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Mind-controlling fungus could cause more crop damage in warmer climates
The combination of a common fungal disease with climate change on ladybirds could cause aphids to become more of a problem.
Ladybirds infected with the fungus Beauveria bassiana were less tolerant of extreme temperatures than their aphid prey, which could see serious crop damage increase across the world.
Fungal infections manipulate insects so that they stay in temperatures that are just right for them, researchers have found.
An international team of scientists found that aphids and ladybirds infected by fungi were less able to deal with high and low temperatures. This could influence how the insects behave as the climate changes, with the potential to impact both human agriculture and natural ecosystems.
Professor Edward Rajotte, a co-author of the study, says, 'This is the first time, to our knowledge, that infection has been observed to shift the behavioural response of both a predator and prey in ways that reduce exposure to heat.
'As the effects of climate change on insects are poorly understood, our research suggests that we cannot expect to understand how an organism will respond to environmental changes by studying the insect species alone. We need to also consider their pathogens.'
The research, led by scientists at Pennsylvania State University, was published in Scientific Reports.
Fatal fungi
Alongside bacteria and viruses, fungi are one of the main disease-causing groups of organisms. Of the millions of species of fungi, around 8,000 are known to cause diseases in plants while 300 can infect humans.
Fungal infections can have devastating impacts. For instance, 95% of European ash trees are threatened by ash dieback, which is believed to have arrived on the continent as just one fruiting body. Meanwhile, almost 10% of the world's most important agricultural crops are lost annually to fungal plant pathogens.
Animals aren't immune to fungal infections either. Chytridiomycosis is a fungal disease which infects most of the world's amphibians and has caused over 40% of species to be lost in some areas of Central America.
Beyond simply killing infected animals, fungi can also have more unusual effects. Ophiocordyceps unilateralis infects carpenter ants and affects their behaviour, causing them to carry it to a location with a suitable climate for the fungus.
It is this demand for a suitable climate that limits many fungi from spreading across the world, with species preferring a narrow range of temperatures. In this study, the researchers wanted to find out if, and how, their temperature preference would influence that of their insect host.
Ladybirds were tested on their ability to cross areas of extreme heat and cold. Image © Pennsylvania State University
Dead heat
For the new study, aphids and ladybirds were separated into separate groups, with one group remaining healthy while the others were infected with either a small or large amount of Beauveria bassiana spores. The researchers then tested the temperature at which the insects were unable to flip themselves over as a measure of their heat tolerance.
Dr Mitzy Porras, the lead author of the study, says, 'We found that the heat tolerance of fungus-infected aphids and beetles was reduced by 6.7⁰C and 4.5⁰C respectively while cold tolerance was only reduced in the beetles.
'In addition, survival was significantly reduced for infected aphids and beetles when they were exposed to both warm and cold extreme temperatures.'
The researchers suggest that the fungus is blocking a process known as behavioural fevering to help with its own survival. Behavioural fevering is when infected insects move into warmer temperatures to weaken the disease affecting them and boost their immune system.
It is thought that the fungus is taking control of the beetles to stop them from doing this. There is also some evidence that the fungus can prevent insects from controlling their own body temperature more generally so that conditions can be kept optimal.
The scientists also tested if the insects would cross areas of extreme heat or cold to reach food held in containers on the other side. While 65% of healthy aphids and ladybirds would make the journey, a similar amount of heavily infected insects wouldn't make the crossing.
This may be due to the increased energy needed to fight an infection. Even a short trip across an extremely hot or cold area requires a lot of energy to withstand, and the scientists think the insect could be conserving its energy to tackle the fungus instead.
In the long run, this might have a knock-on effect on the survival of crops across the world.
Aphids are normally kept in check by ladybirds, but if these beetles are infected with the fungus and thus reluctant to cross hot or cold areas, greater variations in temperature due to climate change could prevent them from reaching their prey. This could mean aphids become even more of a pest.
Professor Jesper Sørensen, another co-author of the paper, says, 'We have learned that the relationships between pathogens and hosts are far more complex than we once thought.
'This study focuses on a specific case but has broad implications for the way we see these interactions which can now be seen as another important dimension of ecology.'
