Killer bug behind coconut plague identified

Old-school taxonomy has solved a case of mistaken identity involving an insect pest threatening the coconut industry in the Philippines, the world’s No. 2 producer, scientists report July 23 in Agricultural and Forest Entomology.

Stopping the pest requires knowing the right perp. When the first of an estimated 1.2 million coconut palms started dying five years ago, authorities in the Philippines initially blamed an insect called Aspidiotus destructor. This pest has caused prior coconut die-offs in Indonesia. But researchers’ detective work led them to conclude that a closely related bug, A. rigidus, has been doing the dirty work.

The true killer A. rigidus, an invasive species, has few native enemies in the Philippines. It also lives 1.5 times as long as the wrongly accused A. destructor. Transferring an A. rigidus predator – a type ofladybug — from its native habitat quelled an outbreak that hopped between Indonesian islands in 2005. The study authors believe this tactic could end the infestation in the Philippines.

Five years ago, flat circular bugs called scale insects began sapping life from coconut trees in the Philippines. As many as 60 million of the insectscan encrust the lower leaf surfaces of a single tree. “The infestation rate was so fast that the local government was caught flat-footed, and poor farmers lacked the resources to cope,” says entomologist Candida Adalla of the University of the Philippines Los Banos College of Agriculture, a coauthor of the study.

Covering 10 percent of the nation’s land area, farmed coconuts are the Philippines’ leading agricultural export and supply half the world’s coconut oil.

Fruit yields have plummeted. If the pest continues unabated, the Philippines could lose 60 percent of its coconut crop, denting its $1 billion in annual export earnings, according to Philippines government estimates.

Insect biosystematist Gillian Watson of the California Department of Food and Agriculture in Sacramento thought A. destructor an unlikely culprit.

“A. destructor has lived in the Philippines for over 100 years and has been controlled by natural predators for a very long time,” says Watson, who led the study. Adalla shipped Watson preserved samples of the insect affecting Philippine coconuts.

By comparing the insects to specimens kept in California and London, Watson learned that A. destructor stood falsely accused. She examined scale insects gathered across Southeast Asia, looking for subtle differences in shape and flexibility.

The correct identification of scale insects almost always depends on morphological details, says evolutionary ecologist Penny Gullan of the Australian National University in Canberra, who wasn’t involved in the study. Limited DNA information exists for the approximately 7,500 scale insect species, making genetic identification impractical, Gullan continues.

Watson discovered that the true culprit wasn’t A. destructor. The insects causing the coconut die-off carry a more rigid cuticle, she found, pointing toward its ID as A. rigidus. In pictures of the scale insect taken in the Philippines, she discovered a crescent shape in the laid eggs, another characteristic of A. rigidus.

Scale insects kill coconut palms by injecting a chemical that destroys the chlorophyll in their leaves, leaving the plant unable to absorb energy from sunlight. Both A. destructor and A. rigidus have caused coconut plagues in Indonesia, although flare-ups involving the latter happen less often, with decades passing between occurrences.

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Editor’s Note: This article was updated on August 19, 2014, to correct the affiliation of Penny Gullan.