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Diprion similis

Introduced pine sawfly caterpillars, Diprion similis. (Photo: John Ghent, Bugwood.)
Scientific Name: 
Diprion similis
Common Name: 
Introduced Pine Sawfly
Growing Degree Days (GDD's): 
700–1000 (approx.), Base 50F, March 1st Start Date (Source: Robert Childs, UMass Extension.)
Host Plant(s) Common Name (Scientific Name): 
Austrian Pine (Pinus nigra) *Somewhat resistant.
Eastern white pine (Pinus strobus)
Jack pine (Pinus banksiana)
Red pine (Pinus resinosa)
Scots pine (Pinus sylvestris)
Swiss mountain pine (Pinus mugo)
Insect Description: 

The introduced pine sawfly was accidentally brought to parts of the United States, including New England, from Europe. The first record is from New Haven, Connecticut in 1914. Two generations per year are noted for the northern parts of its introduced range, with three occurring per year in North Carolina. In most of North America, overwintering is thought to be spent in the cocoon in a prepupal form. The emergence of first generation adults may begin in April, and continues through May and June. The adults of the second generation emerge from July to August. The first generation larvae may be present from May to August, and the second generation larvae from late July to early October. There is a great overlap in the generations, and throughout much of the growing season all life stages of the insect may be found on the trees at the same time, depending upon local variability. Eggs are laid in old host plant needles, and covered with a green, frothy substance. An average of 6-10 eggs are laid per needle, with each female laying on average 70 eggs total in her lifetime. Larvae are black or dark brown with two dark stripes on the back (dorsal side) and yellow and white markings on the side of the caterpillars. The head of the caterpillar is shiny black, and the underside is pale yellow or white. Caterpillars may reach up to 1 inch in length. There are 3 larval instars, the first of which feeds gregariously (in groups) but as caterpillars grow in size, they disperse and feed alone. Larvae feed for approximately 30-40 days, depending on temperatures. When it is time to pupate, cocoons are spun on host plant needles, at the ends of branches, and sometimes on the trunk in bark crevices. Some cocoons may enter diapause, which can last 4-5 weeks or in some studies, 1-3 years.

Damage to Host: 

Caterpillars feed on the needles of their hosts. Defoliation from these caterpillars may be most severe on the upper half of the host plant, however, entire trees may be defoliated in high populations. Young larvae feed together in groups, but larger (more mature) larvae will feed individually. Occasionally, some feeding has been reported on the bark of small branches (creating small pits). There are additional sawfly caterpillars that will feed on similar hosts, including the European pine sawfly and the redheaded pine sawfly. 


Monitor for caterpillar feeding beginning in May, however historically in Massachusetts reports of introduced pine sawfly feeding may not increase until late June in some locations (Robert Childs, Personal Communication). Confirm that this species is responsible for defoliation by viewing caterpillars themselves. Other similar species may cause defoliation on similar hosts.

Cultural Management: 

Using gloves, hand pick and destroy larvae by dropping them into a bucket of soapy water, particularly when caterpillars are young and still feeding gregariously. Austrian pine (Pinus nigra) may be somewhat resistant.

Natural Enemies & Biological Control: 

Coppel et al. (1974) summarized North American records of natural enemies of the introduced pine sawfly, and noted 35 species. Four of the five most important parasitoid species listed were probably of European origin and may have been accidentally introduced. The most effective species noted is the chalcid wasp pupal parasitoid, Monodontomerus dentipes, and the ichneumonid wasp larval parasitoid, Exenterus amictorius (Invasive Species Compendium; CABI). Abiotic conditions are important mortality factors for second generation caterpillars as well. Cold winter temperatures may kill up to 50% of second generation caterpillars before they reach the cocoon (pupal) stage.

Chemical Management: 

Acephate (NL)

Azadirachtin (NL)

Bifenthrin+imidacloprid (L)

Carbaryl (L)

Chlorpyrifos (N)

Cyfluthrin (larvae) (NL)

Deltamethrin (larvae) (L)

Dinotefuran (larvae (NL)

Emamectin benzoate (L)

Flonicamid+cyclaniliprole (N)

Gamma-cyhalothrin (L)

Horticultural oil (larvae) (L)

Imidacloprid (larvae) (L)

Insecticidal soap (larvae) (NL)

Lambda-cyhalothrin (L)

Permethrin (L)

Pyrethrin+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).

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: .