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One of the more interesting facts about the biogeography of Hawaii is that mosquitoes were not always present; they only arrived in the early nineteenth century, as stowaways aboard oceangoing ships. The explanation for this is straightforward; Hawaii is not a splinter of a continent; it is a volcanic island that emerged pristine from the ocean, and is thousands of kilometers from the nearest continent; distance protected it, until that protective barrier was breached.

As far as I know, Hawaii is the only such case, i.e. the only nontrivial tropical ecosystem that is known to have been mosquito-free until recently.

Just how much distance is needed to serve as a barrier? I gather mosquitoes can only fly a couple of kilometers normally, but presumably the tail end of the distribution will be much longer thanks to the occasional one being swept up in a freak storm etc.

Or, for concrete numbers: suppose you have two continents, both with wet tropical climates, suitable habitat for mosquitoes, one of which has them, the other not. How wide a stretch of unbroken ocean would suffice for a less than fifty percent probability of mosquitoes reaching and becoming established on the second continent in a million years, assuming no surprising geological events like ice ages, and no intelligent agency?

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    $\begingroup$ There's also the possibility of their being carried on flotsam. A large enough piece of it could have freshwater pools in it. $\endgroup$
    – Mary
    Apr 30 at 0:41
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    $\begingroup$ They're light enough that potentially the whole way if caught in strong winds is not unreasonable, then there's the potential for eggs or larvae to be carried on the feet or body of migratory birds, so if you're looking for the furthest plausible distance for a piece of fiction it's pretty much 'as far as you want', add human dispersal agents like boats to aid transfer and as you observed yourself by mentioning Hawaii there's basically no limit. $\endgroup$
    – Pelinore
    Apr 30 at 6:51
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    $\begingroup$ @Pelinore I'm actually looking for the reverse. I'm trying to find out how close together I can put the two continents, and still have it be reasonable that the second one is still mosquito-free (prior to the invention of boats). $\endgroup$
    – rwallace
    Apr 30 at 16:21
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    $\begingroup$ @Pelinore - there used to be loads of malaria in Europe. Even Britain. ncbi.nlm.nih.gov/pmc/articles/PMC8277918 Italy for sure. Thalassemia is a disease caused by an hemoglobin variant endemic in southern europe that protects against malaria. T $\endgroup$
    – Willk
    Apr 30 at 19:10
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    $\begingroup$ @Willk OK, so it was found in "southern Europe" 🤔 so, the warmer parts then 🤨 so, my point still stands then, colder climates reduce its prevalence tending toward zero as the temperature drops. $\endgroup$
    – Pelinore
    May 1 at 6:07

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Mosquitos can travel indefinitely if caught in a storm (and not killed for some reason or other). Females can become dormant and survive for 6 months in that state.

But plenty of Pacific Islands didn't have them until recently, Hawaii is much closer to a continent than most of them. The Pacific is almost all volcanic Islands that rose from the sea. So it's not a common occurrence.

The problem is not so much how far they can disperse, but how far they can disperse and breed. A pregnant female only lives a few days. Males only live about a week. So even if a female makes it to an Island, quite possibly it won't find anything to eat in time. Or it lay its eggs, but they don't find anything to feed on.

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Iceland style?

Iceland does not have mosquitoes. They are not afraid to brag about it. Clearly Iceland is close enough for mosquitoes to blow there but they cant get established. Colder places like Greenland have mosquitoes.

Iceland likes to credit some circumstance of their weather. But I think the secret is midges. These flies (biting and nonbiting) occupy the same niche as mosquitoes and are very well established in Iceland. I think the midges outcompete mosquitoes in the larval stage. Also midge larvae in some places where the two flies coexist are carnivorous and eat larval mosquitoes.

Have your mosquito free continent be home to a generous variety of midges!

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    $\begingroup$ Not exactly what I'm hoping for – I'm looking to write descriptions of protagonists marveling that they are not plagued by swarms of biting insects :) – but upvoted for something interesting that I wasn't previously aware of! $\endgroup$
    – rwallace
    Apr 30 at 21:29
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    $\begingroup$ That having been said, Ireland is also mosquito-free, and we don't have midges either, so I'm not sure it's not the weather. I used to think mosquitoes were a tropical pest; took a long time to realize they actually occur nearly everywhere in the world, and Ireland is an unusual exception. I still don't understand the reasons; why do Alaska and Siberia get so many? And if they do, why not Ireland? $\endgroup$
    – rwallace
    Apr 30 at 21:31
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    $\begingroup$ But Iceland is also separated by substantial ocean distance from the nearest continent, and was not land-bridged during the Ice Age. Could isolation be a contributing factor? $\endgroup$
    – rwallace
    Apr 30 at 21:32
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    $\begingroup$ @rwallace - I did not know about Ireland and mosquitoes. Ireland and Iceland are relatively close. That makes me think even more that it is something endemic to the area that is keeping mosquitoes at bay. $\endgroup$
    – Willk
    Apr 30 at 21:37

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