Into the woods: Impact of insertion of oil palm plantations on the diversity, abundance, and distribution of mosquito vectors of sylvatic dengue

Background/Question/Methods: Land cover change may accelerate the emergence of vector-borne disease via effects on vector distribution, abundance, diversity, and/or contact rates with hosts. The ancestors of human-endemic dengue virus circulate between non-human primates and sylvatic Aedes mosquitoes in the forests of Borneo. Novel sylvatic viruses spill over to infect humans living in proximity to sylvatic transmission foci where forests are being cleared, primarily for conversion to oil palm plantations. We hypothesized that this land cover change would alter the abundance and distribution of mosquitoes due to the well-documented effects of plantation agriculture on microclimate and vertebrate host diversity. We tested four predictions: (i) mosquito diversity and abundance would decline in oil palm plantations; (ii) mosquito diversity would be greatest at the forest edge iii) the dengue virus vector Aedes albopictus, an opportunist species, would occur in equal abundance in plantations and forest, (iv) a vector of sylvatic dengue virus, Ae. niveus, a canopy mosquito, would be most abundant in forests. Adult mosquitoes were collected at 340 sites at 4 distances from forest edge into oil palm plantations (10, 20, 50, 100 m), and 6 distances from forest edge into forests (0m, 10, 20, 50, 100, 500 m) in Sarawak, Malaysian Borneo.

Results/Conclusions: A total of 894 mosquitoes comprising 25 genera were collected. Mosquito richness and evenness were greater in forests compared to oil palm, although Aedes species, particularly Ae. albopictus, were enriched with distance into oil palm. Mosquito diversity was highest at 100m into the forest, contra to our prediction. The mean number of mosquitoes collected per site differed significantly among the distances sampled, collectively supporting our first prediction. We found that our sampling effort sufficiently explained the richness of species at the distances sampled, further supporting an increase in mosquito diversity with distance into forest. No significant difference in the mean number of Ae. albopictus sampled at any distance was detected, suggesting that, as we predicted, this species occurs evenly in both oil palm and forest land cover types. Ae. niveus were rarely collected; however, all specimens were collected at 10m or deeper into forests, lending limited support to our fourth prediction. These data indicate that spillover of sylvatic dengue is most likely to occur in forests where non-human primate hosts are present and the two potential vectors, Ae. albopictus and Ae. niveus, are abundant, but may also occur in oil palm agriculture due to its high suitability for Ae. albopictus.