Arboretum News

Summer of scale!

Scale insects, which can suck all the sap from plants, are a frustrating pest to deal with, especially on magnolia trees. The Arboretum’s Integrated Pest Management Specialist Erin Buchholz offers some potential solutions to save your magnolias.

Visiting the Arboretum: All members and visitors need to make a reservation in advance of their visit to the Minnesota Landscape Arboretum. We hope to see you soon!

Magnolia scale clusters on the underside of branches. Photo by Erin Buchholz.

By Erin Buchholz, integrated pest management specialist

We’ve received a lot of questions about scale this summer. With the lack of rain and storms, some insect populations have exploded. Heavy wind and strong jets of water help to wash away some of these problems, and we haven’t benefited from nature’s assistance these past few months.

Scale insects are probably the most frustrating pests to deal with. They get their name from the protective coat they wear when it’s time to lay eggs and feed from plants. They have piercing/sucking mouthparts like aphids and whiteflies, and like them, scales excrete honeydew which is sugary and sticky. If left unchecked, scales can quickly multiply and suck all of the sap from plants, causing dieback and possibly death.

Unfortunately, the honeydew attracts other pests to your impacted plant. Ants, bees, and wasps are carb loading this time of year, and have been known to protect scales from predators while foraging for sugar. Stinging insects make people very nervous, and that’s when we see some troubling use of insecticide spray that will also harm our pollinators (please remember that even wasps are valuable pollinators!).

Ladybugs do feast on scale when they are accessible. But they also may enjoy the honeydew, too. Photo by Erin Buchholz.

The honeydew also favors sooty mold, which is exactly what it sounds like. This mold grows on leaf surfaces and is darn near impossible to clean off. While the mold itself will not kill the plant, the lack of photosynthesis from the green part of the leaf which is now covered can cause stress and contribute to its decline.

Sooty mold on magnolia, lady’s mantle, and spirea. The honeydew falls on the leaves and the mold colonizes on the honeydew. Photo by Erin Buchholz.

So, what can we do? In the past, we would drench the plants’ roots with a systemic insecticide, which makes all parts toxic to all insects. That was a bad option. By doing that, we made the pollen and nectar toxic to pollinators, and we are still trying very hard to help them recover from our mistakes. Even worse, some tree companies are recommending the outright removal of infested trees rather than using elbow grease. That’s not an option for us!

At the Arboretum, magnolia scale populations exploded this year. Fortunately, they are still limited to a couple of small pockets and have not yet found our larger magnolia collections and research plots. To suppress, manage, and kill them, we are using mechanical and (gentle) chemical control options.

While the scales are under their protective coating, they can’t be harmed by sprayed insecticides. However, they can be physically removed pretty easily. If your magnolia is still small, you can hand pick them from the branches and throw them on the ground. Adults can’t get back up on the tree and will die quickly. 

If the juiciness of the scale picking is too gross, consider our favorite option – sweeper nozzles! These nozzles are usually used to clean mud from sidewalks, but work well on tough plants in combination with a shut off valve. The valve lets you control the water pressure so you don’t knock off blooms or leaves.

Sweeper nozzle and shut off valve on a garden hose. Photo by Erin Buchholz.

We wash our magnolias once per week for about 15 minutes each to suppress the scale damage while they are impervious to chemical control. We have to wait patiently for the time when they are at their most vulnerable: the crawler stage! [Insert suspenseful music here]

When they reach the crawler stage, their young hatch and emerge from the mothers’ protective coats. They will stay in this stage from fall until the next spring. Crawlers are about the size of dust particles, so they are hard to see. In Minnesota, we start to see them sometime in August. After they emerge, we will wait for a cool day before striking them with horticultural oil. Trust me, spraying oil products on a hot day is a very bad idea unless you enjoy scorched leaves.

In the fall and in the spring, we will coat our trees with a dormant oil spray. That means we read the label of the horticultural oil product to see how strong to mix our spray solution. Usually, we mix 7.5 tablespoons of horticultural oil into one gallon of water. It’s important to hit every inch of the tree, as the crawlers can lurk anywhere. The oil spray coats and smothers them leading to a pretty quick death. We will repeat this spray several times depending on the severity of the infestation, and it’s worked quite well.

Even with regular spraying, scale insects can return. It’s important to stay vigilant. Keep looking for them, keep applying oil during the crawler stage (while watching out for pollinators), and keep loving our magnolias. We can save our plants from this annoying pest. Don’t give up!

%d bloggers like this: