Dance of life and death on coffee plantation focus of UT researcher

September 21, 2010 | Research, UToday
By Cynthia Nowak

Life, even at the nearly microscopic level, is all about relationships. Plants, insects and fungi in every sort of ecosystem have developed over time complex, mutually beneficial interactions that allow delicate coexistence.

Dr. Stacy Philpott showed off insect samples.

Dr. Stacy Philpott showed off insect samples.

Enter the human, bearing chemicals. The pesticide/herbicide/chemical fertilizer revolution of the last 60 years has in many cases overridden ecological relationships in the effort to blitz agricultural pests.

Which approach works best? The question is central to a study published in the July/August issue of the journal BioScience. Dr. Stacy Philpott, UT assistant professor of environmental sciences, has been researching an organic coffee farm in Mexico for some 10 years, along with scientific colleagues from the University of Michigan, Dr. John Vandermeer and Dr. Ivette Perfecto. Their findings give a strong nod to natural relationships.

What their research uncovered is an intricate dance of interdependence between an unlikely set of partners: a feisty ant species (Azteca instabilis); the noisome green coffee scale insect; and the predatory lady beetle. All three — plus some potential players waiting for a cue — play critical roles in bringing the coffee crop successfully to market.

The Azteca ants live in trees that shade the human-size coffee shrubs, but regularly interact with the green coffee scale insects that are a major pest of coffee crops. On the organic farm under study, however, ants and scales form a relationship in which the ants protect the scales from predators and parasites. In return, the scales secrete a sweet liquid, honeydew, that’s eagerly taken by the ants.

Azteca ants tend to the green coffee scale insects.

Azteca ants tend to the green coffee scale insects.

The symbiotic interaction, though, is made more complex by a predatory lady beetle: Both adults and larvae feed on coffee scales. Azteca ants can protect scales by fending off adult beetles, but can’t get past the waxy substance covering the larvae. Thus falls many a coffee scale.

The ants’ success at repelling another scale enemy, a parasitic wasp, inadvertently chases away other wasps that attack beetle larvae, adding to the system’s complexity.

The ants have their own enemy: a parasitic fly that can limit their presence in the ecosystem. Likewise, the lady beetles can make an impact on ant numbers by preying on the scales and limiting the amount of available honeydew. Using simulation models, the research team discovered how the highly patterned dance might end: If ants are widespread over the entire farm, lady beetles disappear because the adults can’t secure sufficient food. Without ants, though, the beetles also are doomed because their larvae are killed by the parasitic wasps.

Perfect balance is achieved when the ants are limited by beetles and parasitic flies. Both ants and beetles thrive, the latter keeping the crop-damaging scale insects under control.

But wait — scale insects also can be attacked and killed by white halo fungus, naturally limiting the ant population in isolated patches of the coffee farm. That same fungus, though, is an enemy of coffee rust, a disease that in the past wiped out entire coffee-growing regions. The rust exists in Central and South America; white halo fungus is a powerful rust eradicator only in places where it’s already mounting a major attack on scales — places most likely to be where the indefatigable Azteca ants are protecting their honeydew-producing scales.

The complexity of the relationships on the successful coffee farm in southern Mexico wouldn’t have become clear without close research, Philpott said. “Studying these interactions is important for understanding how ecosystems work, especially how agricultural systems work,” she added. “Industrial agriculture is largely aimed at the target pests — controlling an insect or fungal disease by applying something. It wreaks havoc on biodiversity, and causes loss of habitat, contamination and related health problems.

“One solution to these problems is looking at this extremely complicated agricultural system that has so many interlinking components and asking how we can achieve natural disease- and pest-control.”

The research paper represents 10 to 12 years of work, she said, for the scientific team, their students and interns who’ve been making yearly visits to the farm in Chiapas. It’s also been a shared educational experience for local farm workers who have years of experience in handling the coffee seedlings. “They began to interact with us and learn more about the biology of the ecosystem, and we’ve been doing educational activities with the children of the 30 families who live on the farm,” Philpott said. “Although there are very few educational opportunities in Chiapas, the families see these organisms daily and have an intuitive understanding of the relationships between them.

“We describe those relationships from a scientific standpoint. It’s interesting to see how the knowledge from their standpoint and ours often reaches the same conclusions.”

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