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Arge pectoralis

Birch sawflies. Photo: Gilles Arbour, Bugguide.
Scientific Name: 
Arge pectoralis
Common Name: 
Birch Sawfly
Growing Degree Days (GDD's): 
Not available at this time.
Host Plant(s) Common Name (Scientific Name): 
Alder (Alnus spp.) *Occasional host.
Birch (Betula spp.)
Hazelnut (Corylus spp.) *Occasional host.
Willow (Salix spp.) *Occasional host.
Insect Description: 

Adults emerge approximately in June and July from overwintering sites as pupae hidden in the ground at the base of their hosts. (In the leaf-litter.) Larvae active from approximately mid-June into September on host plant foliage. Larvae have a dull orange head capsule and a pale, yellowish-green body with several lines of black spots. The larvae of this sawfly (Hymenoptera) are 0.8-1 inch in length when fully matured. The feeding behavior of this insect may resemble that of other sawfly species found on birch.

Damage to Host: 

Larvae may be important defoliators of birch, generally present between mid-June through mid-September. Feeding from this insect is similar to that of Croesus latitarsus, the dusky birch sawfly.


Look for sawfly larvae feeding on the edges of leaves. These hymenopteran caterpillars may hold the end of the abdomen (rear-ends) elevated behind them.

Cultural Management: 

Crush with a gloved hand and remove from plant when found in large numbers.

Natural Enemies & Biological Control: 

This insect occurs through Canada and parts of the Northeastern United States. It has natively occurring natural enemies. Populations are usually managed by natural enemies, and therefore chemical management is typically not necessary. The majority of time, the presence of this insect can and should be tolerated. However, birch sawflies are occasionally present in large numbers and at times cause considerable defoliation to birch hosts.

Chemical Management: 

Acephate (NL)

Azadirachtin (larvae) (NL)

Bifenthrin+imidacloprid (L)

Carbaryl (L)

Chlorpyrifos (N)

Cyfluthrin (larvae) (NL)

Deltamethrin (larvae) (L)

Dinotefuran (NL)

Emamectin benzoate (L)

Flonicamid+cyclaniliprole (N)

Gamma-cyhalothrin (L)

Horticultural oil (larvae) (L)

Imidacloprid (larvae) (L)

Insecticidal soap (larvae) (NL)

Lambda-cyhalothrin (L)

Pyrethrins+sulfur (larvae) (NL)

Spinetoram+sulfoxaflor (larvae) (N)

Spinosad (larvae) (NL)


To entomologists, the term caterpillar refers to the immature or larval life stage of the Lepidoptera - butterflies and moths. However, the public and some entomologists included sometimes use this term interchangeably to refer to the immature or larval life stage of certain Hymenoptera - in particular, sawflies. 

This can cause confusion when selecting management options for pests collectively referred to as caterpillars. The larvae discussed here will mature into an adult sawfly. This means that the active ingredient Bacillus thuringiensis Kurstaki (Btk) will not be effective at managing this insect. Btk will only kill the caterpillars of moth (or butterfly) pest insects. It will not kill sawfly larvae. 

Sawfly larvae can be distinguished from moth or butterfly caterpillars by the presence of or absence of 6 or more prolegs. Some sawfly species will have 6 or more pairs of fleshy abdominal prolegs, whereas some sawfly larvae will have 0 pairs of fleshy abdominal prolegs. Caterpillars that mature into butterflies or moths will have 5 or fewer pairs of fleshy abdominal prolegs. Prolegs are soft, and found behind the hardened 3 pairs of thoracic legs on the insect. In butterfly or moth caterpillars, the prolegs will also have tiny hooks or crochets on the very bottom.

Active ingredients that may be applied systemically include: acephate (injection), azadirachtin (injection, soil drench), dinotefuran (soil drench), emamectin benzoate (injection), and imidacloprid (soil drench).

When used in nurseries, chlorpyrifos is for quarantine use only.

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.

Read and follow all label instructions for safety and proper use. If this guide contradicts language on the label, follow the most up-to-date instructions on the product label. Always confirm that the site you wish to treat and the pest you wish to manage are on the label before using any pesticide. Read the full disclaimer. Active ingredients labeled "L" indicate some products containing the active ingredient are labeled for landscape uses on trees or shrubs. Active ingredients labeled "N" indicate some products containing the active ingredient are labeled for use in nurseries. Always confirm allowable uses on product labels. This active ingredient list is based on what was registered for use in Massachusetts at the time of publication. This information changes rapidly and may not be up to date. If you are viewing this information from another state, check with your local Extension Service and State Pesticide Program for local uses and regulations. Active ingredient lists were last updated: January 2024. To check current product registrations in Massachusetts, please visit: .