Predatory drosophilid larvae affect the outcome of ant-plant mutualism

Background/Question/Methods

In non-symbiotic protective ant-plant mutualisms, the plant usually offers a food reward (e.g. extrafloral nectar) that attracts ants, which in turn protect the plant against herbivores. In the Brazilian savanna, Qualea grandiflora (Vochysiaceae) is protected against herbivores by visiting ants that feed on its extrafloral nectaries (EFNs). We found a new dipteran species (Rhinoleucophenga n. sp., Drosophilidae) whose larvae construct a sticky shelter on top of the EFNs of Qualea. Although the shelter partially blocks the EFN, dipteran-occupied glands still produce nectar and are commonly visited by foraging ants. Dipteran larvae use the extrafloral nectar to attract exudate-feeding prey that get trapped to their shelter, most of which nectar-gathering ants. We examined whether the presence of dipteran larvae affects the mutualism between Qualea and ants. We used paired branches (one dipteran-occupied branch vs. one dipteran-free branch) within individual shrubs to investigate potential effects of ant-preying dipterans on ant activity and ant aggressiveness on Qualea. We recorded the time spent by foraging ants on each type of branch, and their aggressive behavior toward simulated herbivores (live termite-baits on leaves) (N=20). Presence of arthropods and foliar herbivory were also monitored over 8 weeks on experimental paired branches of other 33 plants. 

Results/Conclusions

Qualea branches occupied by ant-preying dipteran larvae had ant visitors for less time and in lower numbers than dipteran-free branches. This negative effect on ant foraging activity resulted in decreased levels of ant aggression to live termites on leaves of dipteran-occupied compared to dipteran-free branches. In addition, branches hosting ant-preying larvae had higher numbers of chewing herbivores and higher levels of foliar herbivory than dipteran-free branches. Results suggest that ant predation by Rhinoleucophenga larvae results in decreased levels of ant foraging on EFN-bearing Qualea and decreased ant patrolling activity on foliage. Cascading effects ultimately result in increased herbivore damage to leaves of dipteran-occupied plants. Ant-eating Rhinoleucophenga larvae are thus exploiters of the facultative mutualism between ants and Qualea (Funds: FAPESP, CNPq).