Tanks, Zika and ornamental plants

The plant group known as bromeliads famously includes the commercially important pineapple. However, that ground-growing member of the Bromeliaceae (the more formal name for the pineapple family of flowering plants) is rather atypical because many (most?) of the bromeliads are epiphytes (plants that grow on other plants).

In the wild we may view bromeliads with some amusement when their vases (or ‘tanks’, water-holding structures formed by their tightly overlapping leaf-bases*) harbour cute, colourful rainforest frogs. In urban settings, as ornamentals, bromeliads can add a welcome splash of colour and the exotic to landscaping projects. However, those same tanks are also home to less desirable fauna such as mosquitoes as demonstrated by André Wilke et al..

This team were particularly interested in identifying breeding sites for Aedes aegypti, which mosquito is the main vector for such human diseases as yellow fever, dengue fever, chikungunya, and Zika. The study was conducted in Miami-Dade County in Florida (USA) which had a Zika virus outbreak in 2016. Seemingly, as scientific proof of the biblical saying, “Seek and ye shall find”, Wilke et al. discovered that the tanks of ornamental bromeliads in that administrative district were sites that harboured Ae. aegypti – along with numerous other mosquito species.

Although that study did not establish whether the mosquitoes were carrying any of those diseases, this is a finding of some concern because Zika, for instance, has been linked to microcephaly and other birth defects in unborn babies when pregnant women contract the disease, and yellow fever can be fatal. A thing of beauty – such as a lovely flowering plant – should be a joy forever, but evidently that’s not necessarily true in the case of the bromeliads and their potentially-disease-carrying mosquito passengers. Having now recognised this ‘hidden in plain sight’ strategy of the mosquitoes, what is to be done about it?

Whilst it’s not yet been suggested that the ornamental bromeliads in Miami-Dade County – and by implication similar floral displays wherever else they are used amongst human populations – should be destroyed, the study concludes that “ornamental phytotelmata* bromeliads are to be considered in future vector-control strategies to control Zika and other arboviruses”, and “removal of plants should be considered when needed”.** Or, perhaps this is a reminder that we really should leave plants where nature intended they should grow and visit them, rather than uproot them and translocate them into our homes, etc. where we may risk mosquito-borne infections?

Maybe a route to go down is to see if the capacity for digestive-enzyme-release – as practised by the water-filled pitcher* of such insectivorous plants as those in the genera Nepenthes and Sarracenia – might be ‘introduced’ (e.g. by a genetic modification [GM] approach) into the vase/tank of the ornamental bromeliads. This has the potential to not only solve the mosquito-breeding issue,*** but also to generate some extra nutrients to fuel the bromeliad’s growth. And, in an attempt to put this bromeliad-mosquito work into some kind of perspective, Dr J. Howard Frank, Professor Emeritus, University of Florida (“one of the world’s leading experts in the biology of mosquitoes in bromeliads and author of many papers on the mosquito-bromeliad connection”) has provided “a concise overview of the Zika (and other mosquito borne diseases) scare related to bromeliads”, and a more in-depth account here.

* Technically, such water-retaining structures on non-aquatic plants are termed phytotelmata.

** Whilst this disease-facilitating dimension to mosquitoes is clearly a cause for concern, another mosquito-related issue has been identified by Rana Al-Jaibachi et al.. They demonstrate that larval stages of insects, such as mosquitoes, can take up microplastics and transfer them to the later flying developmental stage of the insect’s life cycle (a process technically known as ‘ontogenetic transference’). Those insects may subsequently be eaten by other flying animals, such as birds or bats, and the plastics moved up the food chain. In that way polluting plastics can be transferred from aquatic habitats to aerial and other terrestrial habitats spreading the plastic pollution problem. For more on this newly-identified pollutant transfer pathway, read Andrea Thompson.

*** For more on controlling mosquitoes in your bromeliads, so you can continue to enjoy them in your own home – safely – see Taber on Bromeliads.info.