I’m almost positive I have identified dogwood sawfly as the culprit eating the leaves on my cornus alternafolia. I’ve never found caterpillar/larvae though I look. I have found webby things on the undersides of some leaves. But just now I found a few flying insects landing on the leaves and they sure look like mature sawfly images from web. Looking for possible ID of what is eating them and if it is dogwood sawfly, how to treat safely? I’m in Scarborough.
I’m sorry to hear that your pagoda dogwood is being attacked!
It is difficult to identify pests from a description. I’d suggest taking photos or killing a few of the flying insects and bringing these and a sample of a damaged leaf to your local nursery for help in identification.
To help determine whether the dogwood sawfly (Macremphytus tarsatus) is the culprit, it is important to know the life cycle of this insect.
Adult dogwood sawflies are wasp-like, slim, shiny and black. The adults emerge between May and July and lay eggs on the undersides of dogwood leaves.
The eggs hatch between July and September into larvae or caterpillars, and these are the critters that feed on the leaves (not the adult flies). It goes through several stages of development. One of these is as fluffy, white caterpillars, which should be easy to spot. The caterpillars feed in groups, often gathering together to chow down on the same leaf. When not eating, the larvae curl up so they look like bird droppings (and are ignored by predators). Mature caterpillars are yellow with black spots.
My concern with your tentative identification is that it’s now early June, which is very early for the caterpillars to have emerged to start feeding on leaves. So I suspect that another insect may be at work here, but it’s not clear which one. The pagoda dogwood may be susceptible to insect pests like scale, leaf miner and borers – but these would inflict different types of damage than you describe.
However, in case I’m wrong, and it is indeed the dogwood sawfly causing damage to your tree’s leaves, it is important to interrupt the pest’s life cycle, which you could do by killing the caterpillars. The good news is that generally sawflies don’t harm the tree, although the defoliation can be significant.
So, until the end of July, I’d suggest that you watch for the eggs on the undersides of leaves, parallel to leaf veins – this is quite a distinctive pattern. Unfortunately, the eggs are inserted into the leaf, not on the surface, so you can’t just rub them off. You would have to remove leaves from the tree to get rid of the eggs. This would not be a concern if only a few leaves were involved, but if many are affected, and you removed all these leaves – this could affect the tree’s health.
And come July, start to keep an eye out for caterpillars – also on the undersides of leaves. The time to prevent leaf damage is when the sawfly is in the caterpillar (hungry!) stage.
You can shake the caterpillars off the leaves then drop them into a bowl of soapy water or crush them. Insecticidal soaps and horticultural oils can also work if sprayed to cover the undersides of leaves, if the caterpillars are detected early, i.e., when they are less then 2.5 cm (1 inch) long. The older caterpillars stop feeding and start to look for a place where they can prepare cocoons. They drop to the ground in search of rotted wood and if they can’t find this material on the ground, they can bore into wooden structures. It’s important to keep the area around the dogwood tree clear of debris that might attract the caterpillars.
All the best in keeping your lovely tree healthy. Please get back to us to tell us how you fared!
June 3 2021