Q&A: BIOLOGICAL CONTROLS for Purple Loosestrife (Lythrum salicaria

Why worry about the control of purple loosestrife? 
Purple loosestrife, a wetland plant that is not native to North America, is very aggressive, and if given a favorable site, will  dominate that site within a few years, squeezing out native vegetation. There are very few native species which find loosestrife useful for forage or habitat. In Europe, where purple loosestrife is native, there are more than 120 insect predators of the plant. Few exist in North America, and none achieve control of  the plant. 

What kinds of control methods have been tried? 
Cutting doesn't work; burning doesn't work. Digging large plants is difficult, but useful for small infestations. Flooding the areas where loosestrife is prevalent, to a depth of at least 16 inches for a period of at least seven weeks does affect theviability of loosestrife; however this treatment also negatively affects other species of plants. Careful removal of the flowering stalks before seed dispersal, and disposal in black plastic for  refuse removal may be a temporary control for individual plants.  Chemical control can be partially achieved using Rodeo (glyphosate) in wetlands, but spraying is expensive, requires a licensed applicator and permit process, and can harm native species. Chemical sprays will not reach the seeds which may germinate, so repeated chemical applications are necessary.  Chemical herbicides have potential negative effects on other species (including humans). In short, these methods are too costly, not species specific and don't work long-term. 

Are there biological methods to help control purple loosestrife?
Since 1992, researchers have been rearing and and releasing in a few states two European beetles, Galerucella pusilla and Galerucella calmariensis , which eat only purple loosestrife plants. Trained biologists have released them in all Canadian provinces and twenty-seven other states; Vermont and New Hampshire began in 1997. The European root-mining weevil, Hylobius transversovittatus, is also being reared for release, but it has limited availability. Two weevils which feed only on flowers and seeds of L. salicaria have been approved, but are not yet commonly released. Assuming that biological controls survive, it is expected that it will take five to seven years to achieve control of purple loosestrife. 

Are there any success stories for the bio-control of introduced species?
A few early efforts at bio-control had disastrous results, such as the weasels introduced to control rabbits in New Zealand, or the mongoose released on Guam to control rats. These bio-disasters have introduced considerable caution into the scientific community, and a healthy skepticism on the part of the public. Some successes in biological control in the weed world are alligator weed in Florida and southern US, Klamath weed control in Northern California, tansy ragwort control in Oregon, and leafy spurge control in the great plains where hundreds of thousands of acres  have been returned to productive rangeland and pasture. There is hope for biological control for other invasive problem plants such as phragmites, saltcedar and garlic mustard. While there have been some careless bio-control programs (see next question), the program designed for purple loosestrife is considered a "model of a well-conceived, meticulous bio-control endeavor" (DeVine, 1998, page 240) 

Is Rhinocyllus Conicus (a weevil) on native thistles a case of  bio-control gone wrong?
Rhinocyllus Conicus was introduced to control musk thistle, a serious pest plant for farmers. This weevil is known to attack 4 different thistle genera in Europe (Carduus, Cirsium, Silybum, Onopordum) and its attack on native thistles (though much slower) in the genus Cirsium was predicted (Louda,1997).  Some claim that at the time of introduction in the 60's the attitude was "only a dead thistle is a good thistle", whether native or introduced, and that what went wrong was that societal values have changed since the 60's. Any such introduction would not be permitted today, according to David Ragsdale, (Professor of Entomology,University of Minnesota) since the screening process, a protocol required by the US Department of Agriculture's Animal and Plant Health Inspection Service, APHIS, has been tightened significantly since that time. Though it is not risk-free, one must make an evaluation based on what a do-nothing policy or a policy based on broad spectrum herbicide would do to native (or rare) species sharing the same habitat. 

How are insects for biological control selected today? 
In the case of purple loosestrife, researchers identified insects which feed exclusively on the plant. Approximately 15 species of insects were tested for host specificity against 41 species of North American native plants and 7 species of agricultural plants. Test plants included 17 members of the family Lythraceae, including five other species of Lythrum. They were tested first at the International Institute of Biological Control in Europe, and then under quarantine conditions in the US. (Blossey 1994; Kok 1992). The insects were tested on both their feeding preferences and their ability to survive. Although the insects showed some ability to feed upon a few native plants, these plants were not preferred if Lythrum salicaria were nearby.  In no case, were the insects able to complete their life cycle on any of the native plants (Blossy, 1994).  According to an article by Elizabeth McGowan in Brooklyn Botanical Garden's Plant and Garden News, today bio-controls are chosen primarily for their specificity for the target plant. The testing process takes at least five years, and even if given approval the insects may fail to become established. 

Are there native controls for the Galerucella beetles?
Yes, spiders will prey on these beetles and ladybugs prey on the beetles' eggs . Other generalist predators exist. 

What will the beetles do when they run out of purple loosestrife? 
They won't run out. Biologists no longer expect total eradication of an introduced, invasive species. The hope is to have purple loosestrife reach some sort of equilibrium in the wetlands, where it is present but does not dominate. Experimenters hope for a 90% reduction in abundance (but not eradication) of purple loosestrife. 

Will the beetles threaten other native species? 
In our area, there may several native species of Lythrum: Lythrum alatum (winged loosestrife), L. lineare and a close relative, Decodon verticillatas (swamp loosestrife or waterwillow). All were tested and rejected by the insects in the field (although some were nibbled at in the no-choice caged test.) (Blossey, 1994) 

Can the beetles evolve, or change their preferences? 
According to David Ragsdale, Professor of Entomology,University of Minnesota insects are "wired differently than a long-lived animal like a mammal or bird; in the latter, it is an adaptive advantage to broaden one's food source in case the principal food is scarce or not available." In the case of insects, whose reproductive lives might be measured in hours (such as mayflies) or at most a few weeks (like Galerucella), it is an adaptive advantage to narrowly select a host rather than spending time feeding on a wide variety of hosts -- indeed, the more hosts, the more exposed they are to predation and success of eggs deposited on unsuitable hosts is limited, effectively eliminating from the gene pool those individuals that engage in this behavior. There is no case where an introduced insect has either exterminated the target weed, or unexpectedly switched hosts to become a serious pest of other plants. (Harris, 1988) Although Bernd Blossey (Director of Biological Control of Non-Indigenous Plant Species Program, Department of Natural Resources, Cornell University) can not categorically exclude the possibility, he states that the probability for such a shift is in the same order of magnitude as the monarch butterfly switching from milkweeds to a different host plant (personal communication) 

What happens after beetles are released? 
In all states, the release sites must be approved in advance by the responsible state agency. These sites must meet certain criteria such as minimal flooding, minimal human activity, no mowing, permission of property owner, fairly high density of plants in an easily monitored area. The monitoring for all release sites must be done annually, about the same time each year. Bernd Blossey at Cornell has devised a protocol which is widely used for the detailed observation of a few test sites in each state, which will be monitored over a period of many years. 

What has been the experience with beetles elsewhere? 
In Minnesota, there has been a vigorous campaign to rear and release beetles, with the help of citizen volunteers. This program is now in its fourth year, and has more than 80 volunteer groups rearing insects, and releasing them in sites approved by the state, and then annually monitoring these sites according to a state approved protocol. David Ragsdale reports that this is a great public relations and education project. Not a single volunteer group has dropped out in the four year period, and these have been cooperative and reliable in the reporting required by the state. Since they are responsible for beetle rearing, site selection and monitoring, volunteers really take ownership of their release sites. With volunteer help, more than 400 release sites have been established, many more than state employees could do on their own, and now they can observe major success in about a dozen of the sites -- though it is still a bit early in the campaign to gauge results. Michigan and New York state also have citizen volunteers involved in rearing beetles. Several sites in New York, Canada, and Oregon have shown significant reduction in loosestrife coverage. Donna Ellis (University of Connecticut) reports that of the state's ten release locations since 1996, one release site at Storrs had significant reduction in plant flowering by 1998.  In 2001, both New Hampshire and Vermont plan to have citizens involved in rearing beetles and monitoring bio-control sites

Shouldn't we just let nature take its course?
You must provide the answer to this question. 


Compiled by Barbara McIlroy, January 2000. The above information has been gathered from e-mail with Bernd Blossey (at Cornell), David Ragsdale (U of Minnesota) and Donna Ellis (U of Connecticut), as well as some of the references cited on this website's  reference page