“Winter is coming.” This is the motto of House Stark, one of the dynastic noble families in television’s popular Game of Thrones series. It is both a warning to the clan’s many enemies and a reminder to be prepared for both the metaphorical winter seasons of our lives, when everything is going badly, and literal winter when the world is cold and covered in ice.
“Winter is coming” is also the message received by insects in temperate climates—right about now in the Northern Hemisphere—when days get shorter and the quality and quantity of food declines. These changes signal that the environment will soon be inhospitable, and they prompt animals of all types to begin preparing for this bleak season. Some migrate to warmer climes, but others shelter in place. For many of those that stay put, seasonal changes mean it is time to enter a state of suspended animation. For mammals, like ground squirrels, skunks and bats (oh my!) this means preparing for hibernation. And for many, but not all, insects these same cues indicate it is time to enter diapause.
Insect diapause is a lot like hibernation, but there are some differences. The main difference is that mammals remain active until winter is well established and temperatures are frigid, but insects that enter diapause become dormant in the autumn well before it gets too cold for them to function. Another difference is that mammals can sometimes rouse themselves for short periods of activity during the winter, but once an insect enters diapause it will remain dormant until spring even though the weather might be suitable for normal growth and development.
As a comparative physiologist, I am fascinated by insect diapause. How insects are able effectively to turn themselves off and still preserve tissues and materials needed to turn themselves back on is an interesting puzzle. And understanding the processes and mechanisms that regulate diapause in insects has the potential to benefit society.
When insects enter diapause, they stop developing and spend a prolonged period in a single developmental stage. All insects start as embryos inside an egg and proceed through their life cycle becoming larvae, pupae (when metamorphosis occurs) and finally adults. As a group, insects can enter diapause at any stage, but the stage when each species enters diapause is programmed in their genes. The Asian tiger mosquito, Aedes albopictus, which is involved in spreading diseases like dengue fever, and the silkworm, Bombyx mori, which is important for silk production, both enter diapause as embryos. The corn earworm, Helicoverpa zea, which can put a damper on your summer barbecue if you find one in your ear of corn, enters diapause in the pupal stage. The mosquito Culex pipiens, which spreads West Nile virus, enters diapause during the adult stage; and it is only the females that can survive the winter.
In addition to developmental arrest, entering diapause includes a reduction in metabolic rate that is sometimes so extreme that oxygen consumption is almost undetectable. This, together with arrested development, reduces the insect’s need burn the stores of fats and carbohydrates that are built up before they become dormant. This makes fuel stores last longer and insures there is enough gas in the tank, so to speak, when it is time to restart normal activities in the spring. The energy saved can also be channeled toward producing proteins, sugar alcohols and other substances that act like antifreeze and keep delicate membranes and other structures in the body from being damaged by cold, dehydration or insufficient oxygen.
What we know about diapause comes from the accumulated efforts of scientists who have been studying it since at least 1937 when they first gave the phenomenon its name. In the beginning, studies were descriptive and focused on identifying seasonal cues and the stage of development when each species enters diapause. Since then, studies on diapause have progressed along with our general knowledge of physiology, biochemistry and molecular biology.
Knowledge of genome structure and function has led to some significant breakthroughs in our understanding of the specific genes or gene networks that are turned on and off before, during and after diapause. We know, for example, that changes in how insulin is produced and used is important for diapause. We’ve also learned that changes in the regulation of proteins that govern the circadian clock (the internal system that measures day length) are important for deciding if it is time to prepare for diapause.
My current research is in investigating even finer details about how diapause is controlled. I want to know how microRNAs (tiny RNA fragments that interfere with protein synthesis) might turn specific genes on and off before, during or after diapause. I’m looking at genes that are involved with overseeing metabolism, the timing of specific developmental events, and initiating pathways that will reduce the effects of environmental stress.
Fully understanding how insects overwinter has the potential to benefit society in multiple ways. Scientists like me are using what we know about diapause to try to figure out new ways to control insect pests like mosquitoes and corn earworms. Studying diapause can help scientists more accurately predict the time and place when outbreaks of disease-spreading mosquitoes might occur.
This will help to pinpoint the best time and location to apply mosquito-killing pesticides and make the use of these chemicals more efficient and safer. We are also trying figure out ways to control agricultural pests, like the corn earworm, by interfering with their ability to become or stay dormant. If we can keep them from entering diapause or trick them into awakening too early, these insects would be killed by ice and snow and fewer will be eating our corn next summer.
So take heed, insect pests. Winter is coming.