As watchers know, several of our nests were impacted by blackflies this year. This blog answers some of your questions, looks at the correlation between water flow and hatch at Great Spirit Bluff, and discusses options. 

Are blackflies/buffalo gnats/turkey gnats an old problem that we’re just seeing now thanks to technology and interest, or an emergent threat driven by habitat improvement and climate change-induced extreme weather patterns? Perhaps a little bit of both. While blackflies have been known to parasitize birds for some time, we are recording more blackfly-related disturbances and deaths at our nests now than we did in the early years of our banding program.

Black flies or Bottle flies?

Earlier this year, watchers were concerned about flies on prey remains at N2B and the North nest. Were those black flies? They were not! Members of the family Calliphoridae – aka blow, bottle, or carrion flies – lay their eggs on the carcasses of dead animals and their larvae feed on decaying flesh. No wonder they were attracted to both nests! Calliphoridae eat fruit, vegetables, nectar, carrion, plant secretions, and feces, but they don’t take live blood meals and aren’t a threat to eagles or falcons.

We call black flies (family Simuliidae) flies, but they are more closely related to mosquitoes and midges than bottle flies. Like mosquitoes, black flies lay their eggs in water. Like mosquitoes, female flies require a blood meal to provide protein for egg maturation. And like mosquitoes, black flies often hatch in swarms, cause physical damage, and can carry harmful pathogens that are transmitted via their bite.

Black Fly (Simulium)

Black Fly (Simulium)

Green Bottle Fly

Bottle Fly (Calliphoridae)

How can you tell which fly is which in the nest? Bottle flies are generally metallic green, blue, or black, relatively large, and found on or near prey piles. Black flies are generally black or grey, relatively small, and found on or near eagles. Head twitching and shaking is often a give away in late May and early June as the flies seek blood meals from bare or lightly feathered skin.

Unpacking the data: Production

Blackflies impacted the Decorah Eagles in 2019 and 2014, and the Decorah North Eagles in 2019 and 2018. However, our longest dataset belongs to the Great Spirit Bluff peregrine falcons. The GSB production record looks like this. (Note: Blackfly years – i.e., years where blackflies caused early fledging – are highlighted in gray.  Until 2012, production is listed in the following order: eggs laid, eggs hatched, falcons banded. After 2012, production is listed as eggs laid, eggs hatched, falcons fledged)

Year Male Female Production
2005 Unknown Katrinka 4 | 4 | 4
2006 Unknown Katrinka 4 | 3 | 3
2007 Unknown Unknown 4 | 3 | 3
2008 Unknown Unknown 1 falcon banded
2009 Unknown Unknown 4 | 4 | 4
2010 Unknown Unknown 4 | 4 | 4
2011 Travis Michelle 4 | 4 | 4
* 2012 Travis Michelle 4 | 3 | 2
2013 Travis Michelle 4 | 4 | 2
2014 Travis Michelle 5 | 1 | 1
2015 Travis Michelle 4 | 4 | 4
2016 Newman Michelle 4 | 4 | 4
2017 Newman Michelle 4 | 4 | 2
2018 Newman Michelle 4 | 2 | 2
2019 Newman Michelle 4 | 2 | 1

*2012 was the first year that GSB could be watched online

Water flow and black flies

The data show that we’ve seen more blackfly-induced fledges since we put the nest box online. Is this because we’ve been able to see them, or is something else going on? The chart below shows water flow during May and June in La Crosse, WI, sorted by the sum of the least to the greatest flow in May and June. (Data collected by the USGS National Water Information System:

Year May Jun Sum Total Production
2005 288.5 242.3 530.8 4
2007 323.8 315.6 639.4 3
2010 364.4 431.4 795.8 4
2012 500.2 346.4 846.6 2
2006 468.1 400.1 868.2 3
2016 396.1 515.5 911.6 4
2015 419 505.4 924.4 4
2009 454.4 470.3 924.7 4
2011 543.8 490.7 1034.5 4
2017 586.4 524.3 1110.7 2
2014 538 584.3 1122.3 1
2018 742.2 490.8 1233 2
2008 444.4 800.1 1244.5 1
2013 620.4 679.5 1299.9 3
*2019 716 632 1348 2

*2019 will probably be revised upwards

2008 marked the first year since falcons began nesting at GSB that we saw a discharge or flow of over 1000 cubic feet per second in La Crosse. In 2011, we saw a discharge of 1034.5 feet. Since that time, we’ve had five years with a combined water flow of over 1000 cfps: 2013, 2014, 2017, 2018, and 2019.
During four of those five years (2013, 2014, 2017, and 2019), we saw young falcons driven from the GSB nestbox by blackflies.

Remember, blackflies lay their eggs in flowing water and inhabit streams and rivers ranging from the mighty Mississippi to a trickle. High water flow and wet conditions lead to localized flooding, which vastly increases the amount of habitat (think of the pasture at Decorah North), leading to huge numbers of black flies.

Are high water years becoming more frequent?

So we know that blackfly years are driven by high water levels. But are high water years becoming more frequent? Let’s look at the five-year average flow in May and June since falcons started nesting at Great Bluff, sorted by year…

Year May Jun Sum Total Five-Year Average
2005 288.5 242.3 530.8
2006 468.1 400.1 868.2
2007 323.8 315.6 639.4
2008 444.4 800.1 1244.5
2009 454.4 470.3 924.7 841.52
2010 364.4 431.4 795.8
2011 543.8 490.7 1034.5
2012 500.2 346.4 846.6
2013 620.4 679.5 1299.9
2014 538 584.3 1122.3 1019.82
2015 419 505.4 924.4
2016 396.1 515.5 911.6
2017 586.4 524.3 1110.7
2018 742.2 490.8 1233
2019 716 632 1348 1105.54

See the trend? In the fifteen years we’ve been tracking falcon production at Great Spirit Bluff, high water events have become more common and are lasting longer. More water equals more blackfly habitat equals more eggs equals more flies. Black flies aren’t a new issue, but population numbers are rising with the water and black fly hatches are becoming more intense, and possibly more frequent, during a vulnerable time of life for many nestling birds. Will the trend continue? Rising water levels are predicted with global climate change. How and where that prediction plays out remains to be seen.

What other factors could be involved in explosive blackfly hatches? A 1998 paper on blackfly-induced mortality of Red-tailed Hawks found that temperature is a factor in blackfly activity, with flies less active or not present at ambient temperatures lower than about 58 degrees Fahrenheit. On the plus side, swings in temperature and water level could be used to help predict explosive hatches, which might help us ameliorate their impact. But as the climate warms, blackfly hatch could move even earlier. A longer, wetter season means that blackfly populations will expand in flood-prone areas.

So what are you doing about it?

To control blackflies, we need to know what kind of blackflies we’re  dealing with. Some flies are generalists, while others are specialists. Some species have one generation per year, while others have several. We contacted blackfly expert Dr. Peter Adler and will send him any flies we collect from Peregrine falcons. We’ve also asked regional rehabbers to send him any black flies that come in on their birds and will probably set up dry ice traps near our nests next spring.

Controlling black flies after hatch

Control strategies can be generally grouped into two categories – controlling black flies after hatch and preventing hatch altogether. To control flies after hatch, John installed a fan at Great Spirit Bluff and Neil installed a fan in the Wisconsin kestrel nest. Dr. Laura also applied Endure, a permethrin-based repellent, on the outside of the kestrel nestbox (although not the kestrels themselves). The combination of a high-powered fan with an insect repellent did a very good job at keeping blackflies out of the kestrel nestbox. Endure lasts up to two weeks (and there are similar products that last up to eight weeks), making it a very good candidate for use at our nest boxes.

The two Decorah eagle nests are a little tougher to deal with. We can’t access them without shooting a line, which puts the eagles at risk. Most products we apply during cam work in September will be gone by next May (this includes vanilla and cedar oils, which we tried unsuccessfully at Great Spirit Bluff in 2017). Insecticidal paints last longer, but they aren’t proven to work against blackflies and can’t be allowed to enter or run off into storm drains, drainage ditches, gutters or surface waters. They are extremely toxic to fish and invertebrates, which makes us hesitant to use them. We haven’t given up on a post-hatch control strategy, but the difficulty of reaching the nests without hurting the eaglets makes it a lot harder.

Preventing blackfly hatch

How about preventing blackfly hatch? BTI, or Bacillus thuringiensis israelensis, produces toxins that are effective in killing various species of mosquitoes, fungus gnats, and blackflies. It is believed not to have an effect on non-target organisms, although questions remain about the consequences of wiping out a large number of invertebrates in any given natural system.

BTI is widely used in urban blackfly control, but has it been used to control ornithophilic blackflies in rural settings? It has! Researchers used BTI to control the blackfly population at the Necedah National Wildlife Refuge between 2010 and 2012 as part of a wider study investigating reproductive failure in reintroduced whopping cranes. They found that treating flowing water with BTI helped suppress blackfly hatch, although it isn’t as simple as dumping BTI into every trickle, stream, and river. Blackfly species and sources must be determined to control them effectively and BTI just gets washed downstream if water is flowing high and fast.

We’re currently talking with professor Peter Adler, a blackfly specialist, and others who participated in the Wisconsin study to determine whether BTI could be used safely and effectively near our nests. If we decide to go the BTI route, we’ll be talking with landowners, the Iowa DNR, the City of Decorah, and raptor and blackfly researchers. Any projects would need to be properly permitted and formally documented to determine their success. As an environmental organization, we need to look for solutions that balance the health of the birds we watch with the health of the environment that surrounds and nourishes them.

In short: this problem doesn’t appear to be going away and may become more common in the future as the effects of global climate change deepen. We’re working on a solution that is safe for our birds, minimizes environmental impact, and can be documented and shared with others who are facing the same problem.

We’ve had a lot of followers suggest cedar oil, vanilla, and other homeopathic remedies. We’ve tried vanilla and cedar oils at GSB, but they volatilize or wash away too quickly to provide control throughout the season. This isn’t a bad thing – DDT still remains a problem in some places 47 years after it was banned in the United States– but it means they aren’t a good option for us.

Did you know?

If it’s a good year for mosquitoes, it’s a good year for blackflies, too! More water equals more blackfly habitat equals more eggs equals more flies – and we had a lot of water this year. Blackfly trap counts done by the Metropolitan Mosquito Control District in Minneapolis and Saint Paul this year yielded historically high counts. The agency has seen high numbers before, but not as consistently high for as long, or spread over such a large area.

Blackfly populations have increased thanks to the Clean Water Act of 1972. Like so many other invertebrates that depend on water for reproduction, black flies do best in clean water versus dirty water. In the Decorah area,  the Iowa DNR and groups like Trout Unlimited have worked very hard to turn impaired ag waters into clear streams. This could be another reason we’re seeing more of them at the Decorah nests.

Sources and links

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