Climate-triggered pest and disease invaders threaten US$5 billion cassava industry in Asia

The third largest source of calories after rice and maize in Southeast Asia, cassava supports the livelihoods of an estimated 40 million people in the region. The crop also underpins a US$5 billion regional market in starch, which is used to produce products from paper to biofuel. Southeast Asia is currently the world’s largest trader of cassava starch, with much of the crop produced by smallholder farmers. In Indonesia, cassava is also a key food crop.

The new study contains the most up-to-date assessment of pest and disease threats in the region’s cassava fields. Scientists gathered data from 430 sites across the region to track progress of pests and diseases, estimating the scale of the threat, and the likely drivers.

“Our data suggests that a number of factors have triggered an explosion in pests and diseases in Southeast Asia’s cassava fields, including globalization, climate variability and the changing frequency of droughts,” said Dr. Kris Wyckhuys, an entomologist at the International Center for Tropical Agriculture (CIAT). “We also found that some pests and diseases are far bigger problems than we previously thought, and alarmingly, they’ve already spread further than we thought.”

“It’s vital we act now to safeguard food security, farmer welfare and the long-term sustainability of rural industries.” While its ability to tolerate harsh conditions and poor soils means cassava is an important crop for smallholders, it is particularly prone to pest and disease outbreaks. Two in particular are posing a severe threat in Southeast Asia, the study says.

Pest invaders and disease

Cassava Witches’ Broom disease has already reached the Philippines, Vietnam, Cambodia, Laos and Thailand. Presumed to be spread by an as-yet unknown insect, it causes leaves to discolor and to proliferate, in a distinctive bunch reminiscent of a witch’s broom. The scientists found symptoms of the disease in two-thirds of fields.

A complex of cassava mealybug – pests that followed cassava from its center of origin in South America – can also cripple yields by feeding on and drawing essential nutrients out of the plant. The destructive pests were discovered in 70 percent of fields studied, at highly variable infestation levels.

“Priorities and the level of urgency differ between countries and cassava-growing areas,” continued Wyckhuys. “But right now we know that urgent action is needed to address Cassava Witches’ Broom particularly in Cambodia and central Vietnam, and to halt the spread of cassava mealybug in Indonesia where it’s steadily moving into areas where cassava is a prime food security crop.”

Climate triggers

Dr. Ignazio Graziosi, a CIAT Research Fellow specializing in Invasive Species Ecology, said: “What we’re seeing is that increased climate variability, intensification of cropping systems, and poor crop management practices are aggravating pest activity and disease outbreaks, as well as the crop’s ability to withstand new, emerging threats.”

The study calls for comprehensive pest management, including biological control options – which entail using naturally-occurring insect “enemies” of key pests to combat damage – to try and contain the outbreaks. “What we’re aiming for is practical, environmentally and economically-sustainable options for farmers, so they don’t need to spend much money but get long-term results,” Graziosi said.

 “The aim is to create a balanced ecosystem, which ultimately becomes less vulnerable to pests and the spread of pathogens. Research is crucial to determine how we can modify existing cropping systems and draw upon natural enemies to clamp down on and fight pest invaders, thus boosting the natural health or ‘immune system’ of the whole agro-ecosystem.”

Biological control is already a reality in Southeast Asia, most recently in Vietnam and Indonesia, when a minute killer wasp, Anagyrus lopezi, was released in 2013 and 2014 respectively to control mealybug outbreaks when they were first detected, drawing global media attention.

In key cassava-cropping areas of Thailand and southern Vietnam, introduced parasitoids have drastically lowered mealybug populations. However, researchers also noticed that in areas with prolonged drought and low soil fertility for example, mealybugs appear to have gained an advantage over the wasps, posing a challenge for sustained, cost-free biological control.

Fine-tuning the ecosystems’ immune system

 

Also, in more intensified cassava-cropping areas, growers have embraced insecticide use. While these products do kill off mealybug invaders, they create far larger impacts on the community of natural enemies, kill off parasitoid wasps, and impede effective biological control.

Pest management approaches therefore have to be fine-tuned to a host of pest invaders, and effective pest management constitutes of a bundle of compatible technologies – using both curative and preventative tactics, researchers noted.

Dr Aunu Rauf, Professor of Agricultural Entomology at Indonesia’s Bogor Agricultural University, said: “This study also outlines the real threat that mealybugs pose to poor farmers in Indonesia. Because the pest is from a foreign country, there is no natural enemy so it multiplies quickly.

Urgent action needed

“We’ve already planted the seed for long-term control not only for cassava mealybug, but for other invasive pests in the region. What we need right now is financial and technical support to roll-out a major biological control response program throughout the region, including equipment for diagnosis, detection and quarantine of emerging threats.”

Deputy Director General at Vietnam’s Plant Protection Research Institute, Dr. Trinh Xuan Hoat, said training courses will be critical to equip regional research communities with new knowledge about invading threats, some of which are currently unknown or undetected.

“We consider cassava pests and diseases a new and severe problem for the region, not for any particular country, aggravated through climate change and spread through the informal, regional trade in planting material,” he said. “Broad thinking is therefore needed and cross-border research cooperation is crucial to address these emerging threats,” he added.

This post was updated on March 31st 2016.

The Emerging pests and diseases of cassava in Southeast Asia: Seeking eco-efficient solutions to overcome a threat to livelihoods and industries project was supported by the European Commission through the International Fund for Agricultural Development (IFAD) www.ifad.org

The International Center for Tropical Agriculture (CIAT) is a not-for-profit organization that conducts socially and environmentally progressive research aimed at reducing hunger and poverty and preserving natural resources in developing countries. www.ciat.cgiar.org

CGIAR In collaboration with national agricultural research systems, civil society and the private sector, the CGIAR fosters sustainable agricultural growth through high-quality science aimed at benefiting the poor through stronger food security, better human nutrition and health, higher incomes and improved management of natural resources. www.cgiar.org

The Plant Protection Research Institute (PPRI) is one of eighteen member institutions of the Vietnam Academy of Agricultural Sciences (VAAS), under the Ministry of Agriculture and Rural Development (MARD). The institute conducts research and technology transfer services in plant pathogens, pest diagnosis and identification, and is a CIAT partner. www.ppri.org.vn

Institut Pertanian Bogor (IPB or Bogor Agricultural University) is a state-run agricultural university based in the city of Bogor, Indonesia. CIAT and IPB are working in partnership to establish plans for mealybug control. www.ipb.ac.id