If there is time pressure just read the final paragraph.

July 17, 2014

Tovi Lehmann
Laboratory of Malaria and Vector Research
National Institute of Health
Rockville, Maryland

Dear Sir:
I was thrilled by the article referring to you and your stalwart search for mosquitoes hiding out through the African Sahel dry season.  (Emily Sohn “The Great Mosquito Hunt” NATURE vol. 511 no. 7508 July 10, 2014)  The lead vignette recounting the difficulty of persuading airport guards in Mali that a dog being brought in for research purposes was not in fact a wolf reminds me of a story told me by a friend who had worked in Saudi Arabia.  When someone arrived at an airport with a pet English bulldog the guards, who had never seen even a picture, thought it was a pig, and it was with difficulty that a number of helpful strangers dissuaded them from opening up with their machine guns.  Well you can’t believe every story, can you?

When someone has, as you, worked long and hard at some task without great success there are a number of logical moves.  1) Cut your losses and do something else; that works great with making money, but it is harder to give up on taking a hunk out of malaria.  2) Plug away; this is my own personal favorite. 3) Engage in lateral thinking; of course one is always trying to do that.  4) Listen to someone else’s lateral thinking; this is the obvious loser.  Not many of us are any better at giving up an idea than is the average airport guard.  Thus with modest hope I offer what I can; my version of plugging is to try to help.

When the rain comes the mosquito population explodes.  That’s sort of my field.  The mosquitoes arrive too fast to have been hiding as eggs, and larvae are obviously dependent on water.  They must have over-summered as pupae or adults.  A wet adult should be rehydrated within 24 hours, but things don’t happen that fast.  It seems to me that pupae would be the best bet on first principles; the pupa is quiescent anyway, and its surface volume ratio low compared with an adult.  This flies in the face of the best evidence, which is your single adult documented as having survived the months of dry season when a single month is the ordinary bet.  For the moment let us put this momentous discovery with its implicit Lost Cave of the Mosquitoes aside. 

For pupae to develop into adults in two or three days after getting wet seems a fairly small stretch of the imagination.  And that notion comes with the advantage of suggesting where to find them; they should be where there was water.  Of course you have already covered that possibility by placing nets over the cracked mud of dry lake beds and finding nothing emerging with the rain. 

Now a reasonable and prudent person would place the nets in the biggest dry lake.  That is where by and large the water has been most recently, minimizing the length of time the aridity had to be endured. 

On the other hand, explosive growth after a severe reduction in population size is not surprising.  It is standard.  A. J. Nicholson once pointed out that when one sprays for insects the population falls and then rebounds so that there are more than there were originally and the spraying must be repeated.  J. B. Calhoun raised mice starting with four males and four females, which resulted in explosive growth before they lost their fertility and died out.  The Mouse in Biomedical Researchpoints out that mouse plagues in Australia and New Zealand tend to follow drought.  A paper I was part of “Fluctuation of fertility with number in a real insect population and a virtual population
M.L. Herbert & M.G. Lewis African Entomology 21(1): 119–125 (2013) demonstrated among captive fruit flies that basically when a population is big reproduction drops rapidly and when it is small there is explosive growth.  (The hope is that this phenomenon can be harnessed to take a bite out of malaria.) 

Quite simply, if you are looking at explosive growth then in all probability you are looking at growth from a small rather inbred initial population. 

The article about you mentions that it takes thousands of female mosquitoes to establish a population.  Since four females can reliably establish a fruit fly population it must be the case that the dry season not only kills a lot of mosquitoes but is very hard on the survivors.  This again seems on the face of it to suggest setting up nets over the largest ponds.

But what I envision is this: as the rains end the landscape dries out slowly.  The big ponds maintain a relatively high genetic diversity.  Puddles tend to breed locally and have reduced consanguinity.  As this happens – and this is the key point that few indeed will recognize – their fertility rises.  The number of larvae and pupae increases dramatically.  Perhaps (this is just a guess, but would be favored by selection so it is possible) the pupae within a population of optimum size develop an ability to go into aestivation that they lack when consanguinity is more typical.  At all events, compared with the generous expanse of the pond, the narrow puddle is thick with pupae.  Thence will come the next population explosion and there the monsters skulk. 

My suggestion is to set out net tents over dried up puddles of varying sizes in order to find the one that is just right for the mosquitoes.  If you like, let me know and I’ll try to remember to send you a reminder when the right time comes to move.  (I’m old, so have a backup system.)  The best of luck with your noble task.  Let me know if I can do anything.  In fact, you do good work.  I admire you.  Let me send you a gift of two hundred dollars.  That way I’ll know whether you read this far.  


M. Linton Herbert MD

I am pleased to say that I received a prompt and courteous reply from Dr. Lehman who says he shall proceed.  He gently declined my offer of a gift.  I’m so happy to hear from him.  Maybe it will even work, which would be a good thing for lots of people.

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