Insects represent the most diverse group of animals on earth. The species diversity of insects consists nearly of 1 million described species and form an integral component of the earth’s ecosystems. Nearly 2,000 species of insects are considered to be insect pests of cultivated crops and forest trees. Several of them are vectors of human, animal and plant diseases. However, sporadically, in time or space, some species extend their territories and grow rapidly to damaging numbers, developing outbreaks which may persist for a variable length of time before subsiding. Such large populations may have adverse effects on many aspects of cultivated crops and also forest trees such as growth and survival, yield and quality. Fall Armyworm, (FAW) native to tropical and subtropical regions of the Americas, is the latest example which is presently invading new geographical areas with an uncanny momentum and plaguing wide range of agricultural crops.
Fall Armyworm, locally known as ‘Sena caterpillar’ is a serious insect pest which remained confined to its native territory of the Americas for nearly hundred years. However, in 2016 this insect pest embarked upon a new mission of exploiting other geographical territories. Invasion of FAW in to sub-Saharan Africa was first observed in late January 2016, in a maize field in South-Western Nigeria. Later it spread to the Central and Western Africa and subsequently to Southern Africa. It has now been confirmed in more than 30 African countries. This insect pest was reported in India, in September 2018, infesting a sweet corn field in Gujarat, and subsequently it spread to many other parts of India including Andra Pradesh Maharashtra and Tamil Nadu. During the same year it migrated to Sri Lanka and began to extend its potential range across the country.
I do not spare time to elaborate on control aspects of the problem and its basic biology. Presently, all the possible management strategies are being adapted by the Department of Agriculture for containing the pest problem, and the basic biological attributes of the pest have made known to the public through mass media. My main intention is to summarize its taxonomic status and briefly review efforts to explain the underlying mechanism of its rapid evolution as an invasive pest causing recurrent outbreaks in new territories. In the light of this review, I attempt to examine if there is a possibility of this insect pest becoming lasting threat of the same magnitude to agricultural landscape in Sri Lanka.
FAW, scientifically known as Spodoptera frugiperda, belongs to the Family Noctuidae, the largest family in the Order Lepidoptera which contains nearly 20,000 species of moths some of which are greatly of agricultural importance due to their damage caused to cultivated crops. Nearly 100 species of moths in the Family Noctuidae are reported to be crop pests in Sri Lanka and some of them are of great economic importance. This insect was first described in 1797 by an American insect taxonomist, J. E. Smith as Phaleana frugiperda. In 1958 this species was transferred to a new Genus Spodoptera and the scientific name changed to Spodoptera frugiperda. Genus Spodoptera encompasses 31 species distributed on six continents, and majority of them are considered to be pests of agricultural crops.
In Sri Lanka the presence of seven species insect pests of the Genus Spodoptera has been documented; they are, S. exempta, S.abyssinia, S.compta, S.exigua, S.littoralis, S. maurita and S. litura. Of these S. litura is a seasonal pest occurring on a wide range of crops reaching rarely the economic damage levels. Populations of S. exigua grow rapidly to damaging numbers at irregular intervals causing outbreaks. Outbreaks of S.exigua in Jaffna peninsula in late 70s and in Dambulla area in 2003 and 2007, have had catastrophic impacts on onion cultivations leading to heavy economic losses. Populations of other species present in Sri Lanka remain at a relatively low density, causing little damage, and having negligible impact on growth and vigour of crops.
Invasive nature of FAW in new geographical territories
The invasive insect pests are characterized by rapid reproduction, high dispersal ability and tolerance of a wide range of environmental conditions. In this respect, FAW fulfils all the requirements given above. The rapid spread throughout Africa and India was likely due to adult FAW’s ability to travel very long distances. (However, it should be mentioned here that migration of FAW in to South-Western Nigeria is not an unaided dispersal, and probably it is an introduction as a stowaway on a passenger flight).
Literature documents the ability of adults to travel several hundred kilometres in a single night by flying to and maintaining an elevation of several hundred metres, at which height winds can transport adults in a directional manner. Fall Armyworm’s long distance dispersal ability means that it can undergo seasonal migrations of thousands of kilometres. Adults also appear to migrate several hundred kilometres over the sea. FAW has a short development cycle with and unbelievably high reproductive rate of about 1000 eggs per female. All these attributes qualify FAW to be a highly potential pest of cultivated crops with high capacity of causing outbreaks.
Outbreaks of FAW
The proximate mechanism underlying the outbreaks of FAW has to be investigated in the context of case–specific knowledge of other such insects. Generally all insect populations rise and fall with changes in the seasons, interspecific competition, food quality, and numerous other variables. Insect outbreaks typically occur when the population rises significantly above its general equilibrium level and becomes a threat to human interests or endeavours. Capacity to outbreak is a restricted feature and relatively few species of insects are responsible for outbreaks. However, they do not belong in unique taxa. There are outbreak species across a wide range of insect taxa, and many of the species that are closely related to outbreaking insects do not outbreak. FAW is the latest addition to outbreak category.
Accumulated research findings indicate that increases in food quality, decreases in natural enemies and favourable weather to be the most general, non-exclusive causes of outbreaks. Recent research studies undertaken on number of insect outbreaks reveal that the absence of natural enemy populations has triggered insect outbreaks of pine looper, grey larch moth, black-headed budworm and winter moth in the United Kingdom. In the case of FAW, the initial outbreaks occurred in South West Nigeria, and Ghana. Subsequently, recurrent outbreaks were reported from other locations including Sri Lanka. One of the reasons that triggered the outbreak may be the absence of natural enemies in new environment. On the other hand, all these areas have ideal climatic conditions similar to the region of native distribution of FAW. Recently Sri Lanka experienced a long spell of rains, and as a result a large extent of lands were brought under wide range of agricultural crops including maize which is considered the most preferred host plant of FAW. The climatic conditions in Sri Lanka along with the abundance of preferred host plants and absence of natural enemies may have permitted the rapid multiplication of FAW leading to outbreaks. Nevertheless research studies are necessary to validate the significance of these factors.
Possible host shift of Fall armyworm
Emerging speculations refer to the facts that FAW would shift over to other cultivated plants in Sri Lanka for feeding and ensure its survival for long years to come. Herbivorous insects are remarkably conservative in their host range, and it is hardly possible for the insects to incorporate a new plant in to their fixed host range. Successful host shift entails change in two traits: (1) behavioural adaptation of the female insect to recognize and accept the plant for egg laying, and (2) physiological adaptation of the larva to eat and assimilate the plant tissues. These two traits are genetically independent and evolution of host shift has to entail changes in both of these traits. Rapid evolution of a shift to non target plants cannot be expected to happen without genetic variation of behavioural and physiological adaptations. On some occasions sub optimal host may be favoured for oviposition depending on ecological reasons, especially in the absence of preferred host plants. Under such circumstances, the progeny will perish due to lack of physiological adaptation to the host chosen by the female insect for oviposition.
In the context given above, there is less possibility for FAW to shift over to other crops that does not fall within its host range. However, in the absence of maize, the most preferred host, FAW can ensure its survival, perhaps at low population levels, on any other plant species of within its host range.
FAW has already taken its toll of maize crop. I am not a strong advocate of chemical control. Yet I am of the view that in a catastrophic situation of this nature, careful use of chemicals should be the first line of defense. More environmentally acceptable approaches such as biological control can be major components of long term management strategies. If the populations were brought under control, at the initial stage of infestation, with the use of chemical in an ecologically justifiable manner, huge economic destruction caused by the pest could have been minimized.
No insect outbreak would last long. That is nature’s law. The populations have to subside under the changing circumstances of the environment. Populations of FAW will decline with the end of the cropping season and with the onset of dry spell during next few months. For example, the outbreak of onion caterpillar, Spodoptera exigua, a close associate of FAW, caused heavy yield losses on onion in Jaffna peninsula during late 70s and finally subsided. In later years sporadic outbreaks of onion caterpillar were reported in Dambulla. The same fate may befall FAW. The sporadic populations will survive on the alternate host plants during the off season. It is unlikely that populations of FAW surviving on alternate crops would build up to the same degree of catastrophe the country is experiencing today.
(The writer is a former Entomologist, Department of Agriculture and former Director, Field Crops Research and Development Institute, Maha Illuppallama)
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