We've had black fly problems here in the past. In 2013 and 2014, young falcons were stampeded from the box in mid-June, when they were roughly 30 days of age. But that we know, we've never had a serious problem this early. Why are black flies swarming the box now? It is a more complicated answer than you might think.
What does its life cycle look like? From Purdue: "The length of time it takes an egg to hatch varies greatly from species to species. Eggs of most species hatch in 4-30 days, but those of certain species may not hatch for a period of several months or longer. The number of larval stages ranges from 4-9, with 7 being the usual number. The duration of larval development ranges from 1-6 months, depending in part on water temperature and food supply. The life cycle stage that passes though winter is the last stage larva attached underwater to rocks, driftwood, and concrete surfaces such as dams and sides of man-made channels." In short, the eggs for the 2017 hatch were most likely laid in 2016. The larvae emerged somewhere between one month ago or six months ago. When I compared average April temperature and precipitation for the area for every year between 2013 and 2017, I found that April 2017 was the warmest, if not by much.
|Average Temp (F)||Average Precip (inches)|
The complicated structure of black fly life means that we also need to look at the conditions last fall and winter, which were unusually warm and dry. Did more eggs and larva stay local given the lower river current? Did more larva survive given the unusually mild conditions? Did the slightly warmer April weather lead to an earlier season? Did sun and warm temperatures following days of cold and rain lead to an explosive hatch? John noted that the swarm seemed to blow up and fade very quickly. This video shows 'those dreaded flies': https://youtu.be/Rua_nnLF6TE
Can we control them?
We are looking into it, but we don't have an easy answer yet. We need something that doesn't volatilize since we can't descend to the box every day to spray it. It has to be strong enough to kill flies but not strong enough to harm hatchling and nestling falcons. It can't destroy the integrity of the box or let too much precipitation or wind in through the side. We are contacting the University of Minnesota's insect extension team to pick their brains and have also emailed Dr. Laura Johnson about safe possibilities. John and Susan had an intriguing idea about soaking mesh scrubbies in some known organic repellents and securing them in a safe location inside the nest box, so we may try that as well.
I wish we had more answers for everyone now. We'll do what we can and post more information when we have it.