Why is sap dripping from my magnolia tree?
Scale insects causing sap to drip from trees have a critical time window for control.
Heavy dripping of sap from magnolia trees was a common problem across Michigan this summer. The Michigan State University Extension Lawn and Garden Hotline (1-888-678-3464) received many calls from residents concerned that their magnolia trees were dying. One man asked if it was just easier to cut down the tree and start over. The dripping, called honeydew, is the result of sap feeding insects called magnolia scale.
When scale insects feed, they excrete a sugary sap creating a sticky mess on leaves, branches and anything under the tree. Leaves coated with the honeydew blacken as the sugary substance mildews, creating a condition called sooty mold. Magnolia scale becomes hard to ignore as the insect population increases and the dripping also increases. Loss of sap from the tree can cause yellow, stunted leaves and dying back of smaller branches. Chronic infestations may eventually kill the tree.
Both the star magnolia, Magnolia stellata, and the saucer magnolia, Magnolia x soulangiana, are susceptible to scale infestations. Magnolia scale are found attached to 1- to 2-year-old branches. They are 0.5 inches wide, plump, rounded insects without legs. Populations can grow over several years until nearly all the smaller branches are covered by scale insects.
Managing scale insects is based on an understanding of the insect’s life cycle and when it is most susceptible to control measures. Magnolia scale has one generation each year, with eggs hatching late in the summer. Following egg hatch, the young nymphs (crawlers) emerge from under the female scales and spread along branches, searching for a suitable site to feed. It is this crawler stage where management options can be directed. The nymph stage is more vulnerable to pesticide control measures than the adult scale that has a protective covering. Accurately timed application of contact insecticides such as horticultural oil or insecticidal soap can help reduce scale infestations. Timing is everything in controlling magnolia scale.
MSU’s Enviroweather website helps time the hatch of this pest. This online site operates a series of weather stations across the state that provide localized data on temperature, including growing degree days (GDD). Since stages of an insect’s life cycles are temperature dependent, it is possible to use daily air temperature measurements to accurately time insect development and, in this case, egg hatch. Growing degree days correlate insect development stages with daily accumulation of temperatures above a threshold.
Specific GDD data on timing the hatch of magnolia scale is listed online at the MSU Integrated Pest Management website. The table lists peak hatch of magnolia scale as occurring between 1,925 and 1,950 GDD. This is critical information that helps us predict when the magnolia scale crawlers are out on the foliage and branches and accurately times pesticide use. Pesticide applications timed with growing degree days are not only more effective but help to reduce needless applications and gets us away from the practice of incorrectly using a pesticide on the same date each year.
Through local Enviroweather stations, we can determine when scale hatch will occur across the state. In Flint, Michigan, the local weather station showed that peak magnolia scale hatch occurred between Aug. 16 and 17 in 2019. The previous year, magnolia scale hatch peaked 10 days earlier. The weather station in Traverse City, Michigan, shows growing degree days reached the point for hatch of magnolia scale on Sept. 10, 2019. These localized weather stations can more accurately predict critical stages for scouting for and managing pests including magnolia scale and help reduce their impact on trees across the state.
- Herms, D.A. Using degree days and plant phenology to predict pest activity. Chapter 11, IPM of Midwest Landscapes: Tactics and Tools for IPM