It's a typical summer evening: buzzing bees and chirruping crickets catch the last warm rays before sunset. But how will this idyllic scenario be affected by climate change? Shelley Adamo from Dalhousie University, Canada, says, ‘Insects are poikilotherms, cold-blooded animals, so they are going to have an increase in their metabolic rate and that is going to have a large number of ramifications for ecosystems.’ She explains that one of the consequences of ramping up metabolic rate in temperate zone insects is an increase in reproduction, ‘And the assumption is that global warming is going to increase insect populations,’ she says. However, Adamo was curious: would everything get better for insects in a warmer world? For example, how would an increase in reproductive rate affect their ability to fend off disease? Knowing that heat waves are predicted to become more extreme and occur more frequently with climate change, Adamo and her undergraduate student Maggie Lovett decided to find out how crickets cope with a warm snap (p. 1997).
‘There are two ways their reproductive rate could go,’ says Adamo. Either there could be a trade off and their immune function could decline or, the extra heat could improve the insect's immunity in addition to improving their fertility. ‘No one had asked the question directly before,’ says Adamo, so she and Lovett decided first to find out which temperature crickets prefer.
Offering the insects a choice of temperatures, from a comfortable 26°C to a scorching 38°C, Adamo found that the crickets preferred to settle at 28.1°C. Next, she created a mini heat wave – warming the insects to 33°C for 6 days – and then checked to see how the warmth had affected their fertility. Amazingly, the heat wave females began pumping out eggs, producing up to 66% more than the insects at 26°C. And when Lovett tested the eggs' quality they were every bit as good as those laid by the cooler females: and they developed faster. Even more surprisingly, the females gained weight, despite their colossal productivity.
Next, Lovett tested the crickets' immune function. Collecting haemolymph from the insects, she measured the activity of two key components of the immune response and found that the hot insects had a stronger immune system. Everything appeared to be better for these new super bugs, but how would they cope with a real infection?
Adamo gave the super bugs an injection of bacteria that was strong enough to kill 50% of normal crickets and waited to see how many died. Comparing the death rates of heat wave-treated crickets with those of crickets at normal temperatures, Lovett found that the insects' souped-up immune systems had dramatically improved the crickets' chances of surviving a Serratia marcescens infection, almost halving the death rate from 76.5% at 26°C to 47% at 33°C. However, it was a different matter for the super bugs infected with Bacilus cereus. Their death rate rocketed from 10% at 26°C to over 25% at 33°C. Instead of becoming more resistant, the super bugs had become more susceptible to the pathogen and their fertility suffered also.
Adamo and Lovett had found the super bugs' Achilles' heal. Even though they reproduced better and appeared tougher than normal insects, the additional heat had made them more susceptible to certain pathogens. As well as possibly explaining why the insects prefer to remain at cooler temperatures, Adamo warns that increased susceptibility to specific pathogens could significantly impact future insect populations. While she suspects that some insects will benefit from global warning, others may become more vulnerable, with potentially catastrophic consequences for ecosystems and economies alike.