Two pictures side by side of living trees lining a street followed by dead trees lining the same street.

Calling on nature to combat invasions

Guest post by Ryan Paul, 2018-2019 Sustainability Leadership Fellow and Ph.D. Candidate in the Department of Bioagricultural Sciences and Pest Management and the Graduate Degree Program in Ecology

Pest. It’s a word that often instills anger. For many, the first thing that comes to mind is an insect that destroys crops and raises the price of food. To make things worse, many of the pests that we desperately fend against are introduced by our own doing. As a result of globalization and increased international trade, new pests are introduced and those which become particularly problematic are labeled as “invasive species.” Some of these have widespread ecological impacts. We soon find ourselves asking – how do we mitigate these effects? What management strategies can we use to combat these invasive species?

Integrated pest management (IPM) uses a combination of control measures to manage pest populations. Pest populations are monitored, and when their population level reaches a set threshold, action is taken to control the pest including combinations of pesticides, mechanical removal, or trapping. Another less frequently considered IPM strategy is biological control – indeed, the EPA website on IPM does not even mention biological control. Yet this management strategy often deserves more attention than it receives.

Biological control is the perfect tool to tackle widespread invasive pests because it has a mind of its own! Biological control involves using another organism to control the pest. Agents used in biological control are natural enemies of the pest and include predators, pathogens, parasites, and in the case of weed management – herbivores. Biological control agents disperse on their own as they seek out and consume their pest resource and can form self-sustaining populations which provide continued management year after year. Not only that, but natural enemies are better at locating the pests than we could ever be. Invasive species can quickly become widespread and unmanageable. In many cases, the spread of these species far outweighs the ability of humans to control them effectively.

Historically, biological control involved introducing generalist non-native predators to combat the invasive pest. We’ve seen all too well how horribly that can end (e.g. the cane toad in Australia). Many of these early biological control efforts validate peoples’ skepticism. However, advances in policy and research have led to more practical implementations of biological control and many concerns can be put to rest. (For more background read the blog post by Dhaval Vyas.)

The big question is: why do we need biological control? The answer is simple: because we can’t manage certain pests without it.  In some cases, biological control may be the most practical or maybe even the only practical option for managing some invasive species. Despite the poor outcomes of some early efforts, not all biological control programs have been disasters. The prickly pear (Opuntia spp.) in Australia was a case that dramatically benefited from biological control and ended in great success. In the 1920s, prickly pear covered tens of millions of hectares of land in Australia and was estimated to spread over 320,000 hectares per year. Costs for removing prickly pear were often higher than the total value of the land it occupied. Settlement of new land in much of the country was halted due to the overrunning of prickly pear on those lands. Salvation finally came with the release of the caterpillars of Cactoblastis cactorum, which decimated the prickly pear populations in less than 10 years and still provide a sustainable control for the cactus today1.

Prickly pear infestation in Australia
Prickly pear infestation in Australia before the introduction of Cactoblastus cactorum caterpillars. Photo credit: The Alan Fletcher Research Station, Department of Lands, Queensland
Clear field after prickly pear infestation had been removed
Prickly pear infestation in Australia after introduction and establishment of Cactoblastus cactorum caterpillars. Photo credit: The Alan Fletcher Research Station, Department of Lands, Queensland

Widespread pests like this still exist and need to be managed. Let’s look at an especially devasting example: the emerald ash borer (EAB, Agrilus planipennis). Native to Asia, this beetle was first introduced to North America near Detroit, MI in the 1990s. Since then, it has been introduced to more than 35 states and caused widespread mortality to ash trees, a dominant tree type in urban landscapes and forests across North America. The EAB larvae feed underneath the bark and cut off nutrient transport from the roots. Ash is a major timber tree and many wood products prices have increased from the shortage of trees.  Not to mention, chances are, your neighborhood has ash trees and those dead trees are expensive to remove and decrease property values. Not only has EAB caused tens of billions in tree removal and replacement costs, the death of forest ash trees has left canopy gaps which fundamentally alter plant communities and could threaten the extinction of ash and dozens of species that rely exclusively on ash as a food source. EAB is fundamentally altering diversity in North American deciduous forests. Early eradication efforts for EAB were abandoned out of futility and management of individual trees on such a scale is impossible2. In this situation, we must turn to biological control as the answer.

Two pictures side by side of living trees lining a street followed by dead trees lining the same street.
A street in Toledo in 2006 (left) and 2009 (right) showing the extensive tree death from emerald ash borer. Photo by Daniel A. Herms, the Ohio State University.

For control of EAB, the natural enemies chosen were nonnative parasitoid species. Parasitoids are insects which develop in or on another insect host and ultimately kill it. Think of the movie Alien except the xenomorph is a wasp and the host is EAB instead of a human. Since these parasitoids require hosts to reproduce, they excel at finding them and can locate EAB larvae inside the tree trunk! Most of the time, it takes humans the entire two-year life cycle of EAB to see symptoms of infestation, when it is often too late to remedy, but parasitoids will find larvae within a month or two. Even without human intervention, the high EAB larvae abundance has led a native parasitoid species and woodpeckers to switch to attacking EAB larvae2.

A parasitic insect infecting an invasive species
A native parasitoid (Atanycolus sp.) parasitizing Emerald ash borer. Photos by David Cappaert,
A smaller insect larva parasitizing a larger invasive species larva
A parasitoid larva developing on larval Emerald ash borer. Photos by David Cappaert,

In order to protect our environment from widespread invasive species, we need biological control. Certain pests will inevitably run amok, which would put countless numbers of native species at risk. Biological control provides us with a way to combat this potential overtake of our environment. Biological control inherently has risks, though, so we need research to focus on selecting the right control agents. Many more biological control projects are currently being implemented and will be needed. Despite our advances in detecting introductions, we haven’t seen the last major invasive species and many questions still need to be answered to improve biological control. What areas of research should we focus on first? What changes in regulations for biological control would improve success?

  1. Raghu, S. & Walton, C. Understanding the ghost of Cactoblastis past: historical clarifications on a poster child of classical biological control. Bioscience 57, 699–705 (2007).
  2. Herms, D. & McCullough, D. G. Emerald ash borer invasion of North America: history, biology, ecology, impacts, and management. Annu. Rev. Entomol. 59, 13–30 (2014).
  3. “Integrated Pest Management (IPM) Principles.” EPA, Environmental Protection Agency, 11 Apr. 2019,

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