The Dispersal Ability of the Aspen Hoverfly

Hammerschmidtia ferruginea 

            

 H. ferruginea feeding on Bird cherry Prunus padus

 

In 2001 the first attempts were made to manage dead aspen wood Populus tremula, the habitat of the endangered aspen hoverfly Hammerschmidtia ferruginea.  This involved artificial supplementation by felling aspen trees and manipulating the length of time the breeding habitat was suitable. 

 

 

Fencing around severed aspen trees to protect them from grazers such as sheep

 

The larvae of H. ferruginea feed and develop in the wet layer that builds up under the bark of dead aspen. 

 

The thick rotting cambium layer between the bark and sap wood of an aspen tree

 

Adult male H. ferruginea congregate on the surface of these logs where they defend small territories in order to locate and breed with visiting females.

 

Male H. ferruginea resting on decaying aspen

 

 

Two male H. ferruginea in a territorial feud

 

 

Two male H. ferruginea at the conclusion of a territorial feud: the winner pins the loser down for several seconds before returning to its territory

 

Female H. ferruginea investigate dead aspen logs, and when gravid will oviposit eggs into the crevices in the bark.

 

Female H. ferruginea ovipositing eggs into a crevice on a rotting aspen log

 

In addition to felling existing aspen, due to the scarcity of large aspen woodlands in Scotland which are required to sustain a constant input of dead wood, it is important to link up groups of aspen to encourage dispersion and expansion of both aspen and H. ferruginea populations.  In order to do this effectively and to know to what extent extant populations are isolated, we need to know how far H. ferruginea is capable of dispersing.

 

In the late 1980’s based on the distribution of populations it was estimated that H. ferruginea could disperse up to 4km.  In 2009 a three year study funded by SNH was proposed to confirm this estimate.   

 

In the first year of the project ten aspen logs were deposited along three transects extending out from a central point to a maximum distance of 3km.  One spare log was placed at 4km.

 

Illustration showing basic pattern of aspen logs placed in the field at 1km stepping-stones along 3km long transects

 

 

  

Depositing aspen logs along a 3km transect

 

1066 adults were caught emerging from aspen into traps.  Each individual was marked using non-toxic enamel paint to identify it upon re-sighting, and released at the central point.

 

  

Fallen aspen with an emergence trap constructed over part of its length to capture emerging H. ferruginea

 

Image of a marked female H. ferruginea and illustrations of four possible colour combinations  

 

Each log was observed daily from mid-May until mid-July in search of marked H. ferruginea adults visiting the dead wood. 

 

Volunteer observing logs for H. ferruginea

 

In 2009, 78 marked H. ferruginea were re-sighted.  Seven males and five females were observed 3km from the release point, one marked female was observed at the 4km log along the NE transect. 

In the second year eight decaying aspen logs were set out in different directions at 1 to 7km points from a central release point. 

 

Illustration showing basic pattern of aspen logs placed in the field at 1 to 7km extending out from a central release point

 

In 2010, 1157 adults were caught in emergence traps, marked and released at the central point.   

Line graph illustrating H. ferruginea emergence over time for years 2006, and 2008 to 2010

 

In 2010, 115 marked H. ferruginea were re-sighted.  One male was observed 5km from the central release point.

 

Line graph showing number of H. ferruginea re-sighted at each distance from the central release site (2010)

 

Distance (km)	% of re-sighted H. ferruginea
	2009	2010
0	58	76
1	21	17
2	5	4
3	15	2
4	1	2
5	-	1
6	-	0
7	-	0

Table illustrating percentage of total re-sighted H. ferruginea observed at each distance from the release site

 

Management and further investigation

 

By encouraging the expansion of aspen woodland we can begin to link up this resource which would benefit many animals and plants that depend on it.  By understanding the resource utilisation of one insect we hope to use this as a potential template for other similar species. 

 

Hammerschmidtia ferruginea is capable of dispersing and locating rotting logs up to 5km from where they emerge, but as this investigation shows very few individuals readily disperse more than 1km and this needs to be taken into account in plans to expand aspen woodlands. 

 

In 2009, three 3km long transects with rotting logs positioned at 1km points along each transect were set up to test dispersal and resulted in 15% of re-sighted individuals observed at 3km, which compares with 2% in 2010.  This may indicate the importance of stepping-stones as part of an optimum landscape-scale design for dispersing individuals dependent on aspen in Scotland.  This will be further investigated in the 3rd and final year of this project.