Our insect of the week for August 31, 2026 is the poison ivy sawfly, Arge humeralis [HYMENOPTERA: Argidae]. It was described by the French naturalist Palisot de Beauvois in 1809. Fun fact, his full name/ title was Ambroise Marie François Joseph Palisot, Baron de Beauvois. Fun fact, while traveling to the United States, he was robbed of his worldly goods and arrived destitute in Philadelphia. He joined a circus as a musician to earn money.
Ok, Mark, but what about the sawfly? Adults are observed in central Illinois from June through August. As you can see, adults resemble small wasps (perhaps Braconidae). Nice example of Batesian mimicry since their ovipositor is saw-like and used for laying eggs, not for stinging (unlike braconid wasps which will often readily attempt to sting). Females use their ovipositor to insert eggs along the margins of leaves. The larvae prefer poison ivy (Toxicodendron radicans) and poison sumac (Toxicodendron vernix). Early instars feed in groups; older instars feed individually. Another fun fact – since larvae prefer poison ivy, this species has been evaluated as a control agent for the plant.
Aside – in case you did not know, although we consider this a noxious weed, poison ivy is the preferred food plant for: the poison ivy sawfly (obviously), the showy emerald moth (Dichorda iridaria), the dark Marathyssa moth (Marathyssa basalis), leafblotch miner moths (Caloptilia rhoifoliella and Cameraria guttifinitella), and the woodbine borer beetle (Saperda puncticollis). These insects are, in turn, protein food for birds, reptiles, amphibians, and mammals. In fact, 153 species of invertebrates, 48 species of birds, and 7 species of mammals use or depend on this plant in some way. If you want to learn more, please check out the link below.
The photo below was taken by Jose during our July, 2026, field trip to Jubilee State Park, near Peoria, Illinois.
The photo below was taken by Ruth during that same field trip.
