasing the number of tillers and panicles per plant (Starks and Burton 1979). Sorghum seedlings
can recover more from FAW damage compared to later growth stages (Juarez, Twigg and
Timmermans 2004). In wheat and barley, young FAW larvae have been found hiding in seedling
plants and feeding on the centre of developing leaf whorls (Casmuz et al. 2010; Yang et al. 2021).
It is interesting to note plant damage due to FAW infestation does not necessarily result in yield loss;
pest injury can be inflicted to a certain degree without resulting in significant decrease in yield (Juarez,
Twigg and Timmermans 2004). In addition, plant damage incurred at some growth stages does not
translate to yield loss.
2.4 Hazard identification conclusion
Given that Spodoptera frugiperda:
• has been intercepted multiple times on commodities from several European countries;
• has spread rapidly through Africa, Asia and is currently spreading in Australia;
• is not recorded from New Zealand;
• can potentially establish in New Zealand; and
• can potentially cause unwanted impacts, including damage to multiple fruit and crop species
leading to economic loss;
Spodoptera frugiperda is considered a hazard in this risk analysis.