The pear sawfly is a non-native species introduced from Europe into the United States that feeds on a wide variety of trees and shrubs. Because of the slimy, non-segmented appearance of the immature larvae (caterpillars), these insects are sometimes called slugs. The pear sawfly overwinters in the soil as a mature (fully grown) larva which will undergo pupation in the spring. Adult sawflies emerge from pupation in late-May and June. Adults are glossy black and yellow wasps. Female sawfly adults will lay their 1 mm long, oval, tan eggs one at a time on the underside of the leaf, in a blister-like pocket between the two epidermal layers of the leaf. When eggs hatch, the caterpillars feed initially as skeletonizers on the upper side of the leaf. Young caterpillars are a dark greenish/black color and slimy, lack distinctly visible legs, are approximately 1.4 mm long, and are wider at the front end. Skeletonized leaves turn brown, with premature leaf drop occurring at times. Caterpillars are found feeding on host plant leaves for up to 4 weeks, and pass through 5 instars. Mature caterpillars are yellow, slimy, have 10 pairs of visible legs, and are 1/2 inch long. Upon maturity, caterpillars drop to the ground to form cells within which they pupate in the soil. It is possible for a second generation of adults to emerge by August. Activity of the second generation may continue through September, until the mature larvae of the second generation move to the soil to overwinter. Other slug-like sawfly species are discussed in this guide, including the roseslugs.
Pear slug sawfly caterpillars feed on the foliage of cotoneaster, hawthorn, flowering fruit trees (including ornamental plum), and others. Young caterpillars begin their feeding as skeletonizers, which can lead to the leaf turning brown. Occasionally, completely browned leaves may drop prematurely from the host plant. A second generation of caterpillars feeds into late summer, and may cause defoliation. Often, the second generation population is larger and more damaging.
Monitor for pear sawfly larval (caterpillar) feeding in June/July and again in August/September. Second generation caterpillars may be more abundant, and thus may cause more damage later in the season.
The caterpillars of this species are reportedly extremely susceptible to drying out. Caterpillars may be hand collected, if they can be reached, and dropped into a bucket of soapy water. Syringing, or knocking pear sawfly caterpillars off of their host plant with a strong stream of water, is noted by some as a possible mechanical management option. Certain pear seedlings may exhibit patterns of resistance to pear sawfly feeding. This work has been done in New Zealand (Brewer et al., 2000; Shaw et al., 2003).
Larval and cocoon parasitism is noted for the pear sawfly in areas outside of the United States. A parasitic flagellate (Blastocrithidia caliroae n.spp.) was first described in pear sawfly populations in New Zealand, and found to also be causing high larval mortality and regulating populations in Europe (Lipa et al., 1977). Eight parasites and one hyperparasite and a predacious pentatomid have also been described in Europe (Carl, 1976). Trichogramma minutum is a recorded parasitoid of pear sawflies in Argentina (Bado, 2014). Historically, in Iowa, natural enemies of the pear sawfly have been reported. These include Trichogramma (previously Pentarthron) minutum which was (in addition to in Argentina) also an egg parasitoid of pear sawflies in Iowa, Closterocerus cinctipennis (an egg parasitoid), and Rhyssalus selandriae (a possible larval parasitoid) (Webster, 1912, Agricultural Experiment Station, Iowa State College of Agriculture and the Mechanic Arts). Webster (1912) also discusses Tryphon ratzeburgii and T. gorskii as two hymenopterous parasitoids reported in Europe at the time. Podisus maculiventris (spined soldier bug), Sinea diadema (assassin bug), and Chrysopa spp. (green lacewing) larvae are also mentioned by Webster (1912) as predators of the pear sawfly. Clarification of the efficacy of many of these at reducing pear sawfly populations, or their distributions in North America, is necessary. However, because pear sawfly populations are typically below damaging levels on their host plants (and populations tend to crash following any isolated outbreaks) it is assumed that natural enemies are at work in their populations in much of the United States.
Acephate (NL)
Azadirachtin (NL)
Carbaryl (L)
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) (N)
Spinosad (Larvae) (NL)
Neem oil deters feeding by pear sawfly caterpillars in bioassays conducted with this insect. Reduced feeding, increased mortality, and slower development were observed in these trials (Smirle and Wei, 1996). Trials using Beauveria bassiana also indicate success using that active ingredient to manage this insect, however those were also completed under lab conditions (Aslantas et al., 2008). Make sure sawflies are listed on the label of any products considered for use.
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.
