Invasive species have been associated with billions of dollars in economic and environmental losses in the

United States, including yield and quality losses for farmers and ranchers. Particularly problematic 

invasive species include insects (Mediterranean fruit fly, Emerald ash borer, Asian longhorned beetle),

aquatic invertebrates (zebra mussel), weeds (kudzu, yellow starthistle, purple loosestrife), plant diseases

(Asian soybean rust, citrus canker, sudden oak death), animal diseases (exotic Newcastle disease), and

zoonotic diseases (West Nile virus). Invasive species clog and foul waterways, impair the productivity of

pastures, reduce property values and environmental amenities associated with parks and natural 

ecosystems, and threaten the survival of indigenous wildlife. Moreover, some invasive pathogens pose

health risks to humans.

Programs and policies to minimize the threat of, or mitigate the damages from, invasive species work best

if designed in concert with each other. Whether program emphasis should be on prevention or control

depends on the biological characteristics and size of the invasive species population, ecological 

characteristics of invaded ecosystems, the cost and efficacy of prevention measures relative to control

measures, and the level of prevention costs borne abroad. Because all of these factors are highly variable,

data needs are constant if intervention is to be both effective and economical.
The Invasive Species Management Problem

The value of U.S. agricultural imports grew 7 percent per year between 1995 and 2004. Over the same

period, foreign travel expenditures in the United States and domestic travel expenditures abroad

increased 3 and 4 percent annually. The economic benefits of trade and travel, however, have brought

with them a heightened risk of invasive species, like the Mediterranean fruit fly, becoming established

within our borders (Hlasny and Livingston, 2008). Although most introductions of invasive species

are believed to be facilitated by humans (National Research Council, 2002), harmful organisms like

Asian soybean rust also enter the United States via wind and water currents. Once established, pests

often move between agricultural fields and other domestic areas naturally and via the movement of

contaminated vehicles, packaging materials, and commodities.

Because agricultural pests are difficult to detect and eliminate, and because many are highly mobile,

population control in one area may affect population levels, damage, and control costs in nearby areas.

If the pest control decisions made in one area are based solely on the benefits to individuals making

those decisions, the level of prevention and control effort is likely to be less than desirable from

society’s perspective. When the prevention and control efforts in one area affect pest 

population levels in other areas, coordination of decisions by all of those affected often requires

government intervention. For this reason, government 

agencies around the world regularly devise and implement

prevention and control policies.

Limiting new invasions, controlling outbreaks of established

populations, and restoring native ecosystems in the U.S. 

occupies 21 Federal departments and agencies, with USDA’s

Animal and Plant Health Inspection Service (APHIS) taking the

lead for pests of agricultural significance, forest pests, and

zoonotic diseases. Figure 1 shows how APHIS budget 

expenditures are divided among prevention, control, and other

activities. Monitoring activities encompass both prevention and

control, while emergency spending is for rapid-response 

eradication (control) programs. Designing programs that work

together to deal with invasive species problems requires

economic as well as biological and ecological research. 

The Invasive Species Solution—Creating Synergy

Previous research recognizes the interdependence of policies designed to limit the introduction of

invasive species (prevention) and those designed to reduce or eradicate domestic populations of 

invasive species (control), as well as the practicality of designing such policies simultaneously and

harmoniously. This report synthesizes recent research highlighting the information and data required

when developing policies (for government decisionmakers) or deciding on a course of action

(for agricultural producers). Especially important to consider are the biological characteristics of the 

invasive species, ecological characteristics of the invaded environment, the costs and benefits of

prevention and control policies, the ways in which the actions of decisionmakers in the public and

private sectors combine to influence management outcomes, and whether the costs of prevention are

borne domestically or abroad.

Figure 1

APHIS expenditures during fiscal years 2003-2007

2003 2004 2005 2006 2007

 $ Million

Source: U.S. Department of Agriculture fiscal year budget summaries.

1,600

1,400

1,200

1,000

800

600

400

200

0

Prevention

Monitoring

Control

Emergency programs

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For some invasive species, domestic regulatory authorities and agricultural producers directly 

influence the rate of introduction through preventive efforts. The cost of these efforts is borne 

domestically, by U.S. taxpayers who fund public prevention and control programs and by agricultural

producers and other landowners who implement prevention and control practices to protect their 

operations, businesses, or property. For example, ships and airplanes leaving Guam for Hawaii are

routinely inspected for brown tree snakes. The State of Hawaii pays the Government of Guam for

canine inspections at civilian ports and airports, as well as for the maintenance of snake detection traps

along fences surrounding ports and airports in Guam. Similarly, livestock and broiler producers can

reduce the likelihood of an animal disease outbreak by constructing borders that separate cultivated

animals from wild animals, maintaining sanitary working conditions, and vaccinating animals against

specific diseases (Hennessy, 2007). Research examining this type of invasive species management has

offered a number of policy options.

Prevention and Control Programs Are Substitute Instruments in Invasive Species Management.

If a prevention program—such as inspecting ships, airplanes, and cargo in Guam headed to Hawaii for

brown tree snakes—is more effective and less costly than alternative control programs—such as

searching the difficult Hawaiian terrain for brown tree snakes and eliminating them—devoting more

resources to prevention than control makes economic sense (Burnett et al., 2006; Kim et al., 2006;

Olson and Roy, 2005). In the case of a windborne fungus like Asian soybean rust, which entered the

United States in 2004, the only way to have prevented entry would have been to eliminate the pathogen

from all areas linked atmospherically to the country. Such a program would be extremely costly and

ineffective because the fungus is highly fecund and can live on over 95 plant hosts (Livingston et al.,

2004). APHIS’ response to the threat, therefore, focused on educating U.S. farmers on appropriate

control techniques, viewing the introduction of the pathogen as inevitable (Roberts et al., 2006). 

The relative allocation of resources devoted to prevention and control programs will depend on 

biological characteristics of the invasive species, including its population size and rate of growth (Kim

et al., 2006; Olson and Roy, 2005), the minimum number of organisms required to establish a 

self-sustaining population (Burnett et al., 2007a), the carrying capacity of potential host areas (Kim et

al., 2006), and the cost and efficacy of prevention relative to control. For example, when the rate at

which an invasive species outbreak occurs is high, or varies widely from year to year, it may be more

efficient to focus additional funds on control programs rather than prevention programs (Keohane, 

Roy and Zeckhauser, 2000, 2007).

While drawn from the economics literature dealing with the prevention and control of invasive

species, much of the information presented here was from studies funded by the Program of

Research on the Economics of Invasive Species Management (PREISM). Under this program, the

Economic Research Service conducts research and funds extramural research to support the

economic basis of decision making concerning invasive species of significance to agriculture or

USDA. For more information on PREISM, including detailed activities reports and published

research, visit http://www.ers.usda.gov/Briefing/InvasiveSpecies/.

Invasive Species Research at ERS

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UNITED STATES DEPARTMENT OF AGRICULTURE
Level of Control Needed Is Higher for Larger Initial

Outbreaks. Not surprisingly, the level of resources needed to

control an invasive species outbreak tends to be higher when

the initial size of the pest population is high (Olson and Roy,

2005). On the other hand, the cost of removing an additional

organism is smaller for larger pest populations because it is less

costly to find and deal with invasive organisms in dense pockets

of infestation. The cost of removing an additional organism

generally grows as more organisms are removed because 

the cost of finding and targeting the remaining organisms

increases. Figure 2 demonstrates that the cost of removing an

additional amount of the invasive weed Miconia calvescens on

the island of Oahu (Burnett et al., 2007a) is much more 

sensitive to changes in the population size than to changes in

the level of control (i.e., the number of organisms removed).

Figure 3 demonstrates the opposite case—the additional cost of

control increases more rapidly with the level of control than it

decreases with the population size. For example, pesticide

expenditures increase with the percentage of insects removed

from a given field, especially if a fraction of the population is

resistant to the pesticide.

These results have important implications for the tradeoff

between spending additional funds on prevention or control.

For the M. calvescens case, the level of spending on prevention

declines for larger initial outbreaks. This occurs because the

cost of control decreases relative to the cost of prevention,

highlighting again the way in which prevention and control

measures are substitute policy instruments. However, in the insect management case, the reverse is

true: the level of spending on prevention should increase because the cost of control increases 

relative to the cost of prevention.

Olson and Roy (2005) examine a static environment; however, their basic idea—that it is important to

understand how the additional cost of control depends on both the level of control and the 

population size—is relevant under dynamic management settings. Because the additional cost of 

eliminating a single organism is smaller for larger populations, costs per organism increase as a control

program reduces the invasive population over time. If incremental control costs increase enough 

as the population declines, rather than eliminating the pest, it may be less expensive to 

maintain a small target population level, using control measures to remove new growth and new

entrants from a particular region and prevention measures to reduce the rate of new introductions

(Burnett et al. 2006, 2007a; Kaiser, 2006). Indeed, in many cases, allowing low populations of an 

invasive species to survive may be economical because of the difficulty and extreme cost of finding

and eliminating each organism (Burnett et al. 2006, 2007b).

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Figure 2

Cost of controlling Miconia calvescens in 

Oahu by population size versus level of control

4,000 6,000 8,000 10,000

Population size, y (a = 4,000) Level of control, a (y = 10,000)

Dollars Dollars

140

120

100

80

60

45.1

45.0

44.9

44.8

4,000 6,000 8,000 10,000

Figure 3

Cost of controlling a hypothetical insect infestation, 

by population size versus level of control

10 20 30 40 10 20 30 40

Population size, y (a = 10) Level of control, a (y = 40)

1.4

1.3

1.2

1.1

1.0

0.9

12

10

8

6

4

2

Dollars Dollars
Optimal Spending Increases with More Rapid Infestations and More Conducive Habitats.

Resources needed to control invasive species will be larger if they can grow and spread rapidly 

(Kim et al., 2006; Olson and Roy, 2005). Similarly, spending on prevention (before an invasive

species is discovered) will need to be larger for areas that may allow larger populations to develop—

areas lacking biological and environmental constraints (like natural predators or unfavorable climate)

on the growth of invasive populations. For example, forests with large numbers of dense ash tree

stands would receive larger allocations under an economical Emerald ash borer management program,

not only because those stands are valuable and vulnerable to attack, but also because, if left 

unprotected, the stands would harbor the pest and allow it to spread more quickly. 

Economical Policies Depend on the Stage of an Invasion and Growth Rates. Because pest invasions

are dynamic processes, policies designed to ameliorate their impacts must be dynamic as well. As the

level of a (controlled) invasive population changes, the control cost changes too, so prevention and

control policies must be fluid. Simply recognizing the dynamic nature of the invasion process has

important policy implications. For example, immediate eradication may be economical for smaller

invasions, such as the periodic, contained outbreaks of the Mediterranean fruit fly (medfly) that have

occurred in California and Florida since 1929. However, achieving and maintaining a reduced 

population level may be economically efficient for larger, less contained infestations, such as the 

M. calvescens weed outbreak in Hawaii. Moreover, if eradication is feasible, total damages are likely

to be minimized if control efforts are implemented early, when the population is small. Underfunded

control efforts may be wasteful, having little effect on population growth and related damages. 

Higher initial expenditures may reduce long-term damages and control costs, even if the species 

is not eradicated.

Accounting for the Intertwining Effects of Private and Public Actions Is Essential. It is important

for public decisionmakers to understand how agricultural producers and/or homeowners may respond

to outbreaks of invasive species, and how private sector responses may be affected by government

policies (Finnoff et al., 2005a, 2005b). Otherwise, it is unlikely that the benefits and costs associated

with proposed policies will be estimated accurately. For example, pest control in one area serves as

a preventive measure for areas linked via a pathway. However, individuals typically control pests in

accord with their own costs and benefits and not society’s. In these settings, public monitoring and

control programs, such as APHIS’ Fruit Fly Cooperative Control Program, may help to improve social

welfare by coordinating population management across regions (Baumol and Oates, 1988).

Uncertainty and Risk Perception Can Complicate Decisions. While quick and decisive action can

be cost effective, when the rate of outbreaks varies considerably over time or space, there may be

value in delaying implementation of control measures. It is rare that decisionmakers enjoy perfect

information about the extent of an outbreak. But once committed, control efforts are irreversible 

(i.e., the expenditures cannot be recouped), so until the full extent of the problem is understood, rapid

response may be wasteful (Dixit and Pindyck, 1994).

In addition, the benefits and costs of prevention and control programs are generally uncertain upon

and even during implementation. If individual decisionmakers view the risk of an outbreak differently

(that is, some are more risk averse than others), then the levels of prevention and control provided are

likely to differ depending on who is making the decisions. For example, if the benefits associated with

control are perceived to be more certain than those associated with prevention, decisionmakers will

tend to focus more resources on the control of invasive species (Finnoff et al., 2005a, 2005b; 

Olson and Roy, 2005).

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UNITED STATES DEPARTMENT OF AGRICULTURE
When Prevention Costs Are Borne Abroad, Higher Infestation Rates Call for Heightened 

Restrictions. Sanitary and phytosanitary (SPS) import regulations can affect U.S. infestation rates

only indirectly, and foreign parties generally bear most of the direct costs associated with U.S. 

prevention programs. Therefore, understanding how agricultural producers abroad react to U.S. SPS

import regulations is essential to the effective design of the overall invasive species management

package (Peterson and Orden, 2006). For example, because the medfly is known to exist in 

65 countries that export fresh produce to the United States, USDA regulates the process by which it is

imported. Among the approved and commonly applied ways to eliminate larvae inside infested

produce is cold treatment: refrigeration at a mandatory average temperature for a specific duration.

Livingston (2007) examines cold-treatment schedules that maximize the net benefits of U.S. producers

and consumers of produce using a simulation model, wherein foreign producers control medflies to

maximize their profit. If medflies survive pesticide sprays, cold treatment, and domestic

monitoring/control programs, an outbreak can cause U.S. produce growers to suffer yield losses and

increased production costs.

The efficient number of days to treat imports increases with the severity of medfly outbreaks abroad.

When medfly populations abroad are unusually high, the share of exported commodities infested with

medfly eggs and larvae is higher, requiring heightened population control abroad and more stringent

SPS import restrictions. The appropriate level of prevention—both the stringency of the SPS 

regulations and the effort and direct costs incurred abroad—increases with higher risks of infestation.

When pest pressure is at or below historic levels abroad, economic incentives of foreign producers

may be consistent with U.S. policy; this is less likely when pest pressure is above average. 

This finding demonstrates the importance of monitoring compliance with international cold-treatment

regulations, and provides some justification for USDA’s practice of doing so.

Conclusion

This report synthesizes the implications of a series of studies that describe the information and data

needed by public and private decisionmakers. Keeping detailed records about the estimated size of an

invasion, control costs, and the numbers of organisms removed—or acreage cleared—will enable 

decisionmakers to modify control programs as needed to improve program efficacy and economic 

efficiency. Information about an organism’s ability to spread and a potentially invaded ecosystem’s

carrying capacity is also important. Understanding how agricultural producers and homeowners, both

at home and abroad, will respond to outbreaks and public prevention/control policies is important,

especially when the decisionmaker can affect the risk of infestation only indirectly.

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Baumol, W.J., and W.E. Oates. 1988. The Theory of Environmental Policy, 2nd ed. Cambridge: Cambridge 

University Press.

*Burnett, K., B. Kaiser, and J. Roumasset. 2007a. “Economics Lessons from Control Efforts for an Invasive

Species: Miconia calvescens in Hawaii,” Journal of Forest Economics, Volume 13, Issues 2-3, August. 

*Burnett, K., B. Kaiser, B.A. Pitafi, and J. Roumasset. 2006. “Prevention, Eradication, and Containment of 

Invasive Species: Illustrations from Hawaii,” Agricultural and Resource Economics Review 35: 63-77. 

*Burnett, K.M., S. D'Evelyn, B.A. Kaiser, P. Nantamanasikarn, and J.A. Roumasset. 2007b. “Beyond the Lamppost:

Optimal Prevention and Control of the Brown Tree Snake in Hawaii,” Ecological Economics 52(3).

Dixit, A.J., and R.S. Pindyck. 1994. Investment Under Uncertainty. Princeton, NJ: Princeton University 

Press.

*Finnoff, D., J.F. Shogren, B. Leung, and D. Lodge. 2005a. “Risk and Nonindigenous Species Management,”

Review of Agricultural Economics. 27(3): 475-482.

*Finnoff, D., J.F. Shogren, B. Leung, and D. Lodge. 2005b. “The Importance of Bioeconomic Feedback in Invasive

Species Management,” Ecological Economics. 52: 367-381.

Hennessy, D.A. 2007. “Behavioral Incentives, Equilibrium Endemic Disease, and Health Management Policy for

Farmed Animals,” American Journal of Agricultural Economics 89(3): 698–711.

*Hlasny, V., and M.J. Livingston. “Economic Determinants of Invasion and Discovery of Non-Indigenous Insects.”

Journal of Agricultural and Applied Economics. 40(1)(April 2008): 37-52.

*Kaiser, B.A. 2006. “On the Garden Path: An Economic Perspective on Prevention and Control Policies for an

Invasive Species,” Choices 21(3): 139-142. 

Keohane, N., B. Van Roy, and R. Zeckhauser. 2000. “Controlling Stocks and Flows to Promote Quality: The Environment, with Applications to Physical and Human Capital.” NBER Working Paper 7727. Cambridge, MA, June. 

Keohane, N., B. Van Roy, and R. Zeckhauser. 2007. “Managing the Quality of a Resource with Stock and Flow

Controls,” Journal of Public Economics 91: 541-569.

* Citations preceded with an asterisk are PREISM-funded studies (see box, “Invasive Species Research at ERS”

on p. 3).

This brief is drawn from . . .

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UNITED STATES DEPARTMENT OF AGRICULTURE
*Kim, C.S., R.N. Lubowski, J. Lewandrowski, and M.E. Eiswerth. 2006. “Prevention or Control: Optimal 

Government Policies for Invasive Species Management,” Agricultural and Resource Economics Review

35: 29-40. 

*Livingston, M.J. 2007. “The Mediterranean Fruit Fly and the United States: Is the Probit 9 Level of Quarantine

Security Efficient?” Canadian Journal of Agricultural Economics 55: 517-528. 

*Livingston, M.J., R. Johansson, S. Daberkow, M.J. Roberts, M. Ash, and V. Breneman. 2004. Economic and

Policy Implications of Wind-Borne Entry of Asian Soybean Rust into the United States. OCS-04D-02, Economic

Research Service, U.S. Department of Agriculture, April. 

National Research Council. 2002. Predicting Invasions of Nonindigenous Plants and Plant Pests. Washington,

DC: National Academy Press. 

*Olson, L.J., and S. Roy. 2005. “On Prevention and Control of an Uncertain Biological Invasion,” Review of 

Agricultural Economics 27(3): 491-497.

*Peterson, E., and D. Orden. 2006. “Linking Risk and Economic Assessments in the Analysis of Plant Pest 

Regulations: The Case of U.S. Imports of Mexican Avocados.” Contractor and Cooperator Report No. 25, 

Economic Research Service, U.S. Department of Agriculture. October. 

*Roberts, M.J., D. Schimmelpfennig, E. Ashley, M.J. Livingston, M. Ash, and U. Vasavada. 2006. 

The Value of Plant Disease Early-Warning Systems. ERR-18. Economic Research Service, U.S. Department of

Agriculture, April.

This brief is drawn from (Continued) . . .

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