One of the questions we have the hardest time answering is Where are all the (insert insect of interest) this year? The simple answer is – we don't know. The longer answer is that there is a long history of research in entomology trying to predict insect populations and population cycles. Most of the research has focused on pest insects that eat our crops or defoliate trees. There is a multitude of factors that contribute to insect population cycles. I am not kidding when I say there is a great deal of disagreement among entomologists on whether it is abiotic factors (weather, environment) or biotic factors (predators, parasites and disease) that regulate insect populations more.
Suffice to say, when someone asks me, "Why haven't I been seeing as many monarchs this year?" I start to dither a bit then I mumble about weather, predators, overwintering populations and milkweed selection by adult females. It is easy to make generalizations like – the wet weather was bad for monarchs, but if there is no research then we do not know. Perhaps the wet weather was worse for a predator or parasite and thus it was better for monarch larval survival. Often we do not know. Even in some pest insect systems where we have large sets of data, we still cannot accurately predict populations in the future. We tell farmers what we expect, but always say to go out to the fields and look.
It is great that people care about insects and are worried when we see fewer of our favorites. In the case of the monarch there are many people that care and there is systematic monitoring of the population size. Monitoring populations over many locations and many years is the best way to pick up changes. Seeing fewer monarchs in your yard over one summer is not a cause for concern, but if many people over multiple years start seeing fewer monarchs then there could be a problem.
So what about monarch butterflies in Iowa this year? We don't know. (Surprise!) Casual observation and questions from friends and the public would lead you to believe there are fewer-to-none flying in the state. Or that they are late because of the weather. The Monarch Watch (mentioned below) has a blog where this is addressed. Their estimate is that this season's population is similar to those of the past few years. See the Monarch Watch blog for July 21, 2009.
If you are interested in helping monitor monarchs you should check into the many groups that are involved in conserving this species. A few that I am familiar with are: