Ants throng cacao farms in Indonesia. Where insecticides are absent, thousands of ants patrol the ground and the cacao trees. Some trees host ten or more ant species whereas others are claimed by a few dominant species of large populations.
These trivial differences in ant communities matter to cacao farmers. A study (4 Dec 2013, Proc. Royal Soc. B) reports that ant communities interacting with pests can change cacao yield by 30%.
Lifted by the global demand for chocolate, Indonesia’s cacao production soared in the last 30 years. By 2006, Indonesia was producing almost one-fifth of the world’s cacao but pest attacks and aging trees have plummeted yields since. Trends predict that in 2014, Indonesia will import more cacao beans than it exports.
Worldwide, cacao farmers struggle against a barrage of severe pests which authorities battle to confine. These pests form parts of the complex ecological network in cacao farms and interact with the prevalent group there—the ants.
Farmers often “have a negative attitude towards ants,” says Stacy Philpott of University of California, Santa Cruz, who studies insects in another tree crop, coffee. “There is limited understanding of the ecological roles that ants play.”Arno Wielgoss from University of Göttingen headed a study that examined how ant communities affect cacao yield in Sulawesi, Indonesia. They established four types of ant communities on cacao trees: natural, diverse ant communities; diverse communities with abundant native Dolichoderus ants; communities dominated by abundant Philidris ants (a foreign species brought into Indonesia); no ants. They then recorded cacao yield and damage caused by the region’s three main pests: the fungal-like pathogen Phytophthora, and the insects cocoa pod borer and Helopeltis. The experiment lasted 16 months. Wielgoss found that trees with either natural ant communities or abundant Dolichoderus ants produced the best cacao yield. In contrast, trees without ants or with abundant Philidris ants suffered yield losses of 27% and 34% respectively. Ant interactions with pests explained the yield gap.
“Dolichoderus ants are highly active at cacao pods,” says Wielgoss “and that suppresses cocoa pod borers and Helopeltis.” Dolichoderus however, also protects and encourages lots of mealybugs—soft, white insects that suck plant nutrients and feed sugar excretion to its guardian ants—“that may weaken the tree.” Mealybugs might have prevented trees with abundant Dolichoderus from achieving higher cacao yields despite suffering lower pest damage than trees with natural ant communities.
As natural ant communities improve yields more than Dolichoderus does, Wielgoss suggests that the current laborious use of Dolichoderus ants to control cocoa pod borer in Southeast Asia “may not pay off”. However, Dolichoderus effects could vary, as a Malaysia Cocoa Board officer says that untreated cacao trees produce only half of Dolichoderus-treated trees yields.
Trees with abundant Philidris ants incurred the most Phytophthora infection and the heaviest yield loss. Philidris ants—like Dolichoderus—guard mealybugs, but also collect pieces of rotten
Phytophthora-infected cacao pods to build protective tents over their mealybug herds. Wielgoss showed that Philidris ants and their tent-materials harbour infectious Phytophthora spores with which the ants contaminate fresh cacao pods. The first to confirm that tent-building ants transmit Phyphthora, plant pathologist Harry Evans describes Philidris as “lethal vectors of Phytophthora.”
Wielgoss further warns that insecticide spraying could expedite Philidris dominance as “insecticides harm other ants more than Philidris that are protected in their tents.” The spread of Philidris would likely aggravate Phytophthora infection—and cacao loss.
Despite solid evidence that ants mediate plant-pest interactions, Philpott concedes that “translating scientific results into practice can be difficult.” Farmers face many challenges, and “managing ant diversity may not make it to the agenda.”
However, the assumed trade-offs between managing ant communities and pests are likely misinformed. The intricate links of ecological networks in cacao farms suggest that farmers can also manage pests through managing ant communities.