Biological Control of Weeds in Natural Areas

Bernd Blossey
Department of Natural Resources
Fernow Hall, Cornell University
Ithaca, New York 14853

Note: More slides for this presentation will be added about the end of November.

Introduction

European settlement, western agriculture, forestry, and urbanization have resulted in the sometimes dramatic loss of natural North American habitats. More than 99.9% of the area once covered by oak savannas has been transformed (Henderson and Epstein 1995) and over 99% of the tall-grass prairies have been lost (Swengel and Swengel 1995). Similar but less dramatic losses occurred for the nations wetlands (53%, Dahl 1990) and forests (33%, Darr 1995). The shrinking space in natural areas is shared by an increasing number of non-indigenous species (NIS) introduced (intentionally or unintentionally) by humans. An estimated 17,000 species of native plants and about 5,000 introduced plants occur outside cultivation in the United States (Morse et al. 1995). The extent of change in different floras varies widely (Tables 1, 2) but the loss of the uniqueness of natural areas is of increasing concern. In 1950 20% of the flora of the Northeast consisted of NIS, whereas in 1986 NIS increased to 36% in New York (Morin 1995). Replacement plant communities are often dominated by a few successful cosmopolitan species. This is a direct threat to the integrity of National Parks and natural areas that were otherwise protected because of their unique fauna, flora or landscape. NIS often alter geomorphological, biogeochemical, and hydrological processes or fire regimes, thus, preventing the recruitment of native species and accelerating local and global extinction rates (MacDonald et al. 1989).

Some Examples:

1. Salt cedar (Tamarisk  spp.), introduced from Asia, has invaded some of the most valuable riparian habitats in the Western United States. The native vegetation on more than 500,000 ha has been replaced by dense stands of saltcedar (DeLoach 1990).

   This invasion threatens wildlife and livestock, increases sedimentation and soil salinity. Saltcedar highlights a common dilemma for control of NIS in natural areas. White-winged doves use dense stands for nesting and honey bees frequent the abundant flowers. Thus, conflicts arose with dove hunters and beekeepers who put an economic value on the plant, whereas, it is difficult to express the value of a native ecosystem in monetary terms.

2. Cheatgrass (Bromus tectorum),  a Eurasian annual grass, has replaced native grasses and sagebrush in the intermountain West and now dominates over 6.8 million ha (Ferry et al. 1995). It could expand to another 25 million ha in the sagebrush steppe and has contributed to the alteration of the fire regime which now favors the replacement of native plants by non-native annual grasses (Ferry et al. 1995).
3. Melaleuca (Melaleuca quinquenervia),  an ornamental tree from Australia, has invaded > 460,000 acres of wetlands in Florida. In 1983 its estimated spread was 8 acres/day, a decade later this rate had increased to 50 acres/day (U.S. Congress 1993). Dense stands displacing native vegetation have put especial stress on sawgrass marshes in the Everglades National Park and Big Cyprus National Preserve, endangering the Florida panther and the Big Cyprus fox squirrel (Center et al. 1990).
4. Purple loosestrife (Lythrum salicaria)  is a Eurasian wetland perennial introduced into North America in the early 1800's. Its spread across the temperate part of the continent has degraded many prime wetlands.

   
   Purple loosestrife has become the dominant plant in this wetland.
   Photograph by Richard Malecki

   Large, monotypic stands eliminate native plant communities and threaten endangered plant and animal species (Thompson et al. 1987; Malecki et al. 1993).

In recent years interest in a biological method to control problem plants in natural areas in the United States has grown (US Congress 1993). All federal agencies must now comply with standards to reduce the use and dependence on chemical control of weeds. But biological methodologies are not readily available, nor have they been well endorsed or financially supported. Despite an excellent safety record (Harris 1988; Crawley 1989), skepticism concerning the safety and effectiveness of exotic insect introductions for weed control remains high among the general public, administrators and even scientists. For example, the IUCN established guidelines in 1987 that prohibited introductions of NIS in natural areas (IUCN 1987). This would exclude the release of classical biological control agents.

Much of the concern centers on the host specificity of control agents after their release. In actuality, shifts of control agents to other plant species are extremely rare and can be predicted with the current practice of host specificity screening (Harris 1988). Follow-up studies of the realized field host-range and effectiveness of control agents, however, have not been an integral part of biological control. Many opportunities to investigate the interaction of control agents and their host plants have been missed. Even the obvious benefits of biological control have often been anecdotal rather than scientific (Crawley 1989, Lawton 1990). These evaluations will become even more important as conflict resolution becomes a major factor in biological control. The safety of control agents, ethical concerns (is a NIS less worthy of protection than a native North American species?), and economical concerns will be the major issues requiring resolution. Many NIS were imported because they were valued species in the aquarium or nursery industry and some became abundant enough to allow commercial exploitation (e.g. the Salmon fisheries in the Great Lakes, improved hunter harvests of white-winged doves nesting in saltcedar and honey production of saltcedar or purple loosestrife).

Without scientific evaluation, the safety and impact of biological weed control will remain subject to doubt. Public concerns need to be taken seriously; conflict resolution, guided by sound scientific analysis of costs and benefits, can offer guidance for necessary decisions. Last but not least, biological control needs to become a more predictive science. In the past, programs ran too often on a trial and error basis. Despite an increase in the number of programs initiated, the ability to select and to establish control agents has not progressed to a point where the rate of success has improved (Crawley 1989). Basic questions about the kind of herbivore species to introduce, about the impact of single and multiple species herbivory, and about release strategies remain unanswered (Crawley 1989, Lawton 1990). The Biological Control of Non-Indigenous Plant Species (BC NIPS) program at Cornell is intended to emphasize and contribute basic research to improve the scientific basis of biological weed control. The program will focus on non-indigenous plant species invading natural areas. The following examples of envisioned investigations originate from the purple loosestrife biological control program and highlight pertinent questions regarding agent selection, agent release, impact evaluation, long-term monitoring, and mass production.

What type of agents to introduce?

Detailed investigations by the International Institute of Biological Control in Europe began in 1986 with surveys for potential control agents and investigations about their life-history, distribution, impact, and host-specificity (Blossey 1993; Blossey et al. 1994a, b; Blossey & Schroeder 1995; Blossey 1995b). Existing ecological knowledge did not allow us to rank these species in terms of their prospective control efficacy. Ranking the species according to scoring systems developed for the choice of control agents (Harris 1973, Goeden 1983, Wapshere 1985) and according to the analysis of past control programs (Crawley 1986, 1989) produced contradictory results (Blossey 1995a). Based on the available knowledge, six species were selected as the most promising agents for further investigations. These were a root-mining weevil, Hylobius transversovittatus,  attacking the main storage tissue of purple loosestrife; two leaf-beetles, Galerucella calmariensis  and G. pusilla  capable of completely defoliating individual plants and entire L. salicaria  populations; a flower feeding weevil Nanophyes marmoratus;  a seed feeding weevil N. brevis;  and a gall midge, Bayeriola salicariae,  attacking leaf and flower buds. With the exception of B. salicariae  all agents passed tests for their host specificity and introductions began in 1992 (Hight et al. 1995).

Based on the available knowledge at the time of introduction of the first control agents in 1992 and the information about herbivore impact in Europe the following predictions were put forward (Malecki et al. 1993):

1. All species will become established throughout the current range of L. salicaria  in North America.
2. The root feeder H. transversovittatus  and the two leaf-feeders G. calmariensis  and G. pusilla  will be most important in reducing large populations.

   
   Left: The root feeder, Hylobius transversovittatus ; right: Galerucella  sp.
   Photographs by Bernd Blossey

   The flower and seed feeders will stabilize smaller populations, further reducing seed output in such a way that not every disturbance will lead to a new outbreak of L. salicaria. 
3. Combinations of agents will have greater control effect than any species alone.
4. Control of L. salicaria will be achieved more rapidly in mixed plant communities with competition for space and nutrients.
5. Purple loosestrife abundance will be reduced to 10% of its current level over 90% of its range.

These predictions are the backbone of our current research. Many of the projects outlined below are designed to test these hypotheses.

Release techniques

Despite a long history of using insects for weed control and considerable improvement in procedures, only about 60% of released agents become established (Crawley 1989). The influence of factors (agent taxonomy, climatic pre-adaptations, number of individuals released, number and timing of releases, predators, and weather conditions) in determining the fate of releases lacks scientific evaluation and are largely observational (Crawley 1989; Lawton 1990). We released agents collected from climatically different source populations across North America and started experiments to determine the best release procedure. Agents became established across the entire continent regardless of source populations, number of agents released, time of release, stage released, or whether caged releases or open field releases were conducted (Hight et al. 1995).

In addition, investigations evaluated the potential of predators and parasites to negatively affect establishment and impact of the potential control agents (M. & C. Tauber, unpublished; Blossey & Ehlers 1991), and compared releases of inbred and outcrossed individuals of G. calmariensis  (R. Roush, unpublished data). F. Grevstad has started a larger study on the effect of different founder sizes on establishment success and dispersal. These investigations will continue.

Which agents are successful?

Harris (1981) proposed that biocontrol agents be considered stress factors; the aim being to increase stress load (increase the number of control agent species/plant) until the balance is tipped towards the disadvantage of the target weed population. Recently, agent combinations were reported to be more destructive to plants than a single species alone (Fowler & Griffin 1995; Hallett et al. 1995). Masters et al. (1993), however, found that spatially separated herbivores interact via their common host plant. Root-feeders showed a reduced performance if their host plant was simultaneously attacked by an above ground herbivore. Above ground herbivores on the other hand showed improved performance on plants simultaneously attacked by a root-feeder. Whether these interactions have any influence on the success of weed biocontrol in systems where above and below-ground herbivores were released needs further study. We are currently conducting these experiments for the L. salicaria-Galerucella-Hylobius  system. This is a good example of how an ongoing biological control program can benefit from simultaneously conducted basic research, and vice versa.

Follow-up monitoring

Increased attention is given to follow-up studies to monitor target plant and control agent populations. The future of biological weed control is intimately linked to the demonstrated safety and efficacy of our programs. For example, releases of control agents against L. salicaria  in the state of Wisconsin were only allowed once the Department of Natural Resources agreed on a monitoring plan for insect and plant populations. Thus, an important consideration becomes choosing one of the many different methods of monitoring insect or plant populations. Our goal has been to develop standardized monitoring guidelines sophisticated enough to allow valuable scientific evaluation but at the same time simple enough to allow participation by wildlife managers or their staff with little guidance. Preliminary versions of a monitoring guide have been tested in 1994 and 1995 and a final version will be distributed by the end of 1996. This should allow the comparison of results across the entire distribution of L. salicaria. 

Redistribution programs

Mass rearing is often an integral part of a biological control program; control agents are generally in short supply. A major concern have been potential negative side effects of laboratory mass rearing (e.g. adaptations to rearing conditions) and reduced quality of the produced insects (Hopper et al. 1993). We have experimented with various field and laboratory mass rearing techniques (Blossey & Hunt, unpublished) and found a reduced fecundity and an increased mortality associated with increasing duration of artificial rearing conditions. We now prefer to mass produce all species outdoors for one generation and allow subsequent overwintering. During 1994 and 1995 about 90,000 leaf-beetles were shipped to 23 different states and Canada to collaborators in a wide range of organizations (universities, State Departments of Agriculture and Natural Resources, National Wildlife Refuges, Bureau of Reclamation, Tennessee Valley Authority, Animal Plant Health Inspection Service); many have started their own mass rearing program. We believe that we need to be concerned about the quality of insects released, not the quantity, and recommend outdoor mass rearings. Releasing fewer, but fitter, individuals might be a much more successful approach. Quality control should accompany every mass rearing program.

Conclusion

A number of factors have contributed to the rapid growth of a coordinated biocontrol effort for purple loosestrife in the United States. L. salicaria,  based on its rapid spread, projected range, and severity of impact, was identified among the most harmful non-indigenous species in the United States (US Congress 1993). This designation created interest for improvements in management approaches, including biological control, across the entire continent. People actively involved in the control program against purple loosestrife are often resource managers, essentially a new audience for biological weed control. Their willingness to participate in basic research has enabled us to implement a scientific approach to the entire program with the intention to improve biological control as a science.

From its inception, the biological control program against L. salicaria  has been a multi-agency effort. The overseas exploration by the International Institute of Biological Control was conducted in association with the US Fish and Wildlife Service and the USDA Agricultural Research Service (ARS). The initial success of the interagency effort led to the formation of a scientific advisory group (Purple Loosestrife Working Group, PLWG) with representation from several US federal and state agencies, universities, and Canada. Since 1986 this working group has provided continual guidance on all aspects of our biological control program.

One of the major accomplishments has been to keep federal and state agencies actively involved, informed through internal annual reports and through participation in decision making processes. This broad based involvement has facilitated maintenance of secure funding since 1985. Particularly important was the ability to pool resources from a variety of sponsors. Once the first insects became available in 1992 they were distributed to 7 states and to Canadian cooperators. Workshops held in Colorado and Minnesota in spring 1993 introduced interested agencies to life-history of control agents, mass rearing techniques, follow-up studies and monitoring techniques. Brochures summarizing the available information were produced to encourage active participation of other agencies in the control program. In addition to regular meetings of the PLWG, we now conduct annual planning meetings for the future of the control program. Participants represent 20-30 states (and include Canadian cooperators) and many federal agencies.

References

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Blossey, B. (1995b) Coexistence of two competitors in the same fundamental niche. Distribution, adult phenology and oviposition. Oikos, 74: 225-234.

Blossey, B & Ehlers, R.U. (1991) Entomopathogenic nematodes (Heterorhabditis  spp. and Steinernema anomali)  as potential antagonists of the biological weed control agent Hylobius transversovittatus  Goeze (Coleoptera: Curculionidae). Journal of Invertebrate Pathology, 58:453-454.

Blossey, B. & Schroeder, D. (1995) Host specificity of three potential biological weed control agents attacking flowers and seeds of Lythrum salicaria  (purple loosestrife). Biological Control, 5: 47-53.

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Tables

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