Richard & Diane Van Vleck - Personal Pages

Bluebirds and Blowflies

Larva of Protocalliphora sialia

The larvae feed by night and move downward into the nesting material during the day. This fascinating adaptation has, no doubt, evolved over many centuries of a close association of blowflies with cavity nesting birds. The larva avoids being eaten by the adult bird during the day, and feeds only after dark, when the helpless nestlings are easy prey. Blood is usually drawn from the feet or legs, often from between the toes.

An effective method of dealing with a blowfly infestation is simply to remove the entire nest and replace it with clean dry grass clippings, cupped into the shape of the nest. I have done this many times, and, the adult bluebirds have never seemed disturbed at my meddling. All they ask is that I do it quickly and then get out of their way. My nestboxes are mounted on pipe flanges for easy removal from the post. I place the box on the ground and transfer the young to a small box. Then the nesting material is dumped into a plastic bag and the bag is quickly sealed to avoid loss of other parasites. The new nest is placed in the box and the young returned to the box after a very brief examination. Be sure to check the feet for bites. Later, I examine the nesting material for blowfly larva as well as any other inhabitants.

When bluebirds or tree swallows fledge from your nestboxes, the old nest should be immediately removed. If blowflies are present, the adult flies have likely not yet emerged, and simply destroying the entire nest will get rid of them. However, if you find an empty puparium, it means an adult has left the nestbox to mate and lay eggs in another of your boxes or in a natural cavity occupied by bluebirds or tree swallows. When emptying nestboxes or changing nesting material due to blowfly infestations, I transfer the nesting material to a pail covered with screening which will prevent escape of adult blowflies, but allow passage of any parasitic wasps which may emerge.

Some people are using a hardware cloth bottom in their nestboxes, believing that the blowfly larva will drop through the screen. I haven't seen hard data to support this, and, since the larva would be lost on the ground, it would be hard to test except to compare the percent of infested boxes with and without screen bottoms. If the larva migrates downward to avoid light, this would create a problem, since light would be coming in the bottom of the box. Also, I often find blowfly larva in the middle of the nest as well as on the bottom. However, a hardware cloth insert above the floor might allow larvae to drop through and be trapped in the bottom where they could be counted. I will try several of these inserts next season with a one inch metal flashing around the edges to make sure the larvae can't crawl up the side of the box. They aren't very good crawlers, although I have had them crawl out of a petri dish. If you try a hardware cloth insert next season, let us know how it worked, by recording the number of larvae found in the nesting material and the number trapped in the bottom. If you prefer to use an open bottom, a piece of window screen over the bottom will retain the larvae falling through the hardware cloth so they can be counted.

Luckily, bluebirds have a natural ally in the parasitic wasp, Nasonia vitripennis, which enters the nestbox and lays eggs on the blowfly puparium. The developing wasps devour the blowfly pupa as they mature. Unlike the blowfly, which usually does little harm to it's avian host, the parasitoid, Nasonia always kills the blowfly pupa. While cleaning nestboxes, you may want to collect several blowfly puparia and hold them in an escape proof container to see whether you hatch adult parasitic blowflies or their parasites.

PHOTO - Empty blowfly puparium

PHOTO - Several blowfly larva on floor of nestbox

PHOTO - Adult blowfly

PHOTO - Blowfly larva - the business end

2001 update In the past two years, our bluebirds have been severely affected by blowflies. 1999 was a very dry, hot year, 2000 was unusually wet. Whether environmental factors made the bluebird nestlings more susceptible to blowflies or whether there were just more blowflies is unclear. I didn't find the huge numbers of larvae in the nestboxes that many people report, but all boxes had blowflies, even in the first nesting. In 1999, several broods were lost before I realized the extent of the problem. Now, I do a nest change when the young are about 6 days old. With these single nest changes, there has been no problem. After 15 years of minimal blowfly damage, it seemed strange that they suddenly became lethal. Tree swallows are a rather recent addition to our yard birds, and their presence has resulted in many active nests in our yard, since their territories overlap that of the bluebirds. Perhaps this allows a higher incidence of successful blowfly infestations since the active nestboxes are much closer together. The incidence of blowfly infestation in 1999 and 2000 was 100% in bluebird boxes. However, the number of larvae found in the nests was not higher than in previous years. While all of the bluebird nests had at least a few blowfly larvae, only about 50% of the tree swallow nests had blowflies.

2009 update Blowflies have not been a problem in recent years and few nest changes have been needed.

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