Biological Control Agent: Chaetorellia acrolophi White & Marq.
Invasive Plant Species Attacked: Spotted
knapweed (Centaurea bieberstenii), diffuse
knapweed (C. diffusa)
Type of Agent: Seed feeding fly
Status
Primary
Description and Biology
Adult:
Adults are 4 - 5 mm long and have orange-yellow coloured abdomens with overall spotting.
Their wings are clear with light brown bands and their eyes are bright green. The flies
emerge in early June, coinciding with knapweed bud formation. Mating occurs immediately
and oviposition starts within two days. Females lay eggs during their entire life span,
depositing an average of 69 eggs each. Eggs are placed individually, or, in small groups
of 2 - 4 underneath the bracts of unopened 4 - 5 mm diameter floral buds. Within a lab environment,
adults survive up to four weeks; 17 days in the field. There are two complete generations
(eggs to adults) and part of a third generation (eggs to larvae) each year.
Egg:
The 0.9 x 0.2 mm shiny white, elongated eggs have a long filament which is thickened at
one end. Eggs incubate for 4 - 5 days.
Larva and pupa:
The larvae remain white coloured through the three larval instars which last 10 - 15 days.
The first instar penetrates the bud horizontally and moves towards the bud centre, feeding
on immature florets until it reaches the developing seeds. Second and third instars feed
on bud contents. Mature larvae prepare a puparium, arrange it vertically and pupate.
There are usually two generations per year, however, a rare third generation possible when
ideal habitat conditions are present.
Overwintering stage:
The third instar larvae overwinter in seedheads and pupate the following spring.
Location and effectiveness of attack
Larvae feed on floral buds, sometimes completely consuming the entire contents which significantly
reduces seed production. Unlike other seed feeders, the larvae do not create a gall. Chaetorellia
acrolophi is most effective in areas with low density knapweed which is less preferred by
other seedhead feeders. The highest rate of attack occurs on sites that do not have Urophora species
present.
Predicted and native habitat
Specific C. acrolophi habitat has not been determined. It has the ability to
seek out isolated sites which are normally avoided by competing bioagents. In its native
environment it does not build up high populations in thick stands, showing a desire to be among
widely spaced plants with mixed vegetation. Initially it was predicted that C. acrolophi preferred
very dry south-facing slopes, this may not be the case in North America where establishment
has been found in moist areas of Montana and Oregon.
Native European distribution is from Spain and the European Alps, east to northern Greece,
and north to southwestern (former) USSR. It occurs in the south regions of Austria, France
and Switzerland, the central region of Turkey, the eastern section of Romania and the east
and central portions of Hungary. In western Switzerland, C. acrolophi densities
were greatest on steep, dry, south slopes.
British Columbia Experiences
Origin:
C. acrolophi populations in BC originated from Swiss Valais,
Switzerland.
History:
In 1991 a small number of C. acrolophi were released near Kamloops
into a bunchgrass habitat and another population was released into a propagation
tent. Between 1992 and 1995
several more releases were made as larger populations were made available and
shipped to BC.
Habitat:
C. acrolophi has been released into the Bunchgrass, Interior
cedar-hemlock and Interior Douglas-fir biogeoclimatic zones. Establishment
and dispersal has occurred in the Interior cedar hemlock and Interior
Douglas-fir zones.
Propagation results:
The tented population failed to establish sufficient numbers for
collection and in 2001 the tent was dismantled.
Field results:
C. acrolophi was originally released into
relatively thick patches of knapweed, it is believed that the agent may have
dispersed, seeking its preferred widely spaced infestations. As knapweed infestations continue to reduce in vigour and
density, it is presumed that C. acrolophi may become more evident.
Establishment was found at one site soon after its release, however, subsequent
monitoring had shown no recoveries until 2008. Seedhead sampling in May determined that C. acrolophi
has established at at least one release site and has also been found dispersed
at two locations in the southern interior.
Not available for general distribution at this time.
Notes
Chaetorellia acrolophi may also attack purple starthistle (C. calcitrapa)
and squarrose knapweed (C. virgata spp).
When U. affinis is in high densities, C. acrolophi is a weak competitor.
References
Harris, P. 1989. Feeding strategy, coexistence and impact of insects in spotted
knapweed capitula. In Proc.VII Internat. Symp. Biolog. Contr. Weeds. March 1988,
Rome, Italy. Pp. 39-47.
MFR staff observations and comments
Powell, G. W., A. Sturko, B. Wikeem and P. Harris. 1994. Field guide to the
biological control of weeds in British Columbia. B.C. Min. For. Res. Prog.
Rees, N. E., Quimbly, Jr., P. C., G. L. Piper, E. M. Coombs, C. E. Turner, N. R. Spencer,
L. V. Knutson (editors). 1996. Biological control of weeds in the west.
Wilson, L. M. and C. Bell Randall. Biology and control of knapweed.
| 
