Four insect agents have been released in the USA, at least one of which (the leaf beetle Galerucella calmariensis) is established in California.

Biological control research initially focused on the northern states, where these agents have been very successful at establishing, multiplying and dispersing. Purple loosestrife biomass has been reduced by more than 95% at some sites, but this has often taken more than 10 years. The biological control agents originate from northern Europe. Recent studies that model adaptation of insect life cycles to seasonal temperature and day length indicate that some species are not well adapted to more southerly locations, including much of California, because they enter winter diapause too soon (Grevstad and Coop 2015).

The two leaf beetles (Galerucella calmariensis and G. pusilla, Coleoptera: Chrysomelidae) look very similar and have similar life histories (Figs. 1 & 2). Adults overwinter in soil and vegetation near purple loosestrife plants and emerge from hibernation in the spring when purple loosestrife begins to grow. Adults feed on leaves, forming characteristic 'shot holes'. Eggs are laid on leaves and stems. Small larvae feed inside leaf or flower buds, and larger larvae feed on leaves forming transparent 'window panes' (Fig. 3). Pupation occurs in the soil, or inside the stems when plants are in standing water. These species can have one to three generations per year, depending on latitude and temperature. High densities of larvae can completely defoliate plants. Damage reduces plant growth and seed production. Adults are good dispersers and can find host plants up to 0.6 mi away. Although both species have been released widely in California, only G. calmariensis is known to have established, and only in areas north of Butte County (Fig. 4). The level of control is excellent in Shasta, but low in Butte and Siskiyou counties.

The root weevil(Hylobius transversovittatus, Coleoptera: Curculionidae) overwinters as an adult and is active from the time the host plants start growing until September (Fig. 5). Adults are primarily nocturnal and feed on leaves and stems (Fig. 6). Eggs are laid in stems or nearby in the soil. Larvae mine into the roots and take 1-2 years to develop, depending on temperature and the time of oviposition. Pupation occurs inside the upper root, and adults emerge from June to October. Adults can live more than one year. The principal impact is caused by larval damage to the roots which reduces plant growth and reproduction, and can ultimately kill plants. In Europe, the weevil occurs at all purple loosestrife habitats except those that are permanently flooded. Both adults and larvae can survive extended periods under water, but summer flooding prevents adults from laying eggs. This species has been released at sites in California, but establishment status is unknown.

The seed weevil (Nanophyes marmoratus, Coleoptera: Nanophyidae) overwinters as an adult (Fig. 7). In spring, adults feed on young leaves and then move to the flower spikes where they feed on flower buds. Eggs are laid in flower buds, and oviposition continues into August. Larvae feed inside the developing flower buds, which fail to open or produce seed, and adults emerge in the late summer. The weevil has one generation per year. Feeding damage by the adults and larvae directly reduces seed production. This species has been released at sites in California, but establishment status is unknown.

Biological Control Agents


Common name




Galerucella calmariensis2

black-margined loosestrife beetle

wide in northern CA

high in northern CA

First released in 1998 in CA

Galerucella pusilla2

golden loosestrife beetle

not recovered in CA


First released in 1998 in CA

Hylobius transversovittatus

loosestrife root weevil

unknown in CA


First released in 1996 in CA

Nanophyes marmoratus

loosestrife seed weevil

unknown in CA


First released in 1997 in CA

How the Technique Is Employed

Adult leaf beetles can be collected by sweep net or by beating plants with a stick to knock them off onto a collecting cloth or funnel held below for redistribution.

Adult root weevils, which are nocturnal, can be hand collected at night or in the early morning near sunrise for redistribution. Look for dark green inky droppings and feeding on lower leaves (uniform removal of tissue along leaf edges) (Fig. 3B). Larvae can be reared on artificial diet.

Collect adult seed weevils by sweep net or by beating plants with a stick above a collecting cloth.

For additional information see Blossey et al. (2015).

Special Tips

Collecting adults of these agents and releasing them on purple loosestrife populations that don’t seem to be heavily infested may increase control.


California is further south than the natural latitude of these insects, and it is not clear how well they will adapt to this difference. It would be better to obtain insects from California than from other states further north because they may be better adapted to our low latitude.

The leaf beetles may attack crepe myrtle (Lagerstroemia indica) and ornamental rose bushes that are near heavily infested purple loosestrife, but this ends after beetle populations decrease.

Biological control appears to be more successful at sites with low soil fertility, and in the presence of competing vegetation.

Successful biological control of purple loosestrife may result in increases of other invasive species such as reed canary grass (Phalaris arundinacea) and the European variety of common reed (Phragmites australis).

Continuously flooded sites are not suitable for the leaf beetles or root weevils.

Seed weevils are probably more successful where the leaf beetles are absent.

High summer temperatures and short summer photoperiod limit how far south G. calmariensiscan survive. Because all releases south of Butte County failed to establish, it is recommended that releases of G. calmariensis occur from Butte County north.

Where Can I Get These?

Contact CDFA or your local Agricultural Commissioner to see if they can provide insects. These insects are available from two commercial vendors in Montana:Integrated Weed Control (www.integratedweedcontrol.com) and Weed Busters Biocontrol (www.weedbustersbiocontrol.com).


Blossey, B. 2002. Purple loosestrife. In: R. Van Driesche, S. Lyon, B. Blossey, M. Hoddle and R. Reardon (eds.), Biological Control of Invasive Plants in the Eastern United States. USDA Forest Service Publication FHTET-2002-04, pp. 149-157.

Blossey, B., C.B. Randall, and M. Schwarzländer. 2015. Biology and Biological Control of Purple Loosestrife, second edition. USDA Forest Service. USDA Forest Service. FHTET-2015-3.

Grevstad, F.S. 2006. Ten-year impacts of the biological control agents Galerucella pusilla and G. calmariensis (Coleoptera: Chrysomelidae) on purple loosestrife (Lythrum salicaria) in Central New York State. Biological Control 39: 1-8.

Grevstad, F.S. and L.B. Coop. 2015. The consequences of photoperiodism for organisms in new climates. Ecological Applications 25: 1506-1517.

Hovick, S.M., and W.P. Carson. 2015. Tailoring biocontrol to maximize top-down effects: on the importance of underlying site fertility. Ecological Applications 25(1): 125-139.

Piper, G.L., E.M. Coombs, B. Blossey, P.B. McEvoy, and S.S. Schooler. 2004. Purple loosestrife. In: E.M. Coombs, J.K. Clark, G.L. Piper, and A.F. Cofrancesco, Jr. (eds.), Biological Control of Invasive Plants in the United States. Oregon State University Press, pp. 281-292.

Pitcairn, M.J. 2018. Weed biological control in California, USA: review of the past and prospects for the future. BioControl 63: 349-359.doi.org/10.1007/s10526-018-9884-6

Schooler, S.S., E.M. Coombs, and P.B. McEvoy. 2003. Nontarget effects on crepe myrtle by Galerucella pusilla and G. calmariensis (Chrysomelidae), used for biological control of purple loosestrife (Lythrum salicaria). Weed Science 51: 449-455.

Tomic-Carruthers, N. 2009. Rearing Hylobius transversovittatus and Cyphocleonus acheteslarvae on artificial diets (Coleoptera: Curculionidae). Florida Entomologist 92: 656-658.

Contributing Authors

Dr. Fritzi Grevstad, Assistant Professor, Oregon State University

Dr. Michael J. Pitcairn, Program Manager, California Department of Food and Agriculture

Dr. Lincoln Smith, Research Entomologist, USDA