Biological Control Agent:  Exapion fuscirostre (F.)

Invasive Plant Species Attacked: Scotch broom (Cytisus scoparius (L.) Link)

Type of agent: Seed feeding weevil

Status

Primary - adventive

Description and Biology

Adult:

The weevils are 2-3 mm long, have gray bodies and light brown legs.  A wide, dark coloured band extends down the back.  Their rostrum (nose) is long and curved.  Overwintered adults emerge in early spring to coincide with the plants' new spring growth.  The females need to feed on the spring flowers to stimulate egg production.  Oviposition begins shortly after the blossoms fall.  Each female will lay between five and 10 small white to yellowish eggs into the plants green seed pods.  From this oviposition period, the adults emerge in mid to late summer and then prepare to overwinter.

Egg:

Eggs hatch in 5 - 15 days.

Larva and pupa:

The larvae are white with brown head capsules.  There are three larval instars that feed for 20 - 40 days on developing seeds inside closed seedpods.  The first instars move about inside the pod, seeking a developed seed if the one they hatch nearest is not sufficiently developed.  Once they reach their preferred seed, they penetrate through the seed wall and feed on the cotyledons. Their first moult occurs and the resulting second instars continue to feed.  Usually by the first week of July, the larvae will occur as third instars and little of the seed embryo will be remaining.  The larvae may then begin to feed on the seed coat and continue until the entire upper half is consumed.  If an adjacent seed is nearby, the larvae will also feed on it.  The larvae prepare for pupation by collecting remaining seed parts and their own fecal pellets to build a cocoon.  They chew and position the particles while adding saliva to help build the "cement".  By the third week in July, pupation begins.  The creamy coloured pupae resemble the adults with their legs held close to their body.  The developing pupae gradually darken as they mature.  Pupation completes in 10 - 20 days inside the pods.

Overwintering stage:

Adult weevils overwinter in litter near the plants, on stem crevices and niches, in partially opened seedpods, or on other plants growing in the Scotch broom community.  In warm climates the adults can remain active all winter.

Location and effectiveness of attack

The larvae feed on seeds inside the seedpod.  Studies in the US found that the larvae destroy an average of 85% of the seeds.  The adults feed on flowers, foliage and stems.  In large populations, adults can damage tip growth.

Predicted and native habitat

Exapion fuscirostre prefers meadows and hillsides on south aspects.  Cold, damp, heavy shaded sites, north aspects or areas near the ocean are not desirable.  The weevil is established well in mild climate areas west of the Cascade Mountains.  High temperatures can cause mortality of adults trapped in mature pods which have not yet split.

British Columbia Experiences

Origin:

E. fuscirostre is believed to have crossed the Canadian border sometime since its first introduction into California, US in 1964.  By 1995, it was well established in California, Oregon and Washington and, therefore, presumed to be the original source of the populations now in BC.  The population source in the Pacific Northwestern United States originate from Italy.

History:

During the fall of 2006, the first attempt to locate E. fuscirostre in BC began.  The first collection was made in 2007 from a population mixed with B. villosus and was released into the Kootenays. The adults were collected in unopened pods from established E. fuscirostre infested sites in the lower mainland. The release was made in the central Kootenays in an open site adjacent to water. 

Habitat:

Adventive populations of E. fuscirostre has been found dispersed within the Coastal Douglas-fir and Coastal cedar hemlock biogeoclimatic zones.  Successful establishment has occurred from collections taken from the Fraser Valley and released into the Interior cedar hemlock zone.  It is notably absent on Vancouver Island and near ocean.

Field results:

Dispersal sampling shows E. fuscirostre established as far inland as Hope. The frequency of sightings decreased towards the coastline.  The weevils have not yet been found on Vancouver Island at any Scotch broom sampled sites.  In 2008, release site monitoring in the Kootenays determined that the weevils had survived overwinter and successfully produced a generation.  In 2008, another population was collected from the same Fraser Valley source and was released into a slightly more northern habitat near Upper Arrow Lake.  E. fuscirostre and Bruchidius villosus are frequently found co-existing in unopened seedpods.  Based on the 2007 collection, B. villosus outnumbers E. fuscirostre in the field.  An estimated 2800 pods were collected and the ratio was 5680 B. villosus to 1400 E. fuscirostre , or 80/20% B. villosus to E. fuscirostre per pod.

Collection for redistribution

Not available for general distribution at this time.

Notes

E. fuscirostre adults are active walkers and do so readily when pods open. They have been known to disperse up to 2 km a year from a release point. However, they are not as active as B. villosus.

E. fuscirostre can be confused with B. villosus in the larvae and pupae stages.

They are known to establish easily west of the Cascade Mountains.

The egg stage of E. fuscirostre is less prone to experience ill effects because it is laid into the developing green seedpods, whereas B. villosus is laid on the outer surface of pods.

It was formerly known as Apion fuscirostre.

References

Andres, L. A and E. M. Coombs.  1995.  Scotch broom. Division of Agriculture and Natural Resources, University of California, Publication 3361.  Pages 303-305.

Andres, Lloyd A.  1979.  Biological control - will it solve the broom problem.  Fremontia.  Vol. 7. No. 3. pp. 9-11.

International Pest Control.  1967.  Weevil controls Scotch broom.  Page 9.

MFR staff observations and comments

Oregon Department of Agriculture - Noxious Weed Control/Plant Division.  Eric Coombs's Images.  November 9, 2006.  www.invasive.org/browse/autimages.cfm?aut=19846

Paynter, Quentin, et al.  1996.  Are seed-feeding insects of use for the biological control of broom-  In Proceedings of the IX International Symposium on Biological Control of Weeds.  Pp. 495-501.

Sanz Benito, M. J. and P. Gurrea Sanz.  1999.  Immature stages of five species of the Genus Exapion Bedel (Coleoptera: Brentidae, Apioninae) associated with the seeds of Genista (Tournfourt) and Cytisus L. (Fabaceae).  The Coleoptera Bulletin, 53 (1): 8-26.

Western Society of Weed Science.  1996.  Biological control of weeds in the west.