The brownheaded ash sawfly feeds aggressively on the leaves of green ash trees.
Caterpillar-like insect feeding on Kansas ash trees
K-State entomologist urges homeowners to be on the lookout for the brownheaded ash sawfly
At a glance: A caterpillar-like insect pest known to create pin holes in tree leaves appears to have launched an attack on Kansas’ green ash tree population.
More information: Raymond Cloyd, 785-532-4750, rcloyd@ksu.edu
Related: Brownheaded Ash Sawfly | K-State Horticulture Newsletter | Kansas Garden Guide
May 29, 2025
By Pat Melgares, K-State Research and Extension news service
MANHATTAN, Kan. – A caterpillar-like insect pest known to create pin holes in tree leaves appears to have launched an attack on Kansas’ green ash tree population.
Kansas State University entomologist Raymond Cloyd said he has received several questions about the brownheaded ash sawfly, a yellow-green insect with white and green stripes extending the length of its approximately three-fourths-inch body.
Larvae have a brown head and fleshy leg-like appendages (called prolegs) on every segment of the body, which distinguishes sawfly larvae from caterpillars.
“The larvae feed extensively, causing noticeable leaf damage and producing frass, or fecal material, that can be found on leaves,” Cloyd said.
The larvae can defoliate trees by feeding on leaves. Cloyd said brownheaded ash sawfly pupate in the spring, with adults emerging (eclosing) and females laying eggs inside leaves. Larvae emerge (eclose) from eggs and congregate in groups, feeding from May through June.
“Larvae create shot holes, or pin holes, on young leaves, but as they increase in size, the larvae consume entire leaves, especially terminal leaves, except the main veins,” Cloyd said. “By June, larvae are fully grown and shed a papery-like skin that is attached to the leaf.”
The larvae move toward the base of the tree and construct protective cocoons. Brownheaded ash sawfly overwinter as full-grown larvae, or pre-pupae, within silken lined cells located on the top of the soil and at the base of previously infested trees. There is one generation per year in Kansas.
Cloyd said rainfall will quickly remove larvae from trees. Larvae can also be removed by hand and placed into a container of soapy water to kill them.
“An insecticide application is not warranted unless brownheaded ash sawfly populations are causing extensive damage that compromises the aesthetic value of an ash tree,” Cloyd said.
In that case, he adds, “insecticides with contact activity can be applied, but thorough coverage of the tree canopy, particularly the leaf undersides, is important.”
More information on the brownheaded ash sawfly is available in the publication, Brownheaded Ash Sawfly, available for free online from the K-State Research and Extension bookstore.
Cloyd and his colleagues in K-State’s Department of Horticulture and Natural Resources produce a weekly Horticulture Newsletter with tips for maintaining home landscapes and gardens.
Interested persons can subscribe to the newsletter, as well as send their garden and yard-related questions to hortsupport@ksu.edu, or contact your local K-State Research and Extension office.
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K‑State Research and Extension is a short name for the Kansas State University Agricultural Experiment Station and Cooperative Extension Service, a program designed to generate and distribute useful knowledge for the well‑being of Kansans. Supported by county, state, federal and private funds, the program has county extension offices, experiment fields, area extension offices and regional research centers statewide. Its headquarters is on the K‑State campus in Manhattan. For more information, visit www.ksre.ksu.edu. K-State Research and Extension is an equal opportunity provider and employer.