Population Explosions

Ever find yourself entirely inundated with caterpillars, like reader Liz E. from Alabama? Suddenly and as if overnight, the ground is crawling with them and they are raining down from the trees. This kind of population explosion can often be seen in species like the eastern tent caterpillar (Malacosoma americanum) and other moth species but also with several species of butterflies and other insects like cicadas, ladybugs, locusts and cockroaches.

So, now we are on to the fascinating topic of insect population dynamics!

I imagine that many people just clicked their browser window shut after reading that sentence so if you are still reading, you must be interested in the topic or you are currently knee deep in such an explosion of an insect population and want to know how it may play out. So, let's get on with the science!

Population limiting factors: Usually a population of insects is limited by certain factors: parasites, predators, disease, availability of food and environmental conditions. At all stages of their life moths and butterflies are preyed upon, infected and parasitized which decreases the population. Many of these factors (parasites, predators, disease, availability of food ) tend to increase with the population and are called density dependent factors. If there are only a few moth caterpillars, they are hard for predators to find. If there are thousands of caterpillars, predators like birds can find them with ease and may even flock to an area to consume them with abandon. Environmental conditions are not related to population density and can be considered population density independent factors.

A regular life cycle: Let’s assume that a female moth lays 100 eggs, like the Imperial Moth eggs pictured here. Some of those eggs will likely be infected by a parasite and will never hatch. The remaining caterpillars will be subject to parasites, predators, disease and environmental factors like adverse weather conditions. Additional hardships can be found if the food source for the larvae is scarce. What caterpillars survive to pupation will again be subject to these limiting factors which will leave far fewer adult moths than the original number of eggs. If 10% of the original 100 survive that leaves us with 10 adult moths. If half of those are female and happen to find a mate before being eaten, they will lay more eggs and the cycle will continue.

With each generation the population will grow slightly to moderately, unless one of the limiting factors is lessened. Let us say, for example, that a late frost killed many amphibians and reptiles that would normally prey upon the caterpillars. This suddenly allows many more caterpillars to survive through pupation to adulthood. With more adults there will be more females which can lay more eggs. The next generation of the population will be much larger than usual.

Boom: The lessening of any regularly found limiting factor or even an independent event such as an early mild weather that increases host plants can cause an insect population to increase rapidly. Suddenly, or so it will seem, the world around you will be swarming with insects that cover the ground and greedily consume their hosts, sometimes denuding trees or even sections of forest. With each successive generation the population will increase more and more until either a density dependent factor starts to level out the population explosion event or a density independent factor strikes.

Want to crunch the numbers? I found a great online insect population simulation that was constructed by Hein Bijlmakers. This simulation allows you to control limiting factors and their impact upon your simulated population. Using the pre-set simulation factors and starting with a first generation of just 1000 eggs, the graph shows the simulated increases in the population that can lead to a population explosion. Each generation experiences a near exponential of growth and the population quickly gets out of hand.

Eventually, as with most population explosions, some limiting factor either dependent upon or independent of population density will cause the explosion to collapse.

To learn a little more about populations and the factors that influence them, check out this great AP Biology presentation about ecology and populations put together by a science teacher and his wife from Dublin Scotio High School in Dublin, Ohio. Thanks to Aaron and Candice MacKenzie for having such fine information available on the internet.

Examples of Population Explosion:

Butterflies on Navarro Ridge: In 2005, Navarro Ridge in Mendocino County, California experienced a population boom of the California Tortoise Shell (Nymphalis californica) butterfly. The larvae denuded host plants and pupa were so think on branches that bears began to knock branches over and prey upon the pupa. Imagine that: enough butterfly chrysalis that bears can make a meal. The following spring of 2006 was wet and cold and there were far fewer butterflies to be found. Read more about Navarro Ridge here.

Ladybugs in England: This British invasion of tiny beetles happened in the summer of 2009 when hoards of ladybugs descended upon Somerset and Norfolk England. The ladybugs were feasting upon their favorite food, aphids which were particularly thick before the ladybug population explosion. In their hurry to find food, ladybugs also blanketed yellow objects like cars and posts until they determined that color and flavor didn’t always match.

Cockroaches and Loathing in Las Vegas: Las Vegas, Nevada is currently experiencing a population boom of cockroaches. A particularly wet and mild winter in 2009 did not produce weather cold enough to kill existing cockroaches and their eggs. Current hot temperatures in the area are causing the now much larger population to become very active as the cockroaches move inside and try to seek shelter in moist and dark places like utility boxes and homes of residents.

In summary: When a major environmental event like a hurricane occurs or we just get a really wet spring, we can pretty much lay money on the mosquito population in Florida goings nuts. Every tiny factor in the web of life has to be perfectly balanced for populations to achieve mutual balance. Nature is awesome, but rarely so perfect as to keep precise order. Fluctuations and occasional population booms have always occurred in the incredible chaos of our Earth. Although these population booms can sometimes seem truly biblical in size, they generally play themselves out in short order. Until then, just watch where you step…