Discussion Panelist: Dr. Paula Shrewsbury

Takoma Park Native Tree Selection Guide emphasizes the beneficial role of caterpillars in sustaining biodiversity in an urban ecosystem. Dr. Shrewsbury addresses concerns about caterpillars as pests.

She goes on to explain the many benefits that trees provide to people, other than sustaining biodiversity.

Dr. Shrewsbury is a Professor and Extension Specialist in the Department of Entomology at the University of Maryland.  She runs the Shrewsbury Lab, which focuses on identifying methods to restore plant and insect community dynamics that create sustainable urban landscapes, nurseries, and turf systems.  The emphasis in this work is on biological control of invasive and indigenous pests and the conservation of natural enemies, pollinators, and biodiversity.

References for the citations in this Q&A can be found in the References webpage.

Selecting a native tree species that ranks high on the biodiversity indicator means that the tree will provide habitat for many butterfly and moth (Lepidoptera) species. Will that lead to significant leaf damage and defoliation as a result of hosting all those caterpillars?

The tree species per se is not always the only driver of insect outbreaks.  For example, whether a tree and herbivore have co-evolved together can affect outbreaks.  Trees and insects that have co-evolved. As insects have evolved the capacity to circumvent their host trees’ natural defenses, the hosts have in turn evolved better natural defenses against the insects. This co-evolution goes back and forth over hundreds of thousands or more years. A dynamic balance has emerged between the insect’s capacity to attack a tree and the tree’s natural defenses.

It can be an entirely different situation with a native tree and a non-native insect because they have not co-evolved together resulting in a tree not having any natural defenses against a non-native insect. That insect may find a “defense free space” where the tree’s natural defenses cannot deter an insect’s attack. This is the situation we face now with two Asian insects, the emerald ash borer that is decimating native ash trees and hemlock woolly adelgid that is killing our native hemlocks.

A variant on “defense free space” is when a native insect such as bronze birch borer attacks birches in the U.S. Native birches survive (they co-evolved with the bronze birch borer) and non-native birches suffer significant mortality. No co-evolution results in no defenses.

Increasing and maintaining tree diversity can reduce detrimental impacts of non-native and native pests. A monoculture (many trees of the same species planted together) of any tree species, native or non-native, can definitely foster infestations. Insects can quickly spread if lots of their host species are planted close together, and as a result, the insect population can build up quickly. In addition, if an insect such as emerald ash borer is introduced to an area, if 30% of the trees are ash then 30% of the trees will die (and need to be taken down and replaced), whereas if 5% of the trees are ash, only 5% of the trees will die. Diversity will reduce the environmental and economic impact of pest species. It’s the monoculture, not the species that is driving the impact of the infestation. This can happen with all different sorts of insects, and pathogens for that matter, not just Lepidoptera.

Of course, there are other drivers of pest outbreaks besides monoculture tree plantings and non-native insects and pathogens. Examples include a sudden decrease in natural enemies (predators, parasites, and pathogens) as a result of climate change, simplification of the habitat (low plant diversity and increased amount of hardscape), or inappropriate application of pesticides. One big factor in urban tree infestations is urban heat island effects, that is, the higher temperatures in urban areas due to factors in the human-made urban environment such as built surfaces replacing vegetation and consequently re-emitting heat, reducing water infiltration, etc.

Is there an alternative or additional biodiversity indicator that we should include in our guide?

The Tallamy indicator measures trees’ contribution to the food web, which benefits the natural biological community within an ecosystem. This is very important to food web dynamics and ecosystem function.

However, trees provide other benefits that should also be considered.

A premier tool to assist in assessing a tree (or planting of trees) is i-Tree, a suite of applications that is available for free at itreestools.org.1

In addition, there are many lists of plants, including trees, that are recommended because they provide food and other aspects of habitat that support the conservation of beneficials and biodiversity.

The Xerces Society, for example, maintains a huge online library of plant lists. One list contains pollinator-friendly plants for the Mid-Atlantic region, mostly perennial wildflowers but three trees and three shrubs are included (Adamson et al., 2017). Although there is no quantitative indicator, these three trees score “great” as sources of pollen and nectar for native pollinators.

Michigan State University Extension (Smitley et al 2019) published a bulletin with separate lists of trees and other plants attractive to bees and butterflies respectively. This publication also provides extensive guidance on best pest management practices for key pest problems, including when to apply pesticides and which products have lower non-target impacts on beneficials.

1i-Tree applications

Editor’s note: Among the suite of applications referred to above by Dr. Shrewsbury, the i-Tree Species app in particular allows a homeowner to select a tree species based on the overall benefits it will deliver to the ecosystem. The i-Tree Species app will assess the value of trees based on several ecologically related factors and help to select trees with specific values of importance.  

The ecological factors evaluated in the app are,

  • Air Pollutant Removal
  • Ultraviolet Radiation Reduction
  • Carbon Storage
  • Pollen Allergenicity
  • Storm Water Impacts
  • Wind Reduction
  • Air Temperature Reduction
  • Building Energy Conservation
  • Low Volatile Organic Compound Emissions

For example, if air pollutant removal is the most important service homeowners want from their tree, they can assign a value of 10 to that, and a 0 to the other eight variables.  i-Tree will produce a list of tree species both suitable for planting in their geographic area, and in the top 10% of species in the i-Tree data base for removing air pollutants. 

Some further review of these lists is then needed, as they may include species that should not be planted for other reasons, for example, invasive species, or native trees that should not be planted, such as ashes due to the emerald ash borer problem.

The i-Tree lists will not by any means include only or mostly native trees.  There are any number of non-native trees that will grow well in, say, Takoma Park MD, and perform superbly at one or more of the nine ecological variables measured by i-Tree.

However, none of the apps in the i-Tree toolbox attempts to value the biodiversity services from various tree species to humans, or to the functioning of a natural ecosystem in an urban area.

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