The mountain ash sawfly is found in the Northeast as well as parts of Canada, but also occurs in Europe. Historically, scientists were uncertain of its native origin - whether it came from Europe, or not (Johnson and Lyon, 1991). Natural Resources Canada refers to it as a non-native species. Prepupa overwinter, and starting in late-May the adult sawflies emerge from the soil over a period of approximately 6 weeks. Adults may be found into early July. A second generation of adults occurs by mid-August. In each case, the females lay their eggs in the host plant leaves by cutting a slit in the epidermis with their ovipositor (egg laying structure) at the edges of the leaflet. Adult sawflies live for approximately one week. The first generation eggs hatch by early-June and are present until early-August with second generation eggs hatching and larvae found in September. Larvae feed for approximately 3 weeks in groups - stripping branches of all of their foliage before moving on to the next. Caterpillars are at first pale yellow in color, and eventually yellow with black flecks. Caterpillars will rear up, raising their heads, in an "S" shape if disturbed. The oldest caterpillars may eventually feed alone in the tree. Once mature, the caterpillars drop to the soil and create an earthen cell within which they form a paper cocoon to pupate in. The overwintering life stage (the prepupa) is capable of remaining in a diapause (resting) state for multiple years.
Foliage of mountain ash is fed upon. This sawfly consumes all of the leaf, except the midveins. While noticeable, the damage caused by the mountain ash sawfly often does not impact the overall health of its hosts - thus, this insect typically does not require chemical management and should be tolerated. It is likely because this insect feeds in the middle to the end of the growing season, so host plants still retain enough of their food reserves.
Visually observe the edges of mountain ash leaflets for deposited eggs by late-May or early-June. This may look like tiny, swollen brown ovals on the edges of the serrated leaves. Within a week, search for the early (first) instar larvae which are initially pale yellow in color. Larval feeding may begin in the crown of the tree, moving toward the lower branches later.
Hand pick and destroy infested leaves if the egg stage or early instar larvae from the first generation are detected. It is best to capture and remove them at these stages, as the later instar larvae (mature) are capable of stripping the foliage from entire branches of the host plant.
In Canada, an ichneumonid wasp parasitoid (Olesicampe geniculatae) was released for the biological control of the mountain ash sawfly from 1981-1984 and again in 1986. By 1990, the parasitoid was well established at some of the release sites and the mountain ash sawfly population had dropped below damaging levels (West et al., 1994).
Acephate (NL)
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)
Spinetoram+sulfoxaflor (larvae)
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.
When used in a nursery setting, chlorpyrifos is for quarantine use only.
Active ingredients that may be applied systemically include: Acephate (injection), 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.
