Biological Control Agent: Galerucella pusilla (Duft.).

Invasive Plant Species Attacked: purple loosestrife (Lythrum salicaria L.)

Type of agent: Foliar feeding beetle

Status

Description and Biology

Adult:

The adults are 3 - 5 mm long, light golden-brown and lack the dark coloured wing margins found on Galerucella calmariensis. Overwintered adults appear one week later than G. calmariensis, usually in late May to early June when the plants are 20 cm tall. Adults first feed on young shoots and new budding leaves before feeding on mature leaves. Oviposition starts within one week of adult emergence, beginning in June, peaking around mid month then slowing after the summer solstice and continuing into July. Repeated mating is necessary for viable eggs. The females usually lay the eggs in masses of three on lower parts of plant stems and cover them with a strip of black faecal material. Egg laying is heaviest when temperatures are over 25°C. G. pusilla is strongly affected by low temperatures during oviposition. Over their life, the females will lay between 300 and 400 eggs. G. pusilla disperses less than G. calmariensis and, therefore, creates a local dense population before moving to new locations.

The second generation are more prone to dispersal than those that overwinter and emerge in the spring. Adults emerging in July will mate and oviposit within a week and lay eggs for one month and those that appear later may breed, but, will not oviposit before they hibernate. For two months parent and F1 generation adults are both present, with their oviposition periods overlapping for one month. Adults that emerge and lay to produce the first generation will die by the end of summer. The duration from egg to adult is 24 days at 25°C.

Egg:

Creamy-white, 1 mm diameter, spherical eggs incubate for 12 days.

Larva and pupa:

The larvae are pale yellow coloured. There are three instars, all are similar and have no distinguishing changes. Upon hatching, new larvae move to the leaf buds and flowers concentrating on young foliage before moving onto older leaves. After 14 days the mature larvae will be 3 - 5 mm long and pupation is initiated. The pupae are light brown, 3 - 5 mm long, and will complete pupation in seven days in the top 3 cm of soil. However, at sites which flood, pupation takes place in the plant stems above the water level. The new adults will remain on the soil or lower stems for two days until their bodies become hardened and then appear on the upper plant parts.

Overwintering stage:

Adults overwinter in the soil near the surface. Hibernation is controlled by temperature.

Location and effectiveness of attack

Larvae and adult feeding on buds will stunt plants and reduce seed production. In heavy populations, plants will be destroyed. The spring emerging adults attack early growth, impacting the plants' ability to photosynthesize and restore nutrients. Reduced flowering and seed production is common after several years attack. Delayed flowering by one month also reduces the number of flowers the plant can produce and later blooming limits pollination. Repeated heavy defoliation and attack will reduce the plants vigour the following growing season. Sites with high larvae densities will cause defoliation by the end of July.

Predicted and native habitat

G. pusilla tolerates a variety of habitats where purple loosestrife grows. It can tolerate flooding, but, continual flooding may not suitable. For that reason, sites with a moisture gradient will offer alternative locations of choice habitat. They appear to prefer areas on the margins of wet lands. Galerucella beetles prefer full sun and, therefore, locations with sun exposure will be preferred over shaded or partially shaded sites.

In its native environment, G. pusilla occurs in varying climates within wide ranging ecological habitats. Field surveys indicate their range to be in central and north European countries, and is found in south Sweden and Finland and to the south to north Africa, and with recorded sightings in Siberia, Kazakhstan and central Asia. When G. pusilla and G. calmariensis habitats overlap, G. pusilla occurs when the plants are growing in lower densities.

British Columbia Experiences

Origin:

G. pusilla populations released in BC originate from Germany.

History:

The first G. pusilla treatment in British Columbia was made in 1994 on Vancouver Island. Before its establishment could be confirmed, the site was destroyed by development. Another G. pusilla population was later released into the southern interior and adults were located in 2001 and 2002. However, at this location the collections are suspected to now be mixed Galerucella spp. populations. Since populations are routinely considered mixed, it is possible both species occur on most, if not all, sites.

Habitat:

Treatments have been made into Bunchgrass, Coastal Douglas-fir, Coastal western hemlock, and Interior cedar-hemlock biogeoclimatic zones and all zones have shown establishment.

Field results:

Impressive feeding damage can be found within coastal and interior sites from mid summer onward. On long linear sites, plants are failing to produce flowers near the immediate release point. In the interior, adults can be observed from July into early August, and the seasons' abundant feeding is evident in early September. In the lower mainland, adults and egg masses were seen from mid May through June. Adults were collected from the lower mainland in June well within the incoming tide level showing their tolerance for wet sites. In BC, two generations are normal. Galerucella spp. treatment sites in the lower mainland now have low plant densities and reduced flowering, which have negatively impacted seed production. This same success is starting to show in the southern interior.

Collection for redistribution

Transfers can be successfully done by moving plant material with larvae into new locations or adults can be aspirated from plants during peak periods. When treatments are made with small numbers of beetles released, small populations will usually continue, therefore large releases perform best.

Notes

By combining the two Galerucella species, benefits are gained. G. calmariensis feeds on early plant growths causing the plants to then send out new shoots that then coincide with G. pusilla's feeding requirements.

DNA of these closely related species have been studied and demonstrate a distinct reproductive separation.

References

Blossey, B. and D. Schroeder. 1991. Study and screening of potential biological control agents of purple loosestrife (Lythrum salicaria L.) Internat. Instit. Biol. Control, European Station, Delemont, Switzerland. Final Report. 41 p.

Corrigan, J. and P. Harris. 2003. Classical biological control of weeds established biocontrol agent Galerucella calmariensis (L.) and G. pusilla (Duft.). Defoliating beetles. May 20, 2003. http://res2.agr.ca/lethbridge/weedbio/agents/agalcal_e.htm

Mason, P. G. and J. T. Huber, (editors). 2002. Biological control programmes in Canada, 1981-2000. CAB International.

MFR staff field 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, Linda M., Mark Schwarzlaender, Bernd Blossey, and Carol Bell Randall. No date. Biology and biological control of purple loosestrife. Wash. State Univers. and Oregon Dept. of Agric. 78 p.