Mockorange sawfly – Identification
Q: Can you tell me what type of worm this is and why does it attack my mockorange bush?
A: It’s a “worm” – but it’s not a caterpillar. I counted his fleshy back legs (prolegs) and can distinguish more than five pairs. That’s important because this means you don’t have a caterpillar attacking your bush. Instead, you have mockorange sawfly larvae chewing on the leaves.
The creatures LOOK like caterpillars to the uneducated eye but they are not. A sawfly is a primitive wasp-like insect. The females have a saw-like blade at the tip of the abdomen that is used to cut slits into plant tissue into which they deposit eggs. The resulting larvae, since they are not caterpillars, are not affected by the organic caterpillar insecticide, B. t. (Dipel, Thuricide, etc)
The contact insecticide carbaryl click for sources) offers good control if sprayed on the entire plant. Imidacloprid (click for sources) is a systemic insecticide which can be applied to the soil around the tree before feeding activity is noticed. Both insecticides, however, are moderately toxic to fish.
In your specific situation consider using insecticidal soap (click for sources) every seven days during the few weeks in which the sawfly larvae are present.