The Invasion Curve: Managing Invasive Species in Pennsylvania

Introduction

The invasion curve is a practical framework for understanding invasion stages and choosing the right management strategy at the right time (Figure 1). It is widely used in invasive species management because it helps managers identify which actions are most effective at each stage of an invasion.

Biological invasions typically start slowly. After a species arrives in a new place, there is often a quiet "lag phase" when populations stay small and are easily overlooked. Then population growth takes off, spreading rapidly across the landscape until the species runs out of suitable habitat or resources and levels off. As the invasion progresses, the impacts increases— to ecosystems, economies, and the budgets needed to manage it. At the same time, the chance of eliminating the species for good steadily shrinks.

Pennsylvania's experience with the spotted lanternfly (Lycorma delicatula) shows how quickly an invasion can move along this curve. The insect was first confirmed in Berks County in 2014, but it had likely been present for some time before it was detected — the lag phase, when "Keep it out" and early detection offer the best and cheapest chance of success (Urban 2020). Once it was identified, the state-imposed quarantines and treated infested trees to "Slow it down," yet populations expanded faster than containment could keep pace (Urban 2020; Urban & Leach 2025). Within a few years the species had spread across much of southeastern Pennsylvania and into neighboring states, moving the response into the "Hold the line" stage, where managers now focus on protecting high-value resources such as vineyards rather than on eradication. The spotted lanternfly illustrates a central lesson of the invasion curve: the window for eradication is brief and easily missed, while the cost of action rises sharply once that opportunity has passed.

The four stages of the invasion curve

The "S" shape of the invasion curve comes from classic population growth models. Because many biological invasions follow a similar pattern, the curve provides a practical guide for management decision-making. Managers typically divide the curve into four stages, each requiring a different response (Figure 1). These stages can be summarized with a plain-language action that captures what managers are trying to do:

• Prevention — Keep it out. The species has not yet arrived and stopping it from getting in is the cheapest and most effective option.
• Eradication — Wipe it out. A small, localized population has been detected, and removing it entirely is still possible.
• Containment — Slow it down. The species is spreading; the focus shifts to limiting how far and how fast.
• Long-term management — Hold the line.

The species is established across the landscape, and active, sustained work is needed to reduce its impact and protect what matters most. This is not maintenance or surrender — it is ongoing defense of the places that matter. Every invasion is different. Some species spread rapidly, while others remain localized for years before expanding. Although the pace and shape of an invasion vary, the invasion curve provides a useful framework for deciding when and how to act. Managers use the invasion curve to prioritize species by urgency and likelihood that management will succeed. For example, the Pennsylvania Department of Conservation and Natural Resources uses an "early detection, rapid response" approach that focuses on spotting new invaders quickly and targeting the highest-priority species when eradication is still feasible. The Governor's Invasive Species Council coordinates efforts across state agencies and developed the Pennsylvania Invasive Species Management Plan, which sets out a comprehensive plan for prevention and control. Together, these efforts translate the invasion curve from a simple concept into practical decisions about where to invest limited time, funding, and resources (Table 1). 

Where investment counts the most

Investment of time and money in invasive species management happens at every stage of the invasion curve. Prevention — keeping species out — is consistently the most cost-effective approach and helps avoid irreversible ecological damage. When prevention fails, acting early to wipe out small populations offers the next best return. Slowing the spread of established species buys critical time. And holding the line, where species are already widespread, is what keeps Pennsylvania's forests, waterways, and farmlands resilient over the long run.

The invasion curve helps land managers, agencies, and citizens see where a species falls in the invasion process and what responses will actually work. Matching strategy to stage lets us set realistic goals and distribute resources where they will have the greatest effect. No invasion is too far gone, and no action is too small — Pennsylvania's landscapes are worth every effort it takes to protect them.

What you can do

• Report new invasive species sightings.
• Don't move firewood — buy it where you burn it.
• Plant native species in your landscape and buy plants from local nurseries (Pennsylvania Invasive Replace-ive Program).
• Clean boots, gear, and boats before moving between sites.
• Support local invasive species removal efforts in parks and natural areas.
• Don't dump live bait in waterways or garden waste on land

References 

Victoria Department of Primary Industries. 2010. Invasive Plants and Animals Policy Framework. State of Victoria.

Urban, J. M. (2020). Perspective: shedding light on spotted lanternfly impacts in the USA. Pest management science, 76(1), 10-17.

Urban, J. M., & Leach, H. (2023). Biology and management of the spotted lanternfly, Lycorma delicatula (Hemiptera: Fulgoridae), in the United States. Annual Review of Entomology, 68(1), 151-167.