Sawflies - What Are They?

Recently, a homeowner reported what appeared to be an insect infestation on a tree at her home in southwest Lakeland.  She indicated that this infestation had occurred for the last 7-8 years in March and the tree was usually defoliated. I thought oak tree caterpillars, but the homeowner said definitely not an oak. She further indicated that the larvae were everywhere - on the tree, falling out of the tree to the ground and in her hair, crawling up the house, on trash cans, and on her other landscape plants. This all seemed very unusual and required investigation!

Blackheaded Ash Sawfly Larvae

I visited the homeowner and saw the tree and the infestation – the tree was pretty much white with these caterpillar-like creatures. I thought this insect was a sawfly, but didn’t know which one and could not identify the tree. With the help of the Forest Service and the UF Department of Entomology, the tree was identified as a white ash (not common in Polk County) and the bug as a blackheaded ash sawfly. I had not seen the bug or the tree in Polk County before. This huge infestation had been occurring on this single homeowner tree for the past seven years. Apparently this white ash was able to withstand the repeated injury.



Infested White Ash Tree

So, what are sawflies? They are not flies at all, but are a primitive group of insects in the wasp, ant and bee order – Hymenoptera. The adult sawfly looks nothing like adult moths and butterflies as they are wasp-like, often brightly colored, and are usually found on flowers searching for host plants, mates or prey - some adults are predators. The sawfly larvae are quite different from the typical bee or wasp larvae which are grub-like. Sawfly larvae look like moth or butterfly caterpillars at the first glance as they all have leg-like structures on the abdomen called prolegs for locomotion. However, moth and butterfly caterpillars have not more than five pair of abdominal prolegs with crochets (hooks) for grasping while sawflies have six or more pair of prolegs which do not have crochets. So, sawflies are pretty easy to distinguish from moth and butterfly larvae.

Control of this infestation for the homeowner probably will be quite difficult. The tree could be sprayed with a conventional insecticide in March to control the larvae, but this treatment would be quite expensive – probably not an option for the homeowner. A possible treatment in this case is a fall application of a systemic insecticide like Bayer Advanced Tree and Shrub Insect Killer. Making the application in the fall would give the chemical time to translocate into the upper branches and foliage of the tree by spring-time. The larvae hopefully will then die after feeding on the foliage. No guarantee that this systemic treatment will work. Keeping the area under the tree clean may also help reduce adult numbers in the spring and reduce the infestation.  

Blackheaded Sawfly Larvae Clustered on Trunk

 

 

Blackedheaded Sawfly Larvae on Understory Plant