Diagnosing damage on oak trees

By Melanie Mathieson
Gardening Guru

I have been getting some calls from gardeners this year finding that their oak trees have insects damaging them this year. There are three common pests in our region that will cause damage to oak trees and the damage seems to be quite prevalent this season. Below are descriptions of the damage you may see and the insect so you can diagnose the problem yourself. Unlike human pests and diseases there are not many treatments for trees when it comes to treating specific pests and diseases. There are some commercial insecticide formulas available in your nursery supply store that will kill the insects but remember that these are just general-purpose insecticides that may or may not kill the insects. These insecticides are not specific to the insects themselves but just work because they are poisonous. These insecticides are also poisonous to humans, pets and birds as well, so great care and caution must be exercised if you choose to use a commercial insecticide.
Insect damage can be more widespread in different years for two reasons. Just like forest tent caterpillars many other tree damaging insects have multi-year population cycles that build up, peak and then drop off. Favourable weather conditions, with moderate temperature and moist conditions allow these insects thrive in their population cycle and become more evident. Fortunately these insects are specific to oak trees, unlike the forest tent caterpillar that will eat almost any green leaf in sight. Follow the instructions for treatment and diagnosis and you may be able to interrupt the population cycle of these insects and help your oak tree regain its vigour.
Oak Leafshredder – this insect prefers the red oak but has known to also devour the bur and white oaks in the District. This pest overwinters in the egg stage on tree branches. Harsh winter and spring conditions with cycles of warming and severe freezing usually keep the insects localized or hardly noticeable. But a mild winter and a cool moist spring can jumpstart the population cycle and more damage will be noticeable. The eggs hatch in early May as the tree is coming into leaf and the larvae soon devour the tender new leaves as they develop. The larvae are yellow-brown caterpillars with shiny black heads, about 2 cm in size. The insect will shred the new leaves on the tree, leaving partial leaves hanging in a shredded manner and then will roll the remaining leaves on the tree creating tunnels of leaves for protection. There will be strong evidence of spider-like webbing all over the tree as well. The tree takes on the appearance of a Halloween decoration in June, with the shredded leaves and webs hanging from the branches. The pupae will change to small yellow moths in late June to mid- July. The insects will use leaf litter on the ground or the rolled leaves for protection when they pupate. Once the moths hatch, they lay eggs on the tree branches for next year’s population. The best solution to get rid of the insects is to mix one tablespoon of Sunlight dish soap into your hose attachment fertilizer sprayer and spray as much of the tree with the soapy solution as you can reach. Add more soap if you see the bubbles are no longer forming and you have more of the tree to spray. The soap will dissolve the webs and cause the caterpillars to dry out in the heat of summer. Good housekeeping is a must here too. Pick off any worms you see by hand and destroy, pick off any rolled leaves that obviously are harbouring insects in the folds and destroy. Clean up all leaves that have fallen off the tree throughout the season and in the fall and place in your garbage and remove from your property. Spray the tree again in the late summer, with the soap solution to try and destroy the eggs.
Oak Sawfly – just about every tree species has a sawfly specific to its species. In the oak species, there are a few different sawflies that will infect the different oak species. Oak infecting sawflies all resemble the small green inchworm but have dark brown or black heads. The sawfly larvae overwinter in cocoons in the ground and emerge as small four-winged insects the following year that lay the eggs for the new population. The damage from all sawflies on all tree species looks pretty much the same. The sawfly caterpillar feeds on the green tissue of the leaves and removes the live tissue from in between the veins of the leaves. Once the damage becomes noticeable the leaves will appear light brown but on closer inspection you will see the green areas have been removed and it is the only brown veins that remain. Usually control measures have not been necessary but again the soap solution can be used while the larvae are feeding on the tree in mid-May to late July and of course good housekeeping practices by removing all damaged leaves from the ground in the fall and disposing of them in the garbage, will help to control the population next year.
Oak Leaf Miner – The oak leaf miner is an inconspicuous insect that has a life cycle of two generations of tiny moths each season. In between the moths, the larvae hatch and make brown patches of damaged tissue on the leaves of the tree. The areas of damage are pale brown and have squiggly line shapes resembling “mines” on the leaf. Control of this insect is not necessary and the damage is not often detected until the insect has left the tree. Again rake up all leaves in the fall and dispose of them in the landfill to minimize damage for the next year. Leaf miners, also each specific to the tree species, will also be found on other tree species causing the same damage. Almost every deciduous tree species in Canada has a leaf miner named for it.
The last important question you may be asking is if these insects will kill the tree. The answer is no. The damage caused by sawflies and leafminers is usually minimal and does not have any lasting effect on the tree. The leafshredders will make the tree look ugly until the population cycle dies out and because the young leaves were damaged, the growth of the tree is slowed tremendously. The damage caused by leafshredders could cause only a very young tender new sapling or a tree with other pest and disease problems already, to actually die.
This column will hopefully put your mind at ease a bit, now that you know what is going on with your oak tree and what to do about it. Remember that good housekeeping of your yard and garden is the best form of defence from damaging insects and a little Sunlight soap doesn’t hurt too.