If you have plants with insect problems, the worst action you can take is not watering the plants.
Many insects only target weak plants. When a plant goes into any kind of stress, it has a different color. This sends a different vibration that is picked up by insects.
This is why one plant that is not doing well can have aphids all over it and the same plant nearby that is healthy will not have aphids.
Bronze birch borer
Trees especially need water to keep out insects. We are seeing a lot of damage to birch trees from the bronze birch borer.
“Trees stressed by factors such as sustained drought or defoliation are especially susceptible to attacks by the bronze birch borer,” according to University of Minnesota Extension.
In recent years Seattle has had hotter, dryer summers. This has led many trees and other plants to suffer drought stress.
Other sources of stress could be poor soils, damage to tree roots and trunks, or construction practices such as regrading the landscape without protecting the tree and its roots.
“Trees under stress have a reduced ability to acquire and distribute water and carbohydrates throughout their canopy, which leads to reduced tree defense against the borer larvae,” said UM Extension.
Watering trees is most important step
“The most important management step that can be taken is to keep trees well-watered.” The article also suggests adding organic mulch to keep soil temperatures cooler, slow the rate of moisture evaporation and increase the capacity of the soil to hold water.
“BBB cannot kill healthy trees and is most likely to attack old, stressed, or drought-stricken trees,” according to Oregon State University Extension Service.
“While many homeowners may not place a high priority on watering mature trees, this can be a key strategy for preventing BBB damage in dry areas. Birch trees tend to be shallow-rooted and respond well to watering,” said OSU Extension.
As its name suggests, the insect bores its way into the bark. If a plant has enough water, it keeps out the borer. The water creates pressure on cell walls. This is called turgor pressure.
If a boring insect tries to enter into the bark, tree sap is pushed out the hole and the insect is denied entrance. But if the tree does not have enough water, the borer moves in and the tree will begin to decline slowly. It can take many years before the tree finally succumbs to the insect.
Azalea lace bugs
Lace bugs are a big problem in landscapes now. Usually the main difference between having a few lacebugs or having an epidemic? Water.
Ciscoe Morris’ recent column in the Seattle Times said that more and more gardeners are asking him about insect problems on rhododendrons and azaleas. The azalea lace bug is causing severe damage to these plants.
Rhododendron lace bugs have been around for many years, but they were fairly easy to deal with, Morris said. They only had one generation per year, so only one treatment was necessary.
“Unfortunately, in 2008, the azalea lace bug moved into Washington.” This insect has multiple generations per year, so it is much more damaging and harder to manage.
Drought makes problem worse
Our recent trend toward hot, dry summers has exacerbated the problem, Morris said. “Studies have shown that drought-stressed plants are much more susceptible to lace-bug attack.”
Plants growing in full sun are nearly twice as likely to get infested as those growing in partial shade. Most rhodies and azaleas prefer shady conditions, so sun stresses the plant.
It also dries them out more quickly than would shady conditions. Move them to shadier areas if possible.
“Reduce stress by watering and fertilizing appropriately,” Morris said. You can attempt to manage the pest by spraying with natural neem oil and hand picking eggs from the underside of rhododendron leaves.
If damage is too great, you may need to replace the plant with one that is not susceptible.
If other plants in your landscape have insect problems, pay attention to whether they are getting enough water to thrive. Water slowly and deeply to reach the entire root zone. Reducing stress by watering deeply will help them fight off problem insects.
Find out more about watering
We have written several blog posts about watering this year, all based on thoughts and ideas developed by Ladd Smith.
- Smart watering: how much, when and how long to water.
- Plants need water to survive and thrive.
- Invest in watering for landscape dividends.
- Choose drought-tolerant plants and reduce time you spend watering.
- Summer lawn care: watering, mowing.
Our YouTube playlist on Watering Your Lawn and Garden has several videos on the basics of watering, watering systems, watering new plants and trees, and more.
The Saving Water Partnership has information on watering systems and other watering tips.
Swanson’s Nursery has a great article on best watering practices.
University of Minnesota Extension, Bronze birch borer and twolined chestnut borer in Minnesota.
href=”https://catalog.extension.oregonstate.edu/sites/catalog/files/project/pdf/em9189.pdf”>Oregon State University Extension Service, Homeowner Guide to Managing Bronze Birch Borer in the Upper Klamath Basin.
Ciscoe Morris, Seattle Times, Azalea lace bugs are even peskier than the familiar rhododendron ones.