Soil fertility and plant health offer clues why cassava entices certain pests

In 2008, a number of invasive mealybugs started wreaking havoc on the USD5 billion cassava industry in Southeast Asia, where smallholder farmers cultivate the crop for industrial use and food. In Indonesia, in particular, more than half of locally grown cassava is used for human consumption.

Outbreaks of the cassava mealybug are “effectively” under control in many parts of mainland Southeast Asia, following the introduction in late 2009 of a tiny wasp (Anagyrus lopezi) that attacks the pest, said lead scientist Kris AG Wyckhuys, who has been working in the region to examine cassava pest management. This particular mealybug virtually destroys the whole plant, deforming the leaves and hampering root development.

However, the A. lopezi wasp “appears at a slight disadvantage in cassava plots on sandy, low-fertile soils, and these thus continue to be affected by mealybug outbreaks,” added Wyckhuys.

Beyond the cassava mealybug, the crop consistently faces threats from other pests, including papaya and long-tailed mealybugs, and different kinds of mites.

To better understand why some of the mealybugs proliferate on cassava and evade natural enemies such as the A. lopezi wasp, CIAT researchers and partners in Cambodia, Laos and Vietnam homed in on soil characteristics. They also examined how a plant’s infection with cassava witches’ broom (CWB) bacteria can play a role.

One study, published in Agriculture, Ecosystems and Environment, involved analyzing mealybug-infested fields with different soil fertility profiles across the three countries. The researchers also performed greenhouse experiments, using plants infested with the cassava mealybug and treated with different levels of nitrogen or potassium – and compared the results with those under field conditions.

The research highlights the importance of well-fertilized, healthy soils. Specific findings include:

• Cassava mealybugs thrive on cassava grown in sandy soil with minimal nitrogen, some available phosphorus and organic carbon.
• Cassava infected by CWB and grown in soil with low amounts of nitrogen and high levels of organic carbon and available phosphorus tend to attract other pests, such as long-tailed or papaya mealybugs.
• Though fertilizer addition speeds up the development of both cassava mealybug and the lopezi wasp, the action of the latter also hinges on field management, the surrounding landscape, or crop variety, which were not taken into consideration.

Given the findings, Wyckchuys sees value in selecting and conserving natural enemies, such as lady beetles, lacewings or other parasitic wasps, which could work well in soils with low fertility.

A second study, carried out in Cambodia and published in PLOS ONE, confirms the link between cassava witches’ broom infection and the surge in numbers of papaya mealybugs in cassava fields. As such, beyond changing the appearance and affecting productivity of the plant, CWB infection also modifies the entire insect community surrounding cassava. The researchers thus dubbed CWB as “an ecosystem engineer.”

As to why that happens, the scientists could only postulate the likely effects of a plant’s weakened immune system and modifications in specific traits, such as leafiness, nutritional composition, or production of chemical compounds that attract certain insects.

“These results are quite exciting,” Wyckhuys said. “They also further underline that pest management has to be inclusive, and entomologists cannot operate in isolation. They need to engage with soil scientists and plant pathologists, because unless they take into account the disease status of the plant or the composition of soil, their work only sheds light on some aspects of a pest problem.”

The research, he added, shows how smallholders in Asia, who often grow diseased cassava plants on depleted soils, are more likely to face mealybug problems.

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Additional information

The paper, Soil fertility regulates invasive herbivore performance and top-down control in tropical agroecosystems of Southeast Asia is published in Agriculture, Ecosystems & Environment. The study was part of a European Commission-funded program managed by the International Fund for Agricultural Development (IFAD), and carried out in collaboration with Sok Sophearith, Lao Thao, Dr. Nami Minato, Thort Chuong and David Pick, along with experts from the provincial office of Vietnam’s Plant Protection Department and district-level forestry and agriculture offices in Cambodia, Laos and Vietnam.

The paper, Phytoplasma infection of a tropical root crop triggers bottom-up cascades by favoring generalist over specialist herbivores is published in PLOS ONE and was funded by a European Union-IFAD grant and the CGIAR Research Program (CRP) on Roots, Tubers and Banana (RTB). We’d like to acknowledge the assistance of Kui Hout, Leang Seng and Kim Sophal from the Cambodian Provincial Department of Agriculture; Sok Sophearith; Dr. Michael Gates of the U.S. Department of Agriculture; and Theresa Cira from the University of Minnesota.

The two studies are part of a five-part research series examining cassava crop protection in Southeast Asia through different lenses, embracing disciplines such as landscape ecology, plant pathology, agronomy, soil science and even agricultural anthropology.