Brownheaded ash sawfly, Tomostethus multicinctus
Adults of the brownheaded ash sawfly resemble small dark black wasps that do not sting. They have a saw-like ovipositor used to lay eggs in the edge of developing ash leaves. Larvae of the brownheaded ash sawfly have brown heads and yellow or green bodies with green stripes on the back and measure 13-22 mm (1/2-7/8 inch) in length. These larvae resemble caterpillars since they have a brown head and prolegs on each abdominal segment. However, they lack the gripping, hook-like structures called crochets, which are found on the prolegs of caterpillars.
A close relative of this pest, called the blackheaded ash sawfly (Tethida barda), is commonly confused with larvae of the brownheaded ash sawfly. The color of the head capsule is used to distinguish between these two species. Larvae of the blackheaded ash sawfly have a black head capsule, while larvae of the brownheaded ash sawfly have a brown head capsule.
- Sawflies are a large and important family of pests. Many are herbivores that develop within host plants.
- The brownheaded ash sawfly is native to the eastern United States. This insect was first documented in the Arkansas Valley in the 1980s. Since then, it has become a common pest of ash trees along the Front Range.
- Larvae of the brownheaded ash sawfly are yellow or green with a brown head and striping on the back. They also have prolegs like caterpillars. They usually emerge and start eating leaf matter in May and June, causing more defoliation as they develop. Common feeding characteristics include pinhole damage on the leaf and defoliation, leaving only the main veins.
- Control of the brownheaded ash sawfly is easy and can be done using common insecticides or dislodging the larvae from the tree using a strong stream of water.
Larvae of brownheaded ash sawfly migrating to pupation sites on a green ash tree. Notice the green striping on the back. Image credits: William M. Ciesla, Forest Health Management International, Bugwood.org
Larva of the blackheaded ash sawfly. Note the black head capsule, which is a distinguishing feature of this closely related species. Image credit: Lacy L. Hyche, Auburn University, Bugwood.org
Feeding injury caused by brownheaded ash sawfly to a green ash tree. Notice the pinholes, which are produced by feeding of young stage larvae. Image credit: William M. Ciesla, Forest Health Management International, Bugwood.org.
Life history and habits
Eggs are inserted into growing leaves of green and white ash trees in May when the weather begins to warm. After hatching, newly emerged larvae feed on the inside of leaves. As they grow, larvae consume the entire leaf except for the main veins.
Brownheaded ash sawfly becomes a fully grown larva in two to three weeks by early June. Once larvae are ready to pupate, they fall to the ground by shedding a paper-like skin that remains attached to the leaf. They then burrow into the soil to form an overwintering cocoon. As temperatures rise the following spring, they pupate in the soil and typically emerge in late April. Swarms of adult sawflies appear around trees to mate and lay eggs in leaves, producing one generation each year.
Feeding by young larvae creates small pinhole wounds in leaves. Mature larvae consume entire leaves except for the main vein. Heavy infestations can lead to extensive defoliation, which can be especially problematic when it occurs repeatedly. Egg laying behaviors can distort the shape of leaves.
Control may be warranted when trees are in poor health, repeatedly experience heavy defoliation, or are stressed from other pests such as bark beetles, borers, or scale insects. Since larvae of the sawfly can rapidly defoliate trees, applying treatments early is most effective.
Larvae can be removed from plants with a strong jet of water or suffocated with soapy water.
Except for neem and Bacillus thuringiensis, most common insecticides used in gardens are effective against larvae of the brownheaded ash sawfly.
Cloyd, R. 2022. Brownheaded Ash Sawfly. Kansas State University – Extension. Available https://blogs.k-state.edu/kansasbugs/2015/05/14/brownheaded-ash-sawfly/
Baker, J. 2019. Brownheaded Ash Sawfly. North Carolina State – Extension. Available https://content.ces.ncsu.edu/brownheaded-ash-sawfly