What are these insects?
These insects are mountain ash sawflies. They grow to about 3/4 to 1 inch long. Although they look like caterpillars, sawflies grow up to be wasp-like insects. Sawflies typically feed in groups on the branches. When a group finishes feeding on one branch, it moves on to the next one.
Are they killing the mountain ash?
Under most circumstance, no. Despite the loss of leaves, healthy, well-established, vigorously growing mountain ash can tolerate severe defoliation without any lasting effects to its health. It would take several consecutive years of severe defoliation before the tree's health would adversely affected. Young, recently transplanted trees or unhealthy trees are more vulnerable to effects of severe defoliation in a single season.
What action you should take?
There are several things to consider when deciding whether you should try to manage these insects. First consider their size. If they are larger than ¸ inch, they have already done much of their feeding and treating them does not help the tree. If more than half of tree has been defoliated, it also too late to treat the sawflies. If the sawflies are small enough to spray, consider whether the tree can tolerate the feeding damage. If you are just concerned about its health, you can ignore the sawflies when the tree is healthy and mature but may want to protect it if the tree is young or unhealthy. Another consideration is the tree's appearance. You may want to keep the tree looking good even if it is healthy. If you decide to treat the sawflies, the key is to spray them while they are still small (less than ¸ inch long).
Will they come back?
Mountain ash sawflies have two generations a year. If you see them in June and July, you will probably see them again in August and September. It is usually less important to manage sawflies at the end of the season, although if the mountain ash has been severely defoliated several times recently, you may want to protect it against further feeding. Mountain ash sawfly populations are variable. If you discovered large numbers this year, you will not automatically seen large numbers next year. It is still a good idea to check your mountain ash in June to see if sawflies are present.
