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Over the Garden Fence

Where gardeners come to find out what's happening out in the yard.

Troubling Insects in the Landscape

Posted by Richard Hentschel -

Down the Garden Path

Richard Hentschel, Extension Educator

We need to blame something for all the insect problems in the yard, so it might as well be the summer drought. When our landscape plants are stressed from a lack of water, there is dieback of roots and branches. We cannot see the roots, but it is easy to see the dieback in the canopy. These could be small twigs and branches or in some cases an entire branch will fail and dieback to the trunk. We have seen this happening on our evergreens for example. While our plants remain stressed, it is much easier for insects to locate and begin feeding on a stressed tree, shrub or evergreen.

The biggest concern will be the wood boring insects. They have gotten a good start this year and their populations will likely increase before tapering off to a more normal level. Entomologists are forecasting a rise in wood boring insects through 2015 and then begin to taper off and be back to normal in 2018. We will see our landscape plants continue to fail during this entire time frame unfortunately. Landscape plants that were already compromised in some way and already struggling will be the first ones to fail. Landscape plants can take two to three years to die once the pattern starts. Larger trees will take five or more years to fail.

The list of boring insects is extensive and the adults are going to be either a beetle or moth. The typical life cycle would be an adult female lays eggs on a stressed landscape plant, those eggs hatch and immediately bores into and just under the bark or even deeper into the trunk. The larvae feed and grow larger and gets ready to survive the winter as a pupae. The following spring that insect emerges as an adult to start the cycle all over again. There are variations to this, but you get the idea. It is the feeding that disrupts the trees ability to move nutrients and water up and down and causes the dieback and death of a branch and eventually the tree.

The other insect we have seen on the rise because of the drought are scale insects. There numbers increase after a drought event. Since scale insects really do not announce themselves as being here, their number can grow quickly before we notice them over just a couple of years. Scale insects are unique in that as a crawler stage is easily controlled by us and natural predators. Once they are mature, scale insects are protected from any contact treatments and require dormant oil sprays or the use of a systemic insecticide treatment. Scale insects feed by removing sap from the host plant to live on, adding to the stress already present. If scale insects are well established, you will find branch and twig dieback. Mature scales are usually found on the older wood while the crawler stage on the newer foliage and younger twigs.

Gardeners will need to remember that the impacts of this year's drought are far reaching and helping your landscape plants to recover with fertilization and water and scouting for insect pests over the next several years will be very important. Fall fertilization as our plants go dormant will help rebuild root systems and watering into November can be the first steps to help your landscape plants recover.

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