Invasive pests on the move: lessons from recent invasions
How invasive crop pests spread and what recent invasions teach: fall armyworm, tomato leaf miner, papaya mealybug, wheat blast and the response toolkit.
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In May 2018, entomologists in Shivamogga, Karnataka, found unfamiliar caterpillars feeding in maize whorls. They were fall armyworm (Spodoptera frugiperda), a pest native to the Americas that had appeared in West Africa only two years earlier. Within a year it had spread across almost every maize-growing state in India, and through Bangladesh, Sri Lanka, Southeast Asia and China. By 2020 it had reached Australia.
Fall armyworm is the most dramatic recent example of a broader trend. Trade, travel and a warming climate are moving crop pests and pathogens faster than ever. FAO estimates that plant pests and diseases cause up to 40% losses in global crop production, and invasive species, those introduced outside their native range, are among the most damaging, because they often arrive without their natural enemies.
How pests invade
An invasion has four stages:
- Transport: the pest is carried in planting material, fruit, grain, soil, packaging, wooden pallets or vehicles, or on travellers. Some insects, such as fall armyworm moths and locusts, also migrate hundreds of kilometres on the wind.
- Introduction and establishment: the pest survives and reproduces in the new place. Suitable climate, abundant hosts and the absence of natural enemies help.
- Spread: natural dispersal and human movement carry it across the region.
- Impact: crop losses, extra pesticide use, trade restrictions and changes in farming.
Pests that have many hosts, short life cycles and high reproduction, and that tolerate a wide range of climates, make the most successful invaders.
Lessons from recent invasions
Fall armyworm
Fall armyworm larvae feed on more than 350 plant species, preferring maize, sorghum and other grasses. The moths fly long distances and lay several hundred eggs each. In Africa, without control, losses were estimated in 2017 at several million tonnes of maize a year. In India, it attacks maize, sorghum and sugarcane.
The response showed how quickly science can adapt:
- Early warnings and scouting apps, such as FAO's FAMEWS;
- economic thresholds for maize;
- biological control with native egg parasitoids (Telenomus remus, Trichogramma), the entomopathogenic fungi Metarhizium and Beauveria, Bt, and nucleopolyhedrovirus;
- cultural methods: timely sowing, intercropping with pulses, push–pull, and applying sand, ash or soil to the whorls;
- selective insecticides such as emamectin benzoate, chlorantraniliprole and spinetoram, used when thresholds are crossed.
Native natural enemies began attacking fall armyworm within a few years of its arrival, and damage has become more manageable in many areas.
When to spray fall armyworm in maize
ICAR guidelines suggest action when about 5% of plants show fresh damage at the seedling to early whorl stage, 10% at mid-whorl, and 20% at late whorl. A farmer walks a W-shaped path through a 1-ha field and checks 50 plants. Four show fresh windowing and frass at the early whorl stage, which is 4 ⁄ 50 = 8%, above the 5% threshold. The farmer releases egg parasitoids if available, or applies a recommended selective insecticide into the whorl, rather than spraying broad-spectrum products across the whole field.
Tomato leaf miner
Tuta absoluta is a tiny moth from South America. It reached Spain in 2006, spread across the Mediterranean, Africa and Asia, and was detected in India in 2014. Its larvae mine leaves and bore into fruits, and losses can be total if it is not controlled. It developed resistance to several insecticides quickly. Management combines pheromone traps for monitoring and mass trapping, insect-proof nets, removal of infested plants, natural enemies such as mirid bugs, and rotation of insecticide modes of action.
Papaya mealybug: a classical biocontrol success
Paracoccus marginatus, the papaya mealybug, appeared in Tamil Nadu in 2008 and devastated papaya, mulberry, tapioca and other crops. Pesticides were largely ineffective. In 2010, the National Bureau of Agricultural Insect Resources (NBAIR) imported three parasitoids from Puerto Rico, including Acerophagus papayae. Within a year, the mealybug was brought under control across southern India, protecting crops worth large sums every year. This is classical biological control: introducing a pest's natural enemy from its native range after careful testing.
Wheat blast
Wheat blast is caused by a strain of Magnaporthe oryzae (Pyricularia oryzae). In 2016, it was found in Bangladesh, its first appearance outside South America, probably introduced with imported grain. It destroyed wheat on about 15,000 ha. India responded with a wheat holiday, banning wheat cultivation in districts bordering the affected area. Bangladesh released resistant varieties carrying the 2NS translocation, such as BARI Gom 33.
Desert locust
The 2019–2021 desert locust upsurge, fuelled by unusual cyclones and rains over the Arabian Peninsula, spread swarms across East Africa, Southwest Asia and into Rajasthan, Gujarat and Madhya Pradesh in 2020. Control relied on regional early warning through FAO's Desert Locust Information Service, ground and aerial spraying, and, for the first time in India, drones.
The response toolkit
- Prevention: plant quarantine at borders under India's Plant Quarantine (Regulation of Import into India) Order, 2003, pest risk analysis, inspection of imports, and clean planting material.
- Early detection: surveillance networks, pheromone and light traps, farmer reporting, and diagnostic laboratories.
- Rapid response: containment or eradication while populations are small.
- Long-term management: IPM, classical and conservation biological control, resistant varieties, and insecticide resistance management.
- Regional cooperation: pests do not stop at borders. FAO, IPPC and regional organisations coordinate warnings and responses.
Key points
- Invasive pests arrive through trade, travel and migration, establish, spread and cause impact. Arriving without natural enemies makes them especially damaging.
- Fall armyworm spread from the Americas to Africa (2016), India (2018) and Asia and Australia by 2020. It is managed with thresholds, biocontrol, cultural methods and selective insecticides.
- Tomato leaf miner reached Spain in 2006 and India in 2014, and resists many insecticides.
- Papaya mealybug (India, 2008) was controlled by classical biocontrol with Acerophagus papayae from 2010.
- Wheat blast reached Bangladesh in 2016; the response was a wheat holiday and resistant varieties.
- Prevention through quarantine is cheaper than control. Early detection and regional cooperation are critical.
What the exams ask
- Define an invasive species and the stages of invasion.
- Describe the spread and management of fall armyworm.
- Explain classical biological control with the papaya mealybug example.
- Discuss plant quarantine measures in India.
- Explain why invasive pests often cause greater damage than native ones.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Why do invasive pests become worse than in their native range?
In their native range, pests are held in check by co-evolved parasitoids, predators and diseases, and by resistant hosts. In a new region these are absent, an idea called the enemy release hypothesis, and crops may have no resistance. Populations can then explode until local natural enemies adapt or new ones are introduced.
Can invasive pests be eradicated?
Sometimes, if they are detected very early in a small area. Examples include several fruit fly outbreaks eliminated by trapping and sterile insect releases. Once a mobile pest like fall armyworm is widespread, eradication is impractical, and the goal shifts to sustainable management.
Does climate change make invasions more likely?
Yes, in several ways. Warmer winters let pests survive and establish in new areas, more generations per year raise populations, and changing winds and extreme weather can carry migratory pests farther. Climate change also stresses crops, making them more susceptible.
Sources
- Day, R. et al. "Fall armyworm: impacts and implications for Africa." Outlooks on Pest Management 28, 196–201 (2017).
- Sharanabasappa et al. "First report of the fall armyworm, Spodoptera frugiperda (J. E. Smith), an alien invasive pest on maize in India." Pest Management in Horticultural Ecosystems 24, 23–29 (2018).
- Myrick, S., Norton, G. W., Selvaraj, K. N., Natarajan, K. and Muniappan, R. "Economic impact of classical biological control of papaya mealybug in India." Crop Protection 56, 82–86 (2014).
- Islam, M. T. et al. "Emergence of wheat blast in Bangladesh was caused by a South American lineage of Magnaporthe oryzae." BMC Biology 14, 84 (2016).
- FAO. The Global Action for Fall Armyworm Control: Action Framework 2020–2022. FAO, Rome, 2020.
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