There’s been some discussion in the columns of Nature recently concerning the possibility of eradication of mosquitoes. In an article entitled A World Without Mosquitoes, Janet Fang (1) presents a variety of opinions whether large-scale mosquito eradication is practical and if so whether such a course of action is desirable. In the latest issue of Nature, a collection of four letters revisit the controversy.
So, there are two issues here: Can we?, and should we?
Some years ago, I had a brief foray in to the world of the Anopheles gambiae species complex, an experience which was quite novel for me as my experiences had been with a very different and more docile fly, Drosophila melanogaster. And for several years I maintained a colony of A. gambiae in my laboratory, which while not in itself particularly difficult was certainly quite arduous and one strain required considerably more commitment and care than keeping several hundred Drosophila strains. At that time, there was quite a deal of excitement about the prospects of driving transgenes through wild populations, using as a model the spread of P-elements through wild populations of D. melanogaster during the 20th century. There have always been concerns about the extent to which the entities known as ‘species’ really are homogeneous populations with consistent gene flow through them across their range: certainly within the A. gambiae species complex, there appear to be ‘incipient species’ which represent fairly hefty gene flow barriers within the individual sibling species of the complex.
One caveat would be that even if one could eliminate an entire species, or a species complex, what would fill the vacant niche? A. gambiae may be the most significant malaria vector in sub-Saharan Africa, but there are others, that I guess could move in an colonise a vacant niche. An argument for species control by transgenic means might be for eliminating a species that’s newly colonised areas in which they haven’t formerly been present: a good case might be if A. gambiae were to be introduced into South America. More subtle interventions might be to introduce transgenes that impact on malaria parasite propagation within the mosquito: the downside would probably be that given the huge scale of the parasite population out there, somewhere there is likely to be parasites that can evade the introduced transgene.
Sterile male release can be effective in controlling insect pests, but is really most effective where the female generally mates only once. Drawbacks can include fitness of the males, either from irradiation used to sterilise them or because the laboratory-bred strains are uncompetitive in the wild. Sophisticated technologies exist by which terile males can be made, so this may not be so major a hurdle in the future.
Other, more traditional, methods of insect control are really very blunt instruments such as insecticide usage. Obvious drawbacks would be collateral damage to other insect species, with the potential for massive ecological damage. There are also health implications. In the past, mosquito control by application of DDT was quite effective in Sri Lanka, then Ceylon, until over-enthusiatic and inappropriate use of DDT caused the appearance of DDT-resistant mosquitoes. In the intervening years, malaria incidence had dropped, with the consequence that the resurgence of the disease as mosquito populations recovered was particularly severe (2). I would be surprised if global eradication by insecticide would ever be approved. And I cannot see a means by which wholesale eradication of many different mosquito species could be undertaken. Fang’s article provides a concise overview of the eradication/control techniques that have been tried in the past, and some that may be options in the future.
In the context of should we eradicate mosquitoes, I’m really less well informed about the ecological implications of such a course. Much of the online discussion seems to revolve around the ecological consequences of eliminating all mosquitoes, across all ecosystems, even in sub-Arctic tundra regions. It would not be surprising to me if the ecological effects would be severe, but it also occurs to me that those consequences might well be unrecoverable. At the end of the article, Fang concludes that in her penultimate paragraph that:
Given the huge humanitarian and economic consequences of mosquito-spread disease, few scientists would suggest that the costs of an increased human population would outweigh the benefits of a healthier one. And the ‘collateral damage’ felt elsewhere in ecosystems doesn’t buy much sympathy either. The romantic notion of every creature having a vital place in nature may not be enough to plead the mosquito’s case. It is the limitations of mosquito-killing methods, not the limitations of intent, that make a world without mosquitoes unlikely.
The conclusions seems to be that (a) we couldn’t do it, but that (b) if we could, we should, and it wouldn’t make much difference ecologically. Well, actually, I’d rather not try the ecological experiment!
And anyway, would the world really be better without the beauty pictured above, a species in the genus Sabethes? Sabethes species are often regarded as the “butterflies” of the mosquito world, frequently bearing brightly coloured or iridescent paddles on their legs.
Here’s another image (left), this time of a couple of Sabethes cyaneus, from the BBC website (click on the image for the full-size version).
Finally, I’ve scanned in a plate (below) from F. V. Theobald’s Monograph of the Culicidae or Mosquitoes, published in 1901 by the British Musem (Natural History) – The Plates volume. These are (from left to right) Megarhinus inornatus (male), M. inornatus (female), M. separatus and M. immisericors.
2. Gordon Harrison (1978) Mosquitoes, Malaria and Man, Chapter 26 Pub John Murray (record at Amazon.com)