Friday, 17 August 2012

Pity the poor mosquito!

You are probably, dear reader, a little prejudiced against mosquitoes. Take Aedes scapularis. Whilst it´s true that it is a vector for yellow fever, and  human and Venezuelan equine encephalitis, poor A. scapularis has a hard life. In 2002, Casanova and do Prado studied them in the pastureland around a farm near Campinas, in Sao Paulo state, Brazil.

During the hot rainy season (October to March) muddy puddles form all over the land, and the drought resistant eggs of Aedes scapularis hatch, little larvae swimming off hopefully into the water. It seems a good life in a hot bath full of rich organic matter, but little scapularis does not realise the horrors ahead.

For a start, it might rain, it might not, it might rain a LOT. Aedes only needs 9 days from hatching to adulthood, but even that might be too long. Of the 58 populations studied by Casanova and do Prado, 27 were wiped out by their ponds drying up, and 15 were washed away by floods, they never stood a chance.

Life in the 16 remaining pools was better, but not by much. The good news was that starvation was almost unknown, the bad news that all those mosquitoes ensured that starvation was unknown for their predators too. Estimates of total mortality from tiny larvae to emerging adults varied from 68 to a massive 96% per pool!

Predators included........

Giant water bugs "Baratas d' aqua" (Belostomatidae), aggressively predaceous insects who feed on, well, anything, injecting a digestive saliva and sucking out the remains. One of the most painful bites of any insects to us, let alone to a little mosquito.

Giant Water Bug larva (Wikipedia)

Water scorpions (Nepidae) - actually another type of insect

Diving beetles (Dytiscidae) -  you can eat these, if you want to,  in Mexico the adults are roasted and salted and added to tacos

Dragonfly larvae - there are, incredibly, at least 267 species of dragon- and damsel-fly in Sao Paulo state alone, and many of these eat mosquitoes

Last, but not least, larvae of a predatory mosquito, Psorophora ciliata. Don´t start to feel too grateful to this one, they feed on us as well, and infact are the largest blood feeding species found in the USA.

So, the little larva hatching into a warm, muddy, pool will almost certainly desiccate, be swept away, or be eaten. No wonder they are so angry!

Key-factor analysis of immature stages of Aedes scapularis (Diptera: Culicidae) populations in southeastern Brazil byC. Casanova and A.P. do Prado (Bull. Entomol. Res. 92, 271-7)

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