Brownheaded ash sawfly, Tomostethus multicinctus
Order: Hymenoptera
Family: Tenthredinidae
Description
Adults of the brownheaded ash sawfly (Tomostethus multicinctus) 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), has a black head capsule and is commonly confused with larvae of the brownheaded ash sawfly, which has a brown head capsule.
Quick Facts
- 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.
- Monitoring is highly recommended to help inform decisions related to management. 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 green ash. 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. 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.
Injury
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 in a very short time, which can be especially problematic when it occurs repeatedly. Egg laying behaviors can distort the shape of leaves.
Monitoring
Newly transplanted trees or trees in poor health are more susceptible to infestations. Early detection is important for effective control. Immature can quickly injure plants, and treating infestations with a high number of adults is unlikely to be effective as the plant will already have sustained injury. For this reason, it is recommended that treatments only be made when an infestation contains a high proportion of young larvae. In addition, defoliation of ash trees during late August-September is not likely to have a lasting effect on plant health since the tree will have already stored sufficient reserves for winter dormancy and is unlikely to produce new growth. Furthermore, the presence of sawflies on one or a few branches typically does not warrant control, although widespread populations can severely defoliate plants. When immature sawflies are still in the earlier instars, treatments can be attempted to help prevent additional plant injury.
Cultural control
The best management approach for brownheaded ash sawfly is to maintain healthy trees, as newly transplanted, unhealthy trees are more susceptible to attack. Selecting the correct plant for site conditions and providing adequate plant care will allow trees to tolerate some injury. At lower densities, larvae can be hand-picked from plants or dislodged with a strong jet of water.
Chemical control
Insecticidal soaps can help control low numbers of young sawfly larvae, and many common insecticides used in gardens are also effective. However, it is important to avoid applying insecticides when larvae have nearly completed their development since the plant will already have sustained injury. Since sawflies tend to feed in groups, insecticides only need to be applied to feeding areas unless the infestation is widespread.
CSU Extension Fact Sheet
Download or view the CSU Extension’s PDF fact sheet for your reference.
References
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
Wawrzynski, R. 1996. Sawflies of Trees and Shrubs. University of Minnesota – Extension. Available https://conservancy.umn.edu/bitstream/handle/11299/219498/Ext_Sawflies.pdf?sequence=1