UMass Garden Calendar Photo Contest
We'll be having a photo contest for the 2019 UMass Extension Garden Calendar, so have your camera handy and keep an eye out this summer for contest-worthy pics! Submission details will be coming in a later issue of Garden Clippings.
Tree Care After Gypsy Moth Defoliation
The numerous gypsy moth caterpillars finishing up their feeding for the 2017 season during the last ten days of June. The falling frass (excrement) could be heard as it fell onto trees, shrubs, patios, decks, driveways, and cars below, creating quite a mess. The caterpillars have now finished feeding for this year.
In some areas of Massachusetts, during those last days of June, before the caterpillars finished feeding and pupated, a great number died, succumbing to the caterpillar-killing fungus, Entomophaga maimaiga (image on left). All the wet weather this past spring activated the Entomophaga maimaiga fungus and it paid off. Those caterpillars that died from the fungus in late June provided a place to produce the fungal resting spores that will last for many years and should help infect and reduce future gypsy moth caterpillar populations in years to come.
The caterpillars began hatching about April 26 and, because of the incredible number of gypsy moth eggs laid last June 2016, many people thought that there would be devastating tree defoliation statewide by the caterpillars in 2017. According to UMass Amherst entomology staff, in some areas of Massachusetts defoliation was significant, especially the areas in central/western Massachusetts in the Holyoke range area and along the Mass. Turnpike (I-90) from Palmer to Worcester, where tree defoliation was massive. However, in other areas of the state, defoliation was sporadic, with some areas showing individual trees being defoliated and partial defoliation on others. Some areas show thinned canopies, but, overall, not as bad as expected giving the extremely high numbers of egg masses laid in 2016.
Gypsy moth caterpillars feed on a number of deciduous trees (especially oak) and shrubs (roses, Fothergilla, etc.), but they will also feed on needled evergreens, like spruce and pine, if caterpillar populations are high enough. Many people have commented that their oaks, apples, and crabapples were seriously defoliated; there are also reports of damage to spruce and white pine (image above).
“Now what happens” is what many people are asking and “is my tree going to die, now that it has lost all of its leaves?
What should I do?”
Deciduous trees will not usually die from one year of defoliation; however, repeated years can certainly have a negative impact on trees and trees have died as a result. After-tree care is therefore very important to trees stressed from defoliation.
If you have trees and shrubs, especially large trees, that were defoliated or damaged by the caterpillars, it is critical that these plants receive sufficient water during the hot, usually dry days of July and August and continue throughout the fall until cold weather sets in and the trees go dormant.
Deciduous trees, unlike needled evergreens like pine and spruce, will often put out a second set of leaves after defoliation, but, in order to do that, they need water - a good deep soaking once or twice a week, depending on the weather. It is important not to rely solely on rainfall. Without water, plants cannot carry on photosynthesis, where in the presence of light, they combine water and carbon dioxide and produce the food (sugars, carbohydrates, etc.) they need to produce more leaves and survive. Water is crucial to plant survival, especially those plants stressed by defoliation.
To ensure that defoliated trees, and other plants, are receiving the amount of water needed, purchase a rain gauge (available at garden centers, hardware stores, online, etc.), place out in the open in the yard away from buildings, trees etc., and check it on the same day each week. If there is less than one inch of water in the rain gauge, then it is time to water.
To determine how much time is needed for the sprinkler to deliver one inch of water, turn the sprinkler on and note how much time it takes for the sprinkler to fill the rain gauge with one inch of water. If it takes 45 minutes, then you will know how long to leave the sprinkler on to provide one inch of water to the root zone of plants.
The defoliated trees will need an inch of water over the root zone once a week; twice a week during hot, dry weather. Remember that in some species of trees, 60% of the root system may extend beyond the dripline. Set up a sprinkler over the root zone of the defoliated tree(s); leave the sprinkler on long enough to deliver that one inch of water. Soaker hoses can also be used as long as they are left in place long enough to saturate the soil around the root zone of the tree. Hand-holding a hose for a few minutes a day over the root zone will not get the job done adequately.
Without sufficient water, the defoliated trees, which are already stressed, will be stressed further by drought. Stressed trees are weakened, their resistance is lowered, and they are more prone to attack from secondary organisms like diseases and insects, which may ultimately kill the weakened tree.
It may sound too simple, but watering defoliated trees is the number one action that should be taken now. Fertilization is not recommended.
For a more in-depth article on tree defoliation and what to do, go to horticulture.oregonstate.edu/system/files/onn030213.pdf
Deborah C. Swanson, Horticulturist