More intense dry spells and rain are favoring the spread of pests and diseases that could threaten the multi-billion dollar cassava industry and food security in Southeast Asia, according to a CIAT-led study in Pest Management Science.

In particular, they found that Cassava Witches’ Broom disease has reached the Philippines, Vietnam, Cambodia, Laos, and Thailand. Presumed to be spread by an as-yet-unknown insect, it can reduce yields by up to 60%, and symptoms of the disease were found in two-thirds of fields studied. The cassava mealybug, a pest that followed cassava from its center of origin in South America and that can also cripple yields, was found in 70 percent of fields.It contained the most up-to-date assessment of pest and disease threats in the region’s cassava fields, with scientists gathering data from 430 sites.

Symptoms of Cassava Witches’ Broom disease were found in two-thirds of the 430 fields studied; the cassava mealybug pest was found in 70 percent of fields.

 “A number of factors have triggered the explosion in pests and diseases in Southeast Asia’s cassava fields, including climate variability and the changing frequency of droughts,” said CIAT entomologist Kris Wyckhuys, one of the report’s authors. “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.

Cassava is the third largest source of calories in Southeast Asia after rice and maize, and supports the livelihoods of around 40 million people in the region. The crop – grown predominantly by smallholders – underpins a US$5-billion regional market in starch, which is used to produce products from paper to biofuel. In Indonesia, cassava is also a staple food. It’s, therefore, vital that we act now to safeguard food security, farmer welfare, and the long-term sustainability of rural industries.

“Right now 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 moving into areas where cassava is a prime food security crop,” continued Wyckhuys.

The study called for comprehensive pest management, including stronger controls on the movement of cassava stakes, which can harbor pests and diseases. Biological control, which includes the use of insects that are natural enemies of mealybugs to try and contain outbreaks was also recommended, along with more research into ways to boost the natural health of agro-ecosystems in general.

Biological control is already a reality in Southeast Asia, with releases of the killer wasp, Anagyrus lopezi in Thailand, Vietnam, and Indonesia in recent years to target mealybug infestations. The wasps have drastically lowered mealybug populations in Thailand and southern Vietnam, but researchers have noticed that, in areas with prolonged drought and low soil fertility, the wasps are less effective. Also, some growers have continued to use insecticides, which kill beneficial insects.