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Are My Plants Okay? How to Respond To Insects and Diseases in Trees and Shrubs

By Sharon Selvaggio on 27. June 2024
Sharon Selvaggio

In the presence of trees, it’s easy to feel calmer, sheltered, and at peace. As a child, I was a frequent visitor to the trees in our yard. Weeping willow or maple – it didn’t matter. I sought out trees as places to climb, but more importantly, as shaded and quiet places to bring my books and read.

The Xerces Society’s new fact sheet Responding to Insects and Diseases on Landscape Trees and Shrubs is designed to help you protect your woody plants—and the local insect communities they evolved with.  
 

Trees and shrubs support an entire community of wildlife

As important as they are for us, trees and shrubs in our towns and cities provide much more. They are vital food and shelter for bees, butterflies and other insects. The resources trees and shrubs provide are impressive! Consider this:  by one estimate, a single sugar maple tree produces a hundred billion pollen grains annually, while an oak tree can supply even more. Tree pollen often makes up 25–100% of a bee’s pollen diet, depending on the species. Even wind-pollinated trees like maples and oaks are often visited by bees gathering both nectar and pollen. 

While we might associate butterflies and moths with wildflowers, many species also depend on trees and shrubs. This is often during the first stage of their lives, as caterpillars. For example, across the country, there are 534 butterfly and moth species that rely on oak trees, and 288 species that use native blueberry shrubs and close relatives. Of course, all of the invertebrates that depend on trees and shrubs are just one part of a connected food web. Whether we get to see our  favorite backyard birds, such as wood warblers, wrens, or bluebirds, depends on how well our plants support a vibrant and healthy insect community.  
 

An Arizona sister butterfly (Adelpha eulalia) resting on the leaves of an oak tree.
This Arizona sister (Adelpha eulalia) is just one of many butterflies and moths that feed on oak leaves as caterpillars. Other great options for supporting wildlife include:  willows (Salix spp.), stonefruit trees (Prunus spp.), pines (Pinus spp.), poplars and cottonwoods (Populus spp.), birches (Betula spp.), blueberries and huckleberries (Vaccinium spp.), maples (Acer spp.), hickories (Carya spp.), or crabapples (Malus spp.).  Be sure to pick a species that is native to your region! (Photo: Katja Schulz, CC-BY 2.0)

 

Minor plant “damage” can be a sign of a healthy ecosystem

Given the vital resources our woody plants provide to the local ecosystem, it’s important to be thoughtful about managing these species. Many common issues seen on trees and shrubs, such as leaf blemishes, galls, or leaves being eaten, are largely cosmetic and can be tolerated. Such sights do not necessarily signal that the plant is dying or even unhealthy. Trees and shrubs evolved to handle these types of minor issues and survive. For example, chewed leaves may simply indicate that some caterpillars had a nourishing breakfast, and your plant is providing needed habitat. 

Understanding that the vast majority of these caterpillars will become food for neighborhood songbirds can help reframe your perspective and assist you to make informed choices. Using a pesticide to keep the plant leaves pristine would short-circuit the system and leave baby birds with less food.  

 

The underside of a leaf with several round growths that are a splotchy red in color.
There’s no need to panic if you spot galls: odd swellings on your plants. Common on willows, oaks and other species, galls serve as food and shelter for over 1,500 different species of arthropods and rarely result in harm to the plant. Many gall-forming bugs are even natural enemies of other “pest” insects. (Photo: Peter Chen CC-BY)

 

Why pesticides should never be your first management choice

While pesticides may be effective at killing, they don’t address the conditions that attracted or maintain the pest. Pesticides may appear to resolve your issue, but chemical fixes are usually temporary. Besides harming indispensable invertebrates like pollinators and fireflies, pesticides pollute water, which harms freshwater mussels and aquatic insects. Some are also linked with human health problems. 

Plants also have their own ways to defend themselves! Scientists have figured out that plants whose leaves are being eaten by herbivorous insects emit chemical signals called HIPVs (herbivore-induced plant volatiles). These waft into the air and lure in the predators of the herbivores to come to the rescue. These “natural enemies” are other insects, such as wasps, that help keep pests naturally under control.

That is just one reason that applying pesticides is often counterproductive. Pesticides also often kill the beneficial insects that act as natural enemies, ultimately worsening a problem, and resulting in repeat infestations.  When you have a healthy and diverse wildlife community, insect populations that seem unsustainably high often crash on their own without intervention. 

 

Several yellow aphids on a leaf. Amidst them is a small black wasp, currently using its ovipositor to lay an egg inside one of the aphids
If you’re worried about aphids, keep an eye out for some big help from tiny wasps! This parasitoid wasp is laying her eggs inside aphids. As the wasp larvae grows, it kills the aphid before metamorphosing into an adult wasp that is ready to hunt more aphids. Parasitoid wasps are excellent beneficial insects that control not just aphids, but also scales, psyllids, stink bugs, beetles, and caterpillars.  (Photo: Mark Yokoyama CC-BY)

 

Learn how to keep your trees and shrubs healthy

You can follow a few easy steps to care for your woody plants in ways that also protect the animals that rely on them.

  • Identify the concerning insect or disease. A correct identification will allow you to pick the best course of action. You can use online identification guides like iNaturalist, or guide books. Professional guidance from your local university extension service is also a great choice.
  • Assess whether the concern is an actual risk to the plant. Many apparent issues, like some chewed leaves, are just the sign of a healthy ecosystem.
  • Treat underlying issues that may be stressing your plant. Healthy plants can generally handle insects feeding on them without worry. Problems like root space, or inadequate or overexposure to sunlight or water, can make a big difference.
  • Use non-chemical strategies, like removing worrisome insects by hand or with a strong jet of water, instead of pesticides.

For more detailed advice and information, read our new fact sheet “Responding to Insects and Diseases on Landscape Trees and Shrubs”. Take a look – and happy reading in the shade. 

 

Authors
Sharon assists Xerces staff, partners, and the public to reduce reliance on pesticides and understand pesticide risk to invertebrates. Sharon previously worked at Northwest Center for Alternatives to Pesticides, the U.S. Fish and Wildlife Service, and the U.S. Forest Service, and integrates her focus on pesticides with her experience managing natural areas and agricultural lands.

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