By Athayde Tonhasca
The body temperature of most insects is approximately the same as that of their surroundings. They are poikilothermic, or ‘cold-blooded’, animals. So the warmer it gets, the more active insects become; they feed more, mate more frequently and lay more eggs.
You can see where this is going. Could global warming be good for insects? To a point, yes. In a warmer planet, diapause (the period of suspended development) will terminate earlier, winter mortality will be lower, spring will come sooner and summers last longer. Most insects will have more time for developing and dispersing.
But before you start singing the Ode to the Coal Power Station, remember that less desirable insects may benefit as well. Numbers of crop pests with high reproductive potential such as aphids and thrips could explode; forests in Scandinavian countries are already facing severe damage by insect outbreaks. Invasive disease-carrying species – affecting humans, livestock and plants – may no longer be killed by the cold weather.
Most insects benefit from higher temperatures, but not all: some, like bumble bees, apparently already live close to their upper levels of tolerance. As temperatures rise, these species are forced to follow shifting cooler habitats by moving towards the poles or to higher altitudes. Indeed, the abundance of many butterflies, beetles, dragonflies, grasshoppers – and plants as well – have contracted at low latitudes and elevations, and increased in more northern and elevated regions. Species incapable of dispersing or which are isolated in pockets of habitats are stuck, and eventually may vanish. In Britain, local populations of cold-adapted butterflies with narrow temperature tolerances have disappeared. As far as we know, no species has gone extinct because of increasing temperatures, but the risk is real.
As the overwhelming majority of reputable climatologists have predicted, the rise of greenhouse gases in the atmosphere is not only turning the world warmer, but also changing precipitation patterns and increasing the frequency of extreme weather events. There will be more and stronger droughts, floods, tornadoes, cyclones and other dangerous, traumatic and costly weather-related phenomena.
Scientists have been looking for possible consequences of these factors to insects, particularly agricultural pests, and they have come up with a maze of scenarios. Plants suffering from water stress during droughts are more vulnerable to damage by herbivore insects, but some of these pests do not survive droughts. A rise in atmospheric CO2 enhances crop photosynthesis thus plant growth and crop production, but it also increases the carbon to nitrogen (C:N) ratio in plant tissue. Insects need nitrogen for protein, so herbivore species have to eat more to compensate for plants’ reduced nitrogen content. But higher C:N ratios slow their development, making them vulnerable to natural enemies for longer. On the other hand, higher temperatures reduce the length of larva and nymph stages, which are the periods of greater exposure to predation. So plant feeders may have a greater chance of escaping natural enemies. The life cycles of plants, the insects that feed on plants, and those insects that feed on the plant feeders, may get out of sync if they respond differently to higher temperatures, with unknown and unpredictable consequences.
Does all of this sound complicated? Well, there’s more.
Diapause is one the most important events in an insect’s life – it allows it to survive unfavourable conditions, which happens during the winter in temperate regions. Diapause is regulated mostly by photoperiod, which is the daily period of light. When the days shorten, insects start to prepare for the onset of winter; when the days lengthen, insects become active again. Photoperiod is a reliable clue because it does not depend on the weather; it follows a stable pattern. The problem is that for many species, photoperiod is not the only trigger for diapause: they may respond to temperature and sometimes to precipitation as well. As we have seen, this is risky in a world changed by climate. The mismatch between temperature and photoperiod cues may induce insects to enter or leave diapause too soon or too late, with potentially disastrous consequences for their development, reproduction, and survival.
We have a vague, unsatisfactory grasp of the effects of climate change to insects, but the few data available tell us that trouble is brewing. And risk to insects means risk for us, since we depend on so many of their ecological services such as maintaining the food chain, decomposition, and recycling of organic matter.
And how about the effect of climate change on pollinators?
We will leave that for next time.
(This is the first in a trio of articles by Athayde looking at climate change, insects and pollinators)