Ecological relationships between Aedes albopictus and Nepenthes spp. from Peninsular Malaysia

Abstract

I investigated the possibility of Aedes using Nepenthes as a breeding site, indicating the ecological importance of Nepenthes to human health, since the Aedes mosquito that is an important vector for the dengue virus has overlapping distributional range with the pitcher plant. Nepenthes traps and digests arthropods using jug-shaped leaf appendages to acquire essential nutrients but at the same time serves as a habitat for specialised arthropods such as Toxorhynchites acaudatus. There have been reports of Aedes larvae in pitcher plants; limited to older and senescent yet functional pitchers, suggesting possible larvicidal effects associated with pitcher environment against Aedes larvae. In this study, I investigated the larval survivorship of Aedes albopictus larvae in three lowland Nepenthes species, N. ampullaria, N. gracilis and N. mirabilis, and possible deterrent effects of N. ampullaria against gravid mosquitoes of Aedes. Food webs of 118 pitchers were compared, along with pitcher dimensions, fluid and environmental parameters. Taxon composition of macroscopic inhabitants was different between the Nepenthes species as a possible function of pitcher dimensions and fluid pH/conductivity. Without taking into account species effects, canopy closure was the most important factor that affected the distribution of inhabitants in N. ampullaria pitchers; a separate analysis indicated that the predators (Tx. acaudatus and Corethrella calathicola) were rarely found in N. ampullaria pitchers at areas without canopy trees. This suggests that N. ampullaria pitchers that are growing at exposed areas have lower probability of being colonized by predators. In the absence of predators, Aedes may colonize pitcher plants, indicating possible epidemiological significance of urban pitcher plants. It is nonetheless noteworthy that none of the pitchers collected contained Aedes larvae which further confirmed the rarity of Aedes in this phytotelm. There are two reasons that can possibly explain the rare occurrence of Ae. albopictus larvae in pitcher plants – 1. the survival rates of Ae. albopictus larvae in pitcher plants are extremely low; 2. gravid mosquitoes of Ae. albopictus avoid laying eggs in pitcher plants. I tested the larval survivorship of Ae. albopictus in various experimental settings that addressed the potential risks of predation, low fluid pH, presence of microbes and enzymes, and larvicidal chemical compounds on Ae. albopictus larvae. It was observed that predators (Tx. acaudatus) preferentially fed on Ae. albopictus larvae in the presence of other mosquito species. Aedes albopictus larvae had difficulties adapting to the harsh pitcher environments of N. gracilis and N. mirabilis such as low fluid pH and presence of potentially larvicidal microbes that are non-native since the open pitchers of N. mirabilis had a different set of microbes compared to the unopened pitchers. It is unknown however which of these microbes had larvicidal effects towards Aedes. Nepenthes ampullaria pitcher fluids were generally harmless to Aedes larvae but none of the functional pitchers contained Aedes larvae, suggesting avoidance behaviour of gravid Aedes towards N. ampullaria pitchers. Plastic ovitraps which were of different sizes and colours (black and green) were used to test the hypothesis that the dimensions and appearance of N. ampullaria pitchers repelled gravid mosquitoes. Only the largest black ovitraps had significantly higher egg counts while green ovitraps effectively deterred gravid mosquitoes, suggesting that colour could be a major factor that affected the oviposition decisions of mosquitoes. In addition, pitcher fluids of N. ampullaria had no effects against gravid mosquitoes since similar egg counts were observed in ovitraps containing tap water. Although it is uncommon for Ae. albopictus to breed in pitcher plants, it is advisable for avid growers to remove senescent or non-functional pitchers in order to prevent vector mosquitoes from developing behavioural adaptations towards these microhabitats at the urban areas. Management of pitcher plants in the wild would be more difficult in the sense that removing dead pitchers from each plant is impractical. However, as predators such as Tx. acaudatus and C. corethrella are more prevalent in the natural habitats, the chances of Aedes larvae surviving in these pitcher plants are expected to be low.