The maple petiole borer is a sawfly species that is non-native to the United States and likely an introduction from Europe. It was first reported in 1899 in Danbury, CT (Britton, 1906). The adult sawflies (non-stinging, wasp-like insects) emerge in May and lay their eggs (colorless, long, slender, and curved) at the base of the petiole of their host plant. Adult females have been dissected to reveal that each may have as many as 19 eggs. Adults are rarely seen, but approximately 1/4 inch in length with a shiny, black colored head and thorax and yellow, black tipped abdomen with yellow legs. The larvae feed by mining the leaf petioles, eating almost all of the tissue within. Eventually, this feeding causes the petiole to break, an inch or so from the leaf itself. Leaf drop from the activity of this insect may be noticed in May and June. Feeding occurs approximately over that one month period. While this activity may be concerning, this insect does not normally cause damage to the overall health of the host plant and management is rarely needed. Fully grown larvae are 8 mm in length and resemble weevil larvae (thoracic legs are so small they may appear legless, white in color with brown heads). Larvae do not drop to the ground with the severed leaf portion - they remain in the petiole, which then eventually drops to the ground and contains the larva. Larvae crawl to the soil surface and burrow 5-8 cm below, where they pupate. The pupa is found in a round to oval, earthen cell that is 5 mm. in diameter. Overwintering occurs as a prepupa in the earthen cell. One generation occurs per year.
Hundreds of new leaves may fall from one tree (primarily sugar maple) in May and June. Prior to leaf drop, leaves may appear green and otherwise healthy. Other times, they wilt and may look "scorched" on the edges. However, only approximately 10% (maybe 15-20%) of the tree's total leaves may be impacted. Each tree has thousands more leaves and should survive nicely. This insect does not cause serious damage to its hosts, and can be tolerated. Managing them can be difficult, and unnecessary. Additionally, populations of this insect fluctuate each season and can be unpredictable. Other stressors, both biotic and abiotic, can cause leaf drop but typically happen later in the growing season.
Look for dropped leaves in May or June. Carefully dissect (slice open) the petiole (the portion remaining on the tree) to look for the tiny sawfly larva or evidence of tunneling within. Tunnels will be filled with loose, granular frass (excrement).
Raking and destroying the fallen leaves will not reduce a local population, because the insects are found in the petioles. Petioles would need to be collected and destroyed before the larvae leave them to enter the soil to pupate. One to two weeks after leaf fall, petioles fall containing larvae. Petioles can be raked up, however the efficacy of this practice at reducing a local population is not completely understood. Daily raking during the period of petiole drop may be necessary.
Two hymenopterous parasitoids have been reported from maple petiole borer populations. Bracon montowesi as well as an unidentified chalcid wasp have been reported in the literature (Britton 1906 and Marsh 1979).
Acephate (NL)
Azadirachtin (NL)
Carbaryl (L)
Cyfluthrin (larvae) (NL)
Deltamethrin (larvae) (L)
Dinotefuran (larvae) (NL)
Flonicamid+cyclaniliprole (N)
Gamma-cyhalothrin (L)
Horticultural oil (larvae) (L)
Imidacloprid (L)
Insecticidal soap (larvae) (NL)
Lambda-cyhalothrin (L)
Spinetoram+sulfoxaflor (N)
Spinosad (NL)
Chemical management of this insect is often unnecessary and a best practice is to tolerate their activity in an ornamental landscape. Active ingredients listed here are labeled for use against sawflies.
Active ingredients that may be applied systemically include: acephate (injection), azadirachtin (injection, soil drench), dinotefuran (soil drench), imidacloprid (soil drench), and neem oil (soil drench).
Make insecticide applications after bloom to protect pollinators. Applications at times of the day and temperatures when pollinators are less likely to be active can also reduce the risk of impacting their populations.
Note: Beginning July 1, 2022, neonicotinoid insecticides are classified as state restricted use for use on tree and shrub insect pests in Massachusetts. For more information, visit the MA Department of Agricultural Resources Pesticide Program.
