Dance of life and death
Plants, insects and fungi in every sort of ecosystem have over time developed complex, mutually beneficial interactions that allow delicate coexistence. The pesticide/herbicide/chemical fertilizer revolution, however, has overridden these ecological relationships.
Natural may be best, however, according to Dr. Stacy Philpott, UT assistant professor of environmental sciences, who’s been researching an organic coffee farm in Mexico. There, an intricate dance of interdependence exists between an unlikely set of partners: a feisty ant species (Azteca instabilis); the 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 ants and scales form a symbiotic relationship in which the ants protect the scales from predators and parasites. In return, the scales secrete a sweet honeydew that’s eagerly taken by the ants. It’s 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 make a dent in the larvae.
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, too: 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 scale and limiting the available honeydew.
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 can also be attacked by the white halo fungus. 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 says. “Studying these interactions is important for understanding how ecosystems work, especially how agricultural systems work,” she adds. “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 forms of disease- and pest-control using complex food webs.”