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Fewer mosquitoes may be a bad thing

Sibohan de Little

Siobhan de Little checking for super mosquitoes (Source: Lubomir Bisevac/Charles Darwin University )


They say the findings are important for authorities trying to control the spread of mosquito-borne diseases like Ross River fever.

Sibohan de Little of the University of Adelaide presented her PhD research at an international ecology conference in Brisbane last week.

In the first Australian study of its kind, de Little studied the population dynamics of Aedes vigilax mosquitoes, under the supervision of Associate Professor Corey Bradshaw.

In a large field experiment in Darwin, she found direct evidence that the density of mosquitoes affects their size.

"In the higher density plots there were smaller mosquitoes than in the lower density plots," says de Little.

She says a reduction of density from 50 to 5 larvae per square metre resulted in an 8% increase in the size of the female adult.

These bigger females lived 48% longer and lay 67% more eggs, says de Little.

She says the fewer mosquitoes, the bigger they are, because the larvae have less competition for food and can grow into larger adults.

"The bigger the mosquito, the longer it can live and the further it can fly and the more blood meals it might take so therefore it might be a better vector for disease," says de Little.

Control programs

De Little says current mosquito control programs tend to target high density populations of mosquito larvae after spring rainfall or high tides.

"Often mosquito control programs might target high density areas because it's easy to identify those from a survey point of view and it's easy to know you're controlling them," says de Little.

But she says her findings mean authorities must also look for the low densities of larvae.

"It's important for control programs to make an effort to detect these low density larval habitats that might slip under the radar," says de Little.

"Otherwise you might end up with a whole lot of much bigger mosquitoes."

Making matters worse

De Little says the findings also mean that ineffective control reduces the density of a mosquito population resulting in the remaining mosquitoes being larger.

"If you're not controlling 100%, then the ones that are left could be bigger and better vectors," she says.

Given that total control is difficult, the findings suggest it might be best to time the control when the larvae are in late development, says de Little.

The idea is to reduce the population density at a time when the larvae have almost reached their maximum size and there is less possibility they will become 'super mosquitoes'.

"Then the ones that are left will also be small," says de Little.

Tags: ross-river-fever, invertebrates-insects-and-arachnids